Semiconductor Components Industries, LLC, 2012
July, 2012 − Rev. 8
1Publication Order Number:
ADT7476/D
ADT7476
Remote Thermal Controller
and Voltage Monitor
The ADT7476 controller is a thermal monitor and multiple PWM
fan controller for noise-sensitive or power-sensitive applications
requiring active system cooling. The ADT7476 can drive a fan using
either a low or high frequency drive signal and can monitor the
temperature of up to two remote sensor diodes plus its own internal
temperature. The part also measures and controls the speed of up to
four fans, so the fans operate at the lowest possible speed for minimum
acoustic noise.
The automatic fan speed control loop optimizes fan speed
for a given temperature. The effectiveness of the system’s thermal
solution can be monitored using the THERM input. The ADT7476
also provides critical thermal protection to the system using the
bidirectional THERM pin as an output to prevent system or
component overheating.
Features
Monitors Up to Five Voltages
Controls and Monitors Up to Four Fans
High and Low Frequency Fan Drive Signal
One On-Chip and Two Remote Temperature Sensors
Extended Temperature Measurement Range Up to 191C
Automatic Fan Speed Control Mode Controls System Cooling Based
on Measured Temperature
Enhanced Acoustic Mode Dramatically Reduces User Perception of
Changing Fan Speeds
Thermal Protection Feature via THERM Output
Monitors Performance Impact of Intel Pentium4 Processor
Thermal Control Circuit via THERM Input
3-wire and 4-wire Fan Speed Measurement
Limit Comparison of All Monitored Values
Meets SMBus 2.0 Electrical Specifications
This Device is Pb-Free, Halogen Free and is RoHS Compliant
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PIN ASSIGNMENT
QSOP−24 NB
CASE 492B
ADT7476RQZ = Specific Device Code
# = Pb-Free Package
YYWW = Date Code
xxxx = Assembly Lot Code
See detailed ordering and shipping information in the package
dimensions section on page 64 of this data sheet.
ORDERING INFORMATION
MARKING DIAGRAMS
ADT7476RQZ
#YYWW
xxxx
+2.5VIN/THERM
PWM1/XTO
VCCP
+12VIN/VID5
+5VIN
D1+
*TACH4/THERM/SMBALERT/GPIO6/ADDR SELECT
VID4/GPIO4
SDA
SCL
GND
VCC
VID0/GPIO0
VID1/GPIO1
VID2/GPIO2
PWM2/
SMBALERT
24
23
22
21
20
19
18
17
8
7
6
5
4
3
2
1
ADT7476
(Top View)
16
9
15
10
14
11
13
12
VID3/GPIO3
TACH3
TACH1
TACH2
D1−
D2+
D2−
PWM3/ADDREN
*
ADT7476
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Figure 1. Functional Block Diagram
ADT7476
PWM REGISTERS
AND CONTROLLERS
(HF AND LF)
AUTOMATIC
FAN SPEED
CONTROL
FAN SPEED
COUNTER
PERFORMANCE
MONITORING
THERMAL
PROTECTION
INPUT
SIGNAL
CONDITIONING
AND
ANALOG
MULTIPLEXER
BAND GAP
TEMP. SENSOR
VCC TO ADT7476
10-BIT
ADC
BAND GAP
REFERENCE
SMBus
ADDRESS
SELECTION
SERIAL BUS
INTERFACE
ADDRESS
POINTER
REGISTER
PWM
CONFIGURATION
REGISTERS
INTERRUPT
MASKING
INTERRUPT
STATUS
REGISTERS
VALUE AND
LIMIT
REGISTERS
LIMIT
COMPARATORS
GND
ADDR
SELECTADDREN SCL SDA SMBALERT
PWM1
PWM2
PWM3
TACH1
TACH2
TACH3
TACH4
THERM
VCC
D1+
D1−
D2+
D2−
+5VIN
VID/GPIO
REGISTER
+12VIN
+2.5VIN
VCCP
VID0/GPIO0
VID1/GPIO1
VID2/GPIO2
VID3/GPIO3
VID4/GPIO4
VID5/GPIO5
GPIO6
ACOUSTIC
ENHANCEMENT
CONTROL
Table 1. ABSOLUTE MAXIMUM RATINGS
Parameter Rating Unit
Positive Supply Voltage (VCC) 3.6 V
Maximum Voltage on +12 VIN Pin 16 V
Maximum Voltage on +5.0 VIN Pin 6.25 V
Maximum Voltage on All Open-Drain Outputs 3.6 V
Input Current at Any Pin 5 mA
Package Input Current 20 mA
Maximum Junction Temperature (TJ MAX) 150 C
Storage Temperature Range −65 to +150 C
Lead Temperature, Soldering
IR Reflow Peak Temperature
Pb-Free Peak Temperature
Lead Temperature (Soldering, 10 sec)
220
260
300
C
ESD Rating 1,500 V
Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the
Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect
device reliability.
NOTE: This device is ESD sensitive. Use standard ESD precautions when handling.
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Table 2. THERMAL CHARACTERISTICS (Note 1)
Package Type qJA qJC Unit
24-lead QSOP 122 31.25 C/W
1. qJA is specified for the worst-case conditions, that is, a device soldered in a circuit board for surface-mount packages.
Table 3. PIN ASSIGNMENT
Pin No. Mnemonic Description
1 SDA Digital I/O (Open Drain). SMBus bidirectional serial data. Requires SMBus pullup.
2 SCL Digital Input (Open Drain). SMBus serial clock input. Requires SMBus pullup.
3 GND Ground Pin.
4 VCC Power Supply. Powered by 3.3 V standby, if monitoring in low power states is required. VCC is also
monitored through this pin.
5 VID0/
GPIO0
Digital Input. Voltage supply readouts from CPU. This value is read into the VID/GPIO register (0x43).
General-Purpose Open Drain Digital I/O.
6 VID1/
GPIO1
Digital Input. Voltage supply readouts from CPU. This value is read into the VID/GPIO register (0x43).
General-Purpose Open Drain Digital I/O.
7 VID2/
GPIO2
Digital Input. Voltage supply readouts from CPU. This value is read into the VID/GPIO register (0x43).
General-Purpose Open Drain Digital I/O.
8 VID3/
GPIO3
Digital Input. Voltage supply readouts from CPU. This value is read into the VID/GPIO register (0x43).
General-Purpose Open Drain Digital I/O.
9 TACH3 Digital Input (Open Drain). Fan tachometer input to measure speed of Fan 3.
10 PWM2/
SMBALERT
Digital Output (Open Drain). Requires 10 kW typical pullup. Pulse width modulated output to control Fan 2
speed. Can be configured as a high or low frequency drive. Digital Output (Open Drain). This pin can be
reconfigured as an SMBALERT interrupt output to signal out-of-limit conditions.
11 TACH1 Digital Input (Open Drain). Fan tachometer input to measure speed of Fan 1.
12 TACH2 Digital Input (Open Drain). Fan tachometer input to measure speed of Fan 2.
13 PWM3
ADDREN
Digital I/O (Open Drain). Pulse width modulated output to control the speed of Fan 3 and Fan 4. Requires
10 kW typical pullup. Can be configured as a high or low frequency drive.
If pulled low on powerup, the ADT7476 enters address select mode, and the state of Pin 14 (ADDR SELECT)
determines the ADT7476’s slave address.
14 TACH4/
THERM/
SMBALERT/
GPIO6/
ADDR SELECT
Digital Input (Open Drain). Fan tachometer input to measure speed of Fan 4.
Alternatively, the pin can be reconfigured as a bidirectional THERM pin. Times and monitors assertions on
the THERM input. For example, it can be connected to the PROCHOT output of Intel’s Pentium4
processor or to the output of a trip point temperature sensor. Can be used as an output to signal
overtemperature conditions.
Digital Output (Open Drain). This pin can be reconfigured as an SMBALERT interrupt output to signal
out-of-limit conditions.
General-Purpose Open Drain Digital I/O.
If in address select mode, the logic state of this pin defines the SMBus device address.
15 D2– Cathode Connection to Second Thermal Diode.
16 D2+ Anode Connection to Second Thermal Diode.
17 D1– Cathode Connection to First Thermal Diode.
18 D1+ Anode Connection to First Thermal Diode.
19 VID4/
GPIO4
Digital Input. Voltage supply readouts from CPU. This value is read into the VID/GPIO register (0x43).
General-Purpose Open Drain Digital I/O.
20 +5.0 VIN Analog Input. Monitors 5.0 V power supply.
21 +12 VIN/
VID5
Analog Input. Monitors 12 V power supply.
Digital Input. Voltage supply readouts from CPU. This value is read into the VID/GPIO register (0x43).
22 +2.5 VIN/
THERM
Analog Input. Monitors 2.5 V supply, typically a chipset voltage.
Alternatively, this pin can be reconfigured as a bidirectional/omnidirectional THERM pin. Can be used to
time and monitor assertions on the THERM input. For example, can be connected to the PROCHOT
output of Intel’s Pentium4 processor or to the output of a trip point temperature sensor. Can be used as
an output to signal overtemperature conditions.
23 VCCP Analog Input. Monitors processor core voltage (0 V to 3.0 V).
24 PWM1/
XTO
Digital Output (Open Drain). Pulse width modulated output to control the speed of Fan 1. Requires 10 kW
typical pullup.
Also functions as the output from the XOR tree in XOR test mode.
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Table 4. ELECTRICAL CHARACTERISTICS (TA=T
MIN to TMAX, VCC =V
MIN to VMAX, unless otherwise noted.) (Note 1)
Parameter Conditions Min Typ Max Unit
Power Supply
Supply Voltage 3.0 3.3 3.6 V
Supply Current, ICC Interface Inactive, ADC Active −1.5 3.0 mA
Temperature-to-Digital Converter
Local Sensor Accuracy
Resolution
0C TA 85C
−40C TA 125C
−
−
−
0.5
−
0.25
1.5
2.5
−
C
Remote Diode Sensor Accuracy
Resolution
0C TA 85C
−40C TA 125C
−
−
−
0.5
−
0.25
1.5
2.5
−
C
Remote Sensor Source Current Low Level
High Level
−
−
11
180
−
−
mA
Analog-to-Digital Converter (Including MUX and Attentuators)
Total Unadjusted Error (TUE) For 12 V Channel
For All Other Channels
−
−
−
−
2
1.5
%
Differential Non-linearity (DNL) 8 Bits − − 1 LSB
Power Supply Sensitivity −0.1 −%/V
Conversion Time
Voltage Input
Local Temperature
Remote Temperature
Averaging Enabled
−
−
−
11
12
38
−
−
−
ms
Total Monitoring Cycle Time Averaging Enabled
Averaging Disabled
−
−
145
19
−
−
ms
Input Resistance For VCCP channel
For all other channels
70
70
120
114
−
−
kW
Fan RPM-to-Digital Converter
Accuracy 0C TA 70C
−40C TA +120C
−
−
−
−
6
10
%
Full-Scale Count − − 65,535
Nominal Input RPM Fan Count = 0xBFFF
Fan Count = 0x3FFF
Fan Count = 0x0438
Fan Count = 0x021C
−
−
−
−
109
329
5,000
10,000
−
−
−
−
RPM
Open-Drain Digital Outputs, PWM1 TO PWM3, XTO
Current Sink, IOL − − 8.0 mA
Output Low Voltage, VOL IOUT = −8.0 mA − − 0.4 V
High Level Output Current, IOH VOUT = VCC −0.1 20 mA
Open-Drain Serial Data Bus Output (SDA)
Output Low Voltage, VOL IOUT = −4.0 mA − − 0.4 V
High Level Output Current, IOH VOUT = VCC −0.1 1.0 mA
SMBus Digital Inputs (SCL, SDA) (Note 2)
Input High Voltage, VIH 2.0 − − V
Input Low Voltage, VIL − − 0.4 V
Hysteresis −500 −mV
Digital Input Logic Levels (TACH Inputs)
Input High Voltage, VIH Maximum Input Voltage 2.0 −3.6 V
Input Low Voltage, VIL Minimum Input Voltage −0.3 −0.8 V
Hysteresis −0.5 −V p-p
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Table 4. ELECTRICAL CHARACTERISTICS (TA=T
MIN to TMAX, VCC =V
MIN to VMAX, unless otherwise noted.) (Note 1)
Parameter UnitMaxTypMinConditions
Digital Input Logic Levels (THERM) ADTL+
Input High Voltage, VIH 0.75 x VCCP − − V
Input Low Voltage, VIL − − 0.8 V
Digital Input Current
Input High Current, IIH VIN = VCC −1−mA
Input Low Current, IIL VIN = 0 V −1−mA
Input Capacitance, CIN −5.0 −pF
Serial Bus Timing (See Figure 2)
Clock Frequency, fSCLK 10 −400 kHz
Glitch Immunity, tSW − − 50 ns
Bus Free Time, tBUF 4.7 − − ms
SCL Low Time, tLOW 4.7 − − ms
SCL High Time, tHIGH 4.0 −50 ms
SCL, SDA Rise Time, tr− − 1,000 ns
SCL, SDA Fall Time, tf− − 300 ms
Data Setup Time, tSU;DAT 250 − − ns
Detect Clock Low Timeout, tTIMEOUT Can be Optionally Disabled 15 −35 ms
1. All voltages are measured with respect to GND, unless otherwise specified. Typical voltages are TA=25C and represent a parametric norm.
Logic inputs accept input high voltages up to VMAX, even when the device is operating down to VMIN. Timing specifications are tested at logic
levels of VIL = 0.8 V for a falling edge, and VIH = 2.0 V for a rising edge.
2. SMBus timing specifications are guaranteed by design and are not production tested.
Figure 2. Serial Bus Timing Diagram
P
S
tSU; DAT
tHIGH
tF
tHD; DAT
tR
tLOW
tSU; STO
PS
SCL
SDA
tBUF
tHD; STA
tHD; STA
tSU; STA
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TYPICAL PERFORMANCE CHARACTERISTICS
Figure 3. Temperature Error vs. Capacitance
Between D+ and D−
Figure 4. Remote Temperature Error vs. PCB
Resistance
Figure 5. Remote Temperature Error vs.
Common-Mode Noise Frequency
Figure 6. Remote Temperature Error vs.
Differential-Mode Noise Frequency
Figure 7. Normal IBDDB vs. Power Supply Figure 8. Internal Temperature Error vs. Power
Supply Noise
CAPACITANCE (nF)
0
TEMPERATURE ERROR (C)
−60
246810
−50
−40
−30
−20
−10
0
12 14 16 18 20 22
LEAKAGE RESISTANCE (MW)
0
TEMPERATURE ERROR (C)
−40
D+ To VCC
20 40 60 80 100
−30
−20
0
10
20
30
D+ To GND
−10
NOISE FREQUENCY (Hz)
0
TEMPERATURE ERROR (C)
−5
100 mV
100M 200M 300M 400M 500M 600M
0
5
10
15
20
25
30
60 mV
40 mV
NOISE FREQUENCY (Hz)
0
TEMPERATURE ERROR (C)
−10
100 mV
100M 200M 300M 400M 500M 600M
0
10
20
30
40
50
60
60 mV
40 mV
70
VDD (V)
3.0
IDD (mA)
0.98
3.1 3.2 3.3 3.4 3.5 3.6
1.00
1.02
1.04
1.06
1.08
1.10
1.12
1.14
1.16
1.18
1.20
FREQUENCY (Hz)
0
TEMPERATURE ERROR (C)
−15
100 mV
100M 200M 300M 400M 500M 600M
−10
−5
0
5
10
15
250 mV
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TYPICAL PERFORMANCE CHARACTERISTICS (Cont’d)
Figure 9. Remote Temperature Error vs. Power
Supply Noise Frequency
Figure 10. Internal Temperature Error vs. Temperature
Figure 11. Remote Temperature Error vs. Temperature
FREQUENCY (Hz)
0
TEMPERATURE ERROR (C)
−12
100M 200M 300M 400M 500M 600M
−10
−8
−6
−4
−2
0
2
4
6
100 mV
250 mV
OIL BATH TEMPERATURE (C)
−40
TEMPERATURE ERROR (C)
−20 0 20 40 60 85 105 125
−1.5
−1.0
−0.5
0
0.5
1.0
1.5
2.0
2.5
3.0
OIL BATH TEMPERATURE (C)
−40
TEMPERATURE ERROR (C)
−1.5
−20 0 20 40 60 85 105 125
−1.0
−0.5
0
0.5
1.0
1.5
2.0
2.5
3.0
−2.0
Figure 12. THERM Input Threshold vs. VCCP Voltage
VCCP (V)
0
TRIP POINT (V)
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
2.5 V Applied to 2.5 V Pin
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Product Description
The ADT7476 is a complete thermal monitor and multiple
fan controller for any system requiring thermal monitoring
and cooling. The device communicates with the system via
a serial system management bus. The serial bus controller
has a serial data line for reading and writing addresses and
data (Pin 1), and an input line for the serial clock (Pin 2). All
control and programming functions for the ADT7476 are
performed over the serial bus. In addition, a pin can be
reconfigured as an SMBALERT output to signal
out-of-limit conditions.
Feature Comparisons Between ADT7476 and ADT7468
Dynamic TMIN, dynamic operating point, and
associated registers are no longer available in the
ADT7476. The following related registers are gone:
Calibration Control 1 (0x36)
Calibration Control 2 (0x37)
Operating Point (0x33, 0x34, and 0x35)
Previously (in the ADT7468), TRANGE defined the slope
of the automatic fan control algorithm. TRANGE now
defines a true temperature range (in the ADT7476).
Acoustic filtering is now assigned to temperature zones,
not to fans. Available smoothing times have been
increased for better acoustic performance.
Temperature measurements are now made with two
switching currents instead of three. SRC is not available
in the ADT7476.
High frequency PWM can now be enabled/disabled on
each PWM output individually.
THERM can now be enabled/disabled on each
temperature channel individually.
The ADT7476 does not support full shutdown mode.
The ADT7476 offers increased temperature accuracy
on all temperature channels.
The ADT7476 defaults to twos complement
temperature measurement mode.
Some pins have swapped/added functions.
The powerup routine for the ADT7476 is simplified.
The ADT7476 has a higher maximum input voltage
TACH/PWM spec, supporting a wider range of fans.
VCORE_LOW_ENABLE has been reallocated to Bit 7 of
Configuration Register 1 (0x40).
Recommended Implementation
Configuring the ADT7476 as shown in Figure 13 allows
the system designer to use the following features:
Two PWM outputs for fan control of up to three fans
(the front and rear chassis fans are connected in
parallel).
Three TACH fan speed measurement inputs.
VCC measured internally through Pin 4.
CPU temperature measured using Remote 1
temperature channel.
Remote temperature zone measured through Remote 2
temperature channel.
Local temperature zone measured through the internal
temperature channel.
Bidirectional THERM pin. This feature allows
IntelPentium4 PROCHOT monitoring and can
function as an overtemperature THERM output. It can
alternatively be programmed as an SMBALERT system
interrupt output.
Figure 13. ADT7476 Configuration
AMBIENT
TEMPERATURE
FRONT
CHASSIS
FAN
REAR
CHASSIS
FAN
ADT7476
PWM1
SMBALERT
TACH2
PROCHOT
TACH1
GND
VID[0:4]/VID[0:5]
D2+
D2−
THERM
SDA
SCL
TACH3
PWM3
D1−
D1+
VCC
+5VIN
+12VIN/VID5
5(VRM9)/6(VRM10)
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Serial Bus Interface
Control of the ADT7476 is carried out using the serial
system management bus (SMBus). The ADT7476 is
connected to this bus as a slave device, under the control of
a master controller. The ADT7476 has a 7-bit serial bus
address. When the device is powered up with Pin 13
(PWM3/ADDREN) high, the ADT7476 has a default
SMBus address of 0101110 or 0x2E. The read/write bit must
be added to get the 8-bit address. If more than one ADT7476
is to be used in a system, each ADT7476 is placed in ADDR
SELECT mode by strapping Pin 13 low on powerup. The
logic state of Pin 14 then determines the device’s SMBus
address. The logic of these pins is sampled on powerup.
The device address is sampled on powerup and latched on
the first valid SMBus transaction, more precisely on the
low-to-high transition at the beginning of the eighth SCL
pulse, when the serial bus address byte matches the selected
slave address. The selected slave address is chosen using the
ADDREN pin/ADDR SELECT pin. Any attempted
changes in the address have no effect after this.
Table 5. HARDWIRING THE ADT7476 SMBUS
DEVICE ADDRESS
Pin 13 State Pin 14 State Address
0Low (10 kW to GND) 0101100 (0x2C)
0High (10 kW Pullup) 0101101 (0x2D)
1Don’t Care 0101110 (0x2E)
Figure 14. Default SMBus Address = 0x2E
ADDR SELECT
ADT7476
14
13
PWM3/
ADDREN
10 kW
ADDRESS = 0x2E
VCC
Figure 15. SMBus Address = 0x2C (Pin 14 = 0)
ADDR SELECT
ADT7476
14
13
PWM3/
ADDREN
10 kW
ADDRESS = 0x2C
Figure 16. SMBus Address = 0x2D (Pin 14 = 1)
ADDR SELECT
ADT7476
14
13
PWM3/
ADDR_EN
10 kW
ADDRESS = 0x2D
VCC
Figure 17. Unpredictable SMBus Address if Pin 13
is Unconnected
DO NOT LEAVE ADDREN
UNCONNECTED! CAN CAUSE
UNPREDICTABLE ADDRESSES.
NOTE THAT IF THE ADT7476 IS PLACED INTO ADDR SELECT
MODE, PINS 13 AND 14 CANNOT BE USED AS THE ALTERNATE
FUNCTIONS (PWM3, TACH4/THERM) UNLESS THE CORRECT
CIRCUIT IS MUXED IN AT THE CORRECT TIME OR DESIGNED
TO HANDLE THESE DUAL FUNCTIONS.
CARE SHOULD BE TAKEN TO ENSURE THAT PIN 13
(PWM3/ADDREN) IS EITHER TIED HIGH OR LOW. LEAVING
PIN 13 FLOATING COULD CAUSE THE ADT7476 TO POWER UP
WITH AN UNEXPECTED ADDRESS.
ADDR SELECT
ADT7476
14
13
PWM3/
ADDREN
10 kW
VCC
NC
The ability to make hardwired changes to the SMBus
slave address allows the user to avoid conflicts with other
devices sharing the same serial bus, for example, if more
than one ADT7476 is used in a system.
The serial bus protocol operates as follows:
1. The master initiates data transfer by establishing a
start condition, which is defined as a high-to-low
transition on the serial data line SDA while the
serial clock line SCL remains high. This indicates
that an address/data stream follows. All slave
peripherals connected to the serial bus respond to
the start condition and shift in the next eight bits,
consisting of a 7-bit address (MSB first), plus a
R/W bit, which determine the direction of the data
transfer, that is, whether data is written to or read
from the slave device.
The peripheral whose address corresponds to the
transmitted address responds by pulling the data
line low during the low period before the ninth
clock pulse, known as the acknowledge bit. All
other devices on the bus now remain idle while the
selected device waits for data to be read from or
written to it. If the R/W bit is a 0, the master writes
to the slave device. If the R/W bit is a 1, the master
reads from the slave device.
2. Data is sent over the serial bus in sequences of nine
clock pulses, eight bits of data followed by an
acknowledge bit from the slave device. Transitions
on the data line must occur during the low period of
the clock signal and remain stable during the high
period. A low-to-high transition, when the clock is
high, can be interpreted as a stop signal. The
number of data bytes transmitted over the serial bus
in a single read or write operation is limited only by
what the master and slave devices can handle.
3. When all data bytes have been read or written, stop
conditions are established. In write mode, the
master pulls the data line high during the 10th clock
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pulse to assert a stop condition. In read mode, the
master device overrides the acknowledge bit by
pulling the data line high during the low period
before the ninth clock pulse. This is known as no
acknowledge. The master then takes the data line
low during the low period before the 10th clock
pulse, and then high during the 10th clock pulse to
assert a stop condition.
Any number of bytes of data can be transferred over the
serial bus in one operation. However, it is not possible to mix
read and write in one operation because the type of operation
is determined at the beginning and cannot subsequently be
changed without starting a new operation. In the ADT7476,
write operations contain either one or two bytes, and read
operations contain one byte.
To write data to one of the device data registers or read
data from it, the address pointer register must be set so the
correct data register is addressed. Then, data can be written
into that register or read from it. The first byte of a write
operation always contains an address stored in the address
pointer register. If data is to be written to the device, then the
write operation contains a second data byte that is written to
the register selected by the address pointer register.
This write operation is illustrated in Figure 18. The device
address is sent over the bus, and then R/W is set to 0. This
is followed by two data bytes. The first data byte is the
address of the internal data register to be written to, which
is stored in the address pointer register. The second data byte
is the data to be written to the internal data register.
When reading data from a register, there are two possibilities:
1. If the ADT7476’s address pointer register value is
unknown, or not the desired value, then it must
first be set to the correct value before data can be
read from the desired data register. This is done by
performing a write to the ADT7476 as before, but
only the data byte containing the register address
is sent, because no data is written to the register
(see Figure 19).
A read operation is then performed consisting of
the serial bus address; R/W bit set to 1, followed
by the data byte read from the data register (see
Figure 20.)
2. If the address pointer register is already known to
be at the desired address, data can be read from the
corresponding data register without first writing to
the address pointer register (see Figure 20).
It is possible to read a data byte from a data register
without first writing to the address pointer register, if the
address pointer register is already at the correct value.
However, it is not possible to write data to a register without
writing to the address pointer register, because the first data
byte of a write is always written to the address pointer
register.
In addition to supporting the send byte and receive byte
protocols, the ADT7476 also supports the read byte protocol.
See Intel’s System Management Bus Specifications
Revision 2 for more information.
If several read or write operations must be performed in
succession, the master can send a repeat start condition
instead of a stop condition to begin a new operation.
Figure 18. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
0
SCL
SDA 10 11A1 A0 D7 D6 D5 D4 D3 D2 D1 D0
ACK. BY
ADT7476
START BY
MASTER
19
1
ACK. BY
ADT7476
9
D7 D6 D5 D4 D3 D2 D1 D0
ACK. BY
ADT7476 STOP BY
MASTER
19
SCL (CONTINUED)
SDA (CONTINUED)
FRAME 1
SERIAL BUS ADDRESS BYTE FRAME 2
ADDRESS POINTER REGISTER BYTE
FRAME 3
DATA BYTE
R/W
Figure 19. Writing to the Address Pointer Register Only
0
SCL
SDA 10
11A1 A0 D7 D6 D5 D4 D3 D2 D1 D0
ACK. BY
MASTER
STOP BY
MASTER
START BY
MASTER FRAME 1
SERIAL BUS ADDRESS BYTE FRAME 2
ADDRESS POINTER REGISTER BYTE
119
ACK. BY
ADT7476
9
R/W
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Figure 20. Reading Data from a Previously Selected Register
0
SCL
SDA 1011A1 A0 D7 D6 D5 D4 D3 D2 D1 D0
NO ACK. BY
MASTER STOP BY
MASTER
START BY
MASTER FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
DATA BYTE FROM ADT7476
119
ACK. BY
ADT7476
9
R/W
Write Operations
The SMBus specification defines several protocols for
different types of read and write operations. The ones used
in the ADT7476 are discussed below. The following
abbreviations are used in the diagrams:
S – START
P – STOP
R – READ
W– WRITE
A – ACKNOWLEDGE
A – NO ACKNOWLEDGE
The ADT7476 uses the following SMBus write protocols.
Send Byte
In this operation, the master device sends a single
command byte to a slave device, as follows:
1. The master device asserts a start condition on SDA.
2. The master sends the 7-bit slave address followed
by the write bit (low).
3. The addressed slave device asserts ACK on SDA.
4. The master sends a command code.
5. The slave asserts ACK on SDA.
6. The master asserts a stop condition on SDA, and
the transaction ends.
For the ADT7476, the send byte protocol is used to write
a register address to RAM for a subsequent single-byte read
from the same address. This operation is illustrated in
Figure 21.
Figure 21. Setting a Register Address for
Subsequent Read
SLAVE
ADDRESS WASAP
REGISTER
ADDRESS
231564
If the master is required to read data from the register
immediately after setting up the address, it can assert a repeat
start condition immediately after the final ACK and carry
out a single byte read without asserting an intermediate stop
condition.
Write Byte
In this operation, the master device sends a command byte
and one data byte to the slave device, as follows:
1. The master device asserts a start condition on SDA.
2. The master sends the 7-bit slave address followed
by the write bit (low).
3. The addressed slave device asserts ACK on SDA.
4. The master sends a command code.
5. The slave asserts ACK on SDA.
6. The master sends a data byte.
7. The slave asserts ACK on SDA.
8. The master asserts a stop condition on SDA,
and the transaction ends.
This operation is illustrated in Figure 22.
Figure 22. Single-byte Write to a Register
SLAVE
ADDRESS WA DATASAAP
REGISTER
ADDRESS
23156784
Read Operations
The ADT7476 uses the following SMBus read protocols.
Receive Byte
This operation is useful when repeatedly reading a single
register. The register address is set up beforehand. In this
operation, the master device receives a single byte from a
slave device, as follows:
1. The master device asserts a start condition on SDA.
2. The master sends the 7-bit slave address followed
by the read bit (high).
3. The addressed slave device asserts ACK on SDA.
4. The master receives a data byte.
5. The master asserts NO ACK on SDA.
6. The master asserts a stop condition on SDA, and
the transaction ends.
In the ADT7476, the receive byte protocol is used to read
a single byte of data from a register whose address has
previously been set by a send byte or write byte operation.
This operation is illustrated in Figure 23.
Figure 23. Single-byte Read from a Register
SLAVE
ADDRESS DATAARSAP
243156
Alert Response Address
Alert response address (ARA) is a feature of SMBus
devices, allowing an interrupting device to identify itself to
the host when multiple devices exist on the same bus.
The SMBALERT output can be used as either an interrupt
output or an SMBALERT. One or more outputs can be
connected to a common SMBALERT line connected to the
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12
master. If a device’s SMBALERT line goes low, the
following procedure occurs:
1. SMBALERT is pulled low.
2. The master initiates a read operation and sends the
alert response address (ARA = 0001 100). This is
a general call address that must not be used as a
specific device address.
3. The device whose SMBALERT output is low
responds to the alert response address, and the
master reads its device address. The address of this
device is now known and can be interrogated per
usual.
4. If more than one device’s SMBALERT output is
low, the one with the lowest device address has
priority in accordance with normal SMBus
arbitration.
5. Once the ADT7476 responds to the alert response
address, the master must read the status registers,
and SMBALERT is cleared only if the error
condition goes away.
SMBus Timeout
The ADT7476 includes an SMBus timeout feature. If
there is no SMBus activity for 35 ms, the ADT7476 assumes
the bus is locked and releases the bus. This prevents the
device from locking or holding the SMBus expecting data.
Some SMBus controllers cannot handle the SMBus timeout
feature, so if necessary, it can be disabled.
Table 6. CONFIGURATION REGISTER 1 (REG. 0X40)
Bit Description
[6] TODIS 0: SMBus Timeout Enabled (Default)
1: SMBus Timeout Disabled
Virus Protection
To prevent rogue programs or viruses from accessing
critical ADT7476 register settings, the lock bit can be set.
Setting Bit 1 of Configuration Register 1 (0x40) sets the
lock bit and locks critical registers. In this mode, certain
registers can no longer be written to until the ADT7476 is
powered down and powered up again. For more information
on which registers are locked see Table 49.
Voltage Measurement Input
The ADT7476 has four external voltage measurement
channels. It can also measure its own supply voltage, VCC.
Pin 20 to Pin 23 can measure 5.0 V, 12 V, and 2.5 V
supplies, and the processor core voltage VCCP (0 V to 3 V
input). The VCC supply voltage measurement is carried out
through the VCC pin (Pin 4). The 2.5 V input can be used to
monitor a chipset supply voltage in computer systems.
Analog-to-Digital Converter
All analog inputs are multiplexed into the on-chip,
successive-approximation, analog-to-digital converter,
which has a resolution of 10 bits. The basic input range is 0 V
to 2.25 V, but the inputs have built−in attenuators to allow
measurement of 2.5 V, 3.3 V, 5.0 V, 12 V, and the processor
core voltage VCCP without any external components. To
allow the tolerance of these supply voltages, the ADC
produces an output of 3/4 full scale (768 dec or 300 hex) for
the nominal input voltage, giving it adequate headroom to
cope with overvoltages.
Input Circuitry
The internal structure for the analog inputs is shown in
Figure 24 The input circuit consists of an input protection
diode, an attenuator, plus a capacitor to form a first-order
low-pass filter that gives input immunity to high frequency
noise.
Figure 24. Structure of Analog Inputs
17.5 kW
52.5 kW
VCCP
35 pF
45 kW
94 kW
+2.5VIN
30 pF
68 kW
71 kW
VCC
30 pF
93 kW
47 kW
+5VIN
30 pF
183.6 kW
30 kW
+12VIN
30 pF
MUX
Table 7. VOLTAGE MEASUREMENT REGISTERS
Register Description Default
0x20 2.5 V Reading 0x00
0x21 VCCP Reading 0x00
0x22 VCC Reading 0x00
0x23 5.0 V Reading 0x00
0x24 12 V Reading 0x00
Voltage Limit Registers
Associated with each voltage measurement channel is a
high and low limit register. Exceeding the programmed high
or low limit causes the appropriate status bit to be set.
Exceeding either limit can also generate SMBALERT
interrupts.
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Table 8. VOLTAGE LIMIT REGISTERS
Register Description Default
0x44 2.5 V Low Limit 0x00
0x45 2.5 V High Limit 0xFF
0x46 VCCP Low Limit 0x00
0x47 VCCP High Limit 0xFF
0x48 VCC Low Limit 0x00
0x49 VCC High Limit 0xFF
0x4A 5.0 V Low Limit 0x00
0x4B 5.0 V High Limit 0xFF
0x4C 12 V Low Limit 0x00
0x4D 12 V High Limit 0xFF
Table 13 shows the input ranges of the analog inputs and
output codes of the 10-bit ADC.
When the ADC is running, it samples and converts a
voltage input in 0.7 ms and averages 16 conversions to
reduce noise; a measurement takes nominally 11 ms.
Extended Resolution Registers
Voltage measurements can be made with higher accuracy
using the extended resolution registers (0x76 and 0x77).
Whenever the extended resolution registers are read, the
corresponding data in the voltage measurement registers
(0x20 to 0x24) is locked until their data is read. That is, if
extended resolution is required, then the extended resolution
register must be read first, immediately followed by the
appropriate voltage measurement register.
Additional ADC Functions for Voltage Measurements
A number of other functions are available on the
ADT7476 to offer the system designer increased flexibility.
Turn-off Averaging
For each voltage/temperature measurement read from a
value register, 16 readings have been made internally and
the results averaged before being placed into the value
register. When faster conversions are needed, setting Bit 4
of Configuration Register 2 (0x73) turns averaging off. This
effectively gives a reading 16 times faster but the reading
can be noisier. The default round robin cycle time takes
146.5 ms.
Table 9. CONVERSION TIME WITH AVERAGING
DISABLED
Channel Measurement Time (ms)
Voltage Channels 0.7
Remote Temperature 1 7
Remote Temperature 2 7
Local Temperature 1.3
When Bit 7 of Configuration Register 6 (0x10) is set, the
default round robin cycle time increases to 240 ms.
Bypass All Voltage Input Attenuators
Setting Bit 5 of Configuration Register 2 (0x73) removes
the attenuation circuitry from the 2.5 V, VCCP
, VCC, 5.0 V,
and 12 V inputs. This allows the user to directly connect
external sensors or rescale the analog voltage measurement
inputs for other applications. The input range of the ADC
without the attenuators is 0 V to 2.25 V.
Bypass Individual Voltage Input Attenuators
Bits [7:4] of Configuration Register 4 (0x7D) can be used
to bypass individual voltage channel attenuators.
Table 10. BYPASSING INDIVIDUAL VOLTAGE INPUT
ATTENUATORS
Configuration Register 4 (0x7D)
Bit No. Channel Attenuated
[4] Bypass 2.5 V Attenuator
[5] Bypass VCCP Attenuator
[6] Bypass 5.0 V Attenuator
[7] Bypass 12 V Attenuator
Table 11. CONFIGURATION REGISTER 2 (REG. 0X73)
Bit Description
[4] 1: Averaging Off
[5] 1: Bypass Input Attenuators
[6] 1: Single-channel Convert Mode
TACH1 Minimum High Byte (0x55)
[7:5] Selects ADC channel for single-channel convert mode.
Single-channel ADC Conversion
While single-channel mode is intended as a test mode that
can be used to increase sampling times for a specific
channel, and therefore helps to analyze that channel’s
performance in greater detail, it can also have other
applications.
Setting Bit 6 of Configuration Register 2 (0x73) places
the ADT7476 into single-channel ADC conversion mode. In
this mode, the ADT7476 can only read a single voltage
channel. The selected voltage input is read every 0.7 ms. The
appropriate ADC channel is selected by writing to Bits [7:5]
of the TACH1 minimum high byte register (0x55).
Table 12. PROGRAMMING SINGLE-CHANNEL ADC
MODE
Bits [7:4], Register 0x55 Channel Selected (Note 1)
000 2.5 V
001 VCCP
010 VCC
011 5.0 V
100 12 V
101 Remote 1 temperature
110 Local temperature
111 Remote 2 temperature
1. In the process of configuring single-channel ADC conversion
mode, the TACH1 minimum high byte is also changed, possibly
trading off TACH1 minimum high byte functionality with
single-channel mode functionality.
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Table 13. 10-BIT ADC OUTPUT CODE VS. VIN
Input Voltage ADC Output
12 VIN 5.0 VIN VCC (3.3 VIN)2.5 VIN VCCP VTT/IMON Decimal Binary (10 Bits)
<0.0156 <0.0065 <0.0042 <0.0032 <0.00293 <0.00220 0 00000000 00
0.0156 to
0.0312
0.0065 to
0.0130
0.0042 to
0.0085
0.0032 to
0.0065
0.0293 to
0.0058
0.00220 to
0.00440
100000000 01
0.0312 to
0.0469
0.0130 to
0.0195
0.0085 to
0.0128
0.0065 to
0.0097
0.0058 to
0.0087
0.00440 to
0,00660
200000000 10
0.0469 to
0.0625
0.0195 to
0.0260
0.0128 to
0.0171
0.0097 to
0.0130
0.0087 to
0.0117
0,00660 to
0.00881
300000000 11
0.0625 to
0.0781
0.0260 to
0.0325
0.0171 to
0.0214
0.0130 to
0.0162
0.0117 to
0.0146
0.00881 to
0.01100
400000001 00
0.0781 to
0.0937
0.0325 to
0.0390
0.0214 to
0.0257
0.0162 to
0.0195
0.0146 to
0.0175
0.01100 to
0.01320
500000001 01
0.0937 to
0.1093
0.0390 to
0.0455
0.0257 to
0.0300
0.0195 to
0.0227
0.0175 to
0.0205
0.01320 to
0.01541
600000001 10
0.1093 to
0.1250
0.0455 to
0.0521
0.0300 to
0.0343
0.0227 to
0.0260
0.0205 to
0.0234
0.01541 to
0.01761
700000001 11
0.1250 to
0.14060
0.0521 to
0.0586
0.0343 to
0.0386
0.0260 to
0.0292
0.0234 to
0.0263
0.01761 to
0.01981
800000010 00
− − − − − − − −
4.0000 to
4.0156
1.6675 to
1.6740
1.1000 to
1.1042
0.8325 to
0.8357
0.7500 to
0.7529
0.5636 to
0.5658
256
(1/4 scale)
01000000 00
− − − − − − − −
8.0000 to
8.0156
3.3300 to
3.3415
2.2000–2.204
2
1.6650 to
1.6682
1.5000 to
1.5029
1.1272 to
1.1294
512
(1/2 scale)
10000000 00
− − − − − − − −
12.0000 to
12.0156
5.0025 to
5.0090
3.3000 to
3.3042
2.4975 to
2.5007
2.2500 to
2.2529
1.6809 to
1.6930
768
(3/4 scale)
11000000 00
− − − − − − − −
15.8281 to
15.8437
6.5983 to
6.6048
4.3527 to
4.3570
3.2942 to
3.2974
2.9677 to
2.9707
2.2301 to
2.2323
1013 11111101 01
15.8437 to
15.8593
6.6048 to
6.6113
4.3570 to
4.3613
3.2974 to
3.3007
2.9707 to
2.9736
2.2323 to
2.2346
1014 11111101 10
15.8593 to
15.8750
6.6113 to
6.6178
4.3613 to
4.3656
3.3007 to
3.3039
2.9736 to
2.9765
2.2346 to
2.2368
1015 11111101 11
15.8750 to
15.8906
6.6178 to
6.6244
4.3656 to
4.3699
3.3039 to
3.3072
2.9765 to
2.9794
2.2368 to
2.23899
1016 11111110 00
15.8906 to
15.9062
6.6244 to
6.6309
4.3699 to
4.3742
3.3072 to
3.3104
2.9794 to
2.9824
2,23899 to
2.2412
1017 11111110 01
15.9062 to
15.9218
6.6309 to
6.6374
4.3742 to
4.3785
3.3104 to
3.3137
2.9824 to
2.9853
2.2412 to
2.2434
1018 11111110 10
15.9218 to
15.9375
6.6374 to
6.4390
4.3785 to
4.3828
3.3137 to
3.3169
2.9853 to
2.9882
2.2434 to
2.2456
1019 11111110 11
15.9375 to
15.9531
6.6439 to
6.6504
4.3828 to
4.3871
3.3169 to
3.3202
2.9882 to
2.9912
2.2456 to
2.2478
1020 11111111 00
15.9531 to
15.9687
6.6504 to
6.6569
4.3871 to
4.3914
3.3202 to
3.3234
2.9912 to
2.9941
2.2478 to
2.25
1021 11111111 01
15.9687 to
15.9843
6.6569 to
6.6634
4.3914 to
4.3957
3.3234 to
3.3267
2.9941 to
2.9970
2.25 to
2.2522
1022 11111111 10
>15.9843 >6.6634 >4.3957 >3.3267 >2.9970 >2.2522 1023 11111111 11
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VID Code Monitoring
The ADT7476 has five dedicated voltage ID (VID code)
inputs. These are digital inputs that can be read back through
the VID/GPIO register (0x43) to determine the processor
voltage required or the system being used. Five VID code
inputs support VRM9.x solutions. In addition, Pin 21 (12 V
input) can be reconfigured as a sixth VID input to satisfy
future VRM requirements.
VID/GPIO Register (0x43)
[0] = VID0, reflects logic state of Pin 5.
[1] = VID1, reflects logic state of Pin 6.
[2] = VID2, reflects logic state of Pin 7.
[3] = VID3, reflects logic state of Pin 8.
[4] = VID4, reflects logic state of Pin 19.
[5] = VID5, reconfigurable 12 V input. This bit reads 0 when
Pin 21 is configured as the 12 V input. This bit reflects the
logic state of Pin 21 when the pin is configured as VID5.
VID Code Input Threshold Voltage
The switching threshold for the VID code inputs is
approximately 1.0 V. To enable future compatibility, it is
possible to reduce the VID code input threshold to 0.6 V.
Bit 6 (THLD) of the VID/GPIO register (0x43) controls the
VID input threshold voltage.
VID/GPIO Register (0x43)
[6] THLD = 0, VID switching threshold = 1.0 V,
VOL < 0.8 V, VIH > 1.7 V, VMAX = 3.3 V.
[6] THLD = 1, VID switching threshold = 0.6 V,
VOL < 0.4 V, VIH > 0.8 V, VMAX = 3.3 V.
Reconfiguring Pin 21 as VID5 Input
Pin 21 can be reconfigured as a sixth VID code input
(VID5) for VRM10 compatible systems. Because the pin is
configured as VID5, it is not possible to monitor a 12 V
supply.
Bit 7 of the VID/GPIO register (0x43) determines the
function of Pin 21. System or BIOS software can read the
state of Bit 7 to determine whether the system is designed to
monitor 12 V or a sixth VID input.
VID/GPIO Register (0x43)
[7] VIDSEL = 0, Pin 21 functions as a 12 V measurement
input. Software can read this bit to determine that there are
five VID inputs being monitored. Bit 5 of VID/GPIO
Register (0x43) always reads back 0. Bit 0 of Interrupt Status
Register 2 (0x42) reflects 12 V out-of-limit measurements.
[7] VIDSEL = 1, Pin 21 functions as the sixth VID code
input (VID5). Software can read this bit to determine that
there are six VID inputs being monitored. Bit 5 of Register
0x43 reflects the logic state of Pin 21. Bit 0 of Interrupt
Status Register 2 (0x42) reflects VID code changes.
VID Code Change Detect Function
The ADT7476 has a VID code change detect function.
When Pin 21 is configured as the VID5 input, VID code
changes are detected and reported back by the ADT7476.
Bit 0 of Interrupt Status Register 2 (0x42) is the 12 V/VC bit
and denotes a VID change when set. The VID code change
bit is set when the logic states on the VID inputs are different
than they were 11 ms previously. The change of VID code is
used to generate an SMBALERT interrupt. If an
SMBALERT interrupt is not required, Bit 0 of Interrupt Mask
Register 2 (0x75), when set, prevents SMBALERTs from
occurring on VID code changes.
Interrupt Status Register 2 (0x42)
[0] 12 V/VC = 0, if Pin 21 is configured as VID5, Logic 0
denotes no change in VID code within the last 11 ms.
[0] 12 V/VC = 1, if Pin 21 is configured as VID5, Logic 1
means that a change has occurred on the VID code inputs
within the last 11 ms. An SMBALERT is generated, if this
function is enabled.
Programming the GPIOs
The ADT7476 follows an upgrade path from the
ADM1027 to the ADT7476. In order to maintain
consistency between versions, it is necessary to omit
references to GPIO5. As a result, there are six GPIOs as
follows: GPIO0, GPIO1, GPIO2, GPIO3, GPIO4, and
GPIO6.
Setting Bit 4 of Configuration Register 5 (0x7C) to 1
enables GPIO functionality. This turns all pins configured as
VID inputs into general-purpose outputs. Writing to the
corresponding VID bit in the VID/GPIO register (0x43) sets
the polarity for the corresponding GPIO. GPIO6 can be
programmed independently as, for example, an input or
output, using Bits [3:2] of Configuration Register 5 (0x7C).
Temperature Measurement Method
Local Temperature Measurement
The ADT7476 contains an on-chip band gap temperature
sensor whose output is digitized by the on-chip, 10-bit ADC.
The 8-bit MSB temperature data is stored in the temperature
registers (Addresses 0x25, 0x26, and 0x27). Because both
positive and negative temperatures can be measured, the
temperature data is stored in Offset 64 format or twos
complement format, as shown in Table 14 and Table 15.
Theoretically, the temperature sensor and ADC can measure
temperatures from −63C to +127C (or −61C to +191C in
the extended temperature range) with a resolution of 0.25C.
However, this exceeds the operating temperature range of
the device, so local temperature measurements outside the
ADT7476 operating temperature range are not possible.
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Table 14. TWOS COMPLEMENT TEMPERATURE DATA
FORMAT
Temperature Digital Output (10-bit) (Note 1)
–128C1000 0000 00 (Diode Fault)
–50C1100 1110 00
–25C1110 0111 00
–10C1111 0110 00
0C0000 0000 00
+10.25C0000 1010 01
+25.5C0001 1001 10
+50.75C0011 0010 11
+75C0100 1011 00
+100C0110 0100 00
+125C0111 1101 00
+127C0111 1111 00
1. Bold numbers denote 2 LSB of measurement in the Extended
Resolution Register 2 (0x77) with 0.25C resolution.
Table 15. EXTENDED RANGE, TEMPERATURE DATA
FORMAT
Temperature Digital Output (10-Bit) (Note 1)
–64C0000 0000 00 (Diode Fault)
–1C0011 1111 00
0C0100 0000 00
1C0100 0001 00
10C0100 1010 00
25C0101 1001 00
50C0111 0010 00
75C1000 1001 00
100C1010 0100 00
125C1011 1101 00
191C1111 1111 00
1. Bold numbers denote 2 LSB of measurement in the Extended
Resolution Register 2 (0x77) with 0.25C resolution.
Remote Temperature Measurement
The ADT7476 can measure the temperature of two remote
diode sensors or diode-connected transistors connected to
Pin 17 and Pin 18, or Pin 15 and Pin 16.
The forward voltage of a diode or diode-connected
transistor operated at a constant current exhibits a negative
temperature coefficient of about –2 mV/C. Unfortunately,
the absolute value of VBE varies from device to device, and
individual calibration is required to null this out. As a result,
this technique is unsuitable for mass production. The
technique used in the ADT7476 is to measure the change in
VBE when the device is operated at two different currents.
This is given by:
(eq. 1)
DVBE +kT
q In(N)
where:
k is the Boltzmann’s constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvin.
N is the ratio of the two currents.
Figure 25 shows the input signal conditioning used to
measure the output of a remote temperature sensor. This
figure shows the external sensor as a substrate transistor,
which is provided on some microprocessors for temperature
monitoring. It could also be a discrete transistor such as a
2N3904/2N3906.
If a discrete transistor is used, the collector is not grounded
and is linked to the base. If a PNP transistor is used, the base
is connected to the D– input and the emitter to the D+ input.
If an NPN transistor is used, the emitter is connected to the
D– input and the base to the D+ input. Figure 26 and
Figure 27 show how to connect the ADT7476 to an NPN
or PNP transistor for temperature measurement. To prevent
ground noise from interfering with the measurement, the
more negative terminal of the sensor is not referenced to
ground, but is biased above ground by an internal diode at
the D– input.
Figure 25. Signal Conditioning for Remote Diode Temperature Sensors
LOW-PASS FILTER
fC = 65 kHz
REMOTE
SENSING
TRANSISTOR BIAS
DIODE
D+
D−
VDD
IBIAS
IN I
VOUT+
VOUT−
To ADC
THERMDA
THERMDC
CPU
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Figure 26. Measuring Temperature by Using
an NPN Transistor
ADT7476
D+
D−
2N3904
NPN
Figure 27. Measuring Temperature by Using
a PNP Transistor
ADT7476
D+
D−
2N3906
PNP
To measure DVBE, the sensor switches between operating
currents of I and N I. The resulting waveform passes
through a 65 kHz low-pass filter to remove noise and through
a chopper-stabilized amplifier. The amplifier performs the
amplification and rectification of the waveform to produce a
dc voltage proportional to DVBE. This voltage is measured by
the ADC to give a temperature output in 10-bit, twos
complement format. To further reduce the effects of noise,
digital filtering is performed by averaging the results of
16 measurement cycles.
A remote temperature measurement takes nominally
38 ms. The results of remote temperature measurements are
stored in 10-bit, twos complement format, as illustrated in
Table 10. The extra resolution for the temperature
measurements is held in the Extended Resolution Register 2
(0x77). This gives temperature readings with a resolution of
0.25C.
Noise Filtering
For temperature sensors operating in noisy environments,
previous practice placed a capacitor across the D+ pin and
the D− pin to help combat the effects of noise. However,
large capacitances affect the accuracy of the temperature
measurement, leading to a recommended maximum
capacitor value of 1,000 pF.
This capacitor reduces the noise but does not eliminate it,
which makes using the sensor difficult in a very noisy
environment. In most cases, a capacitor is not required
because differential inputs by their very nature have a high
immunity to noise.
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7476 is designed to work with substrate
transistors built into processors or with discrete transistors.
Substrate transistors are generally PNP types with the
collector connected to the substrate. Discrete types can be
either PNP or NPN transistors connected as a diode
(base-shorted to the collector). If an NPN transistor is used,
the collector and base are connected to D+ and the emitter
to D−. If a PNP transistor is used, the collector and base are
connected to D− and the emitter is connected to D+.
To reduce the error due to variations in both substrate and
discrete transistors, a number of factors should be taken into
consideration:
The ideality factor, nf, of the transistor is a measure of
the deviation of the thermal diode from ideal behavior.
The ADT7476 is trimmed for an nf value of 1.008. Use
the following equation to calculate the error introduced
at a temperature T (C), when using a transistor whose
nf does not equal 1.008 (see the processor’s data sheet
for the nf values):
DT+(nf *1.008) ǒ273.15 K )TǓ(eq. 2)
To factor this in, the user can write the DT value to the
offset register. The ADT7476 then automatically adds
it to or subtracts it from the temperature measurement.
Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7476, IHIGH, is 180 mA, and
the low level current, ILOW, is 11 mA. If the ADT7476
current levels do not match the current levels specified
by the CPU manufacturer, it could be necessary to
remove an offset. The CPU’s data sheet advises
whether this offset needs to be removed and how to
calculate it. This offset can be programmed to the offset
register. It is important to note that if more than one
offset must be considered, then the algebraic sum of
these offsets must be programmed to the offset register.
If a discrete transistor is used with the ADT7476, the best
accuracy is obtained by choosing devices according to the
following criteria:
Base-emitter voltage greater than 0.25 V at 11 mA, at
the highest operating temperature.
Base-emitter voltage less than 0.95 V at 180 mA,
at the lowest operating temperature.
Base resistance less than 100 W.
Small variation in the current gain, hFE, (approximately
50 to 150) that indicates tight control of VBE
characteristics.
Transistors, such as 2N3904, 2N3906, or equivalents in
SOT−23 packages, are suitable devices to use.
Nulling Out Temperature Errors
As CPUs run faster, it is more difficult to avoid high
frequency clocks when routing the D+/D– traces around a
system board. Even when recommended layout guidelines
are followed, some temperature errors can still be
attributable to noise coupled onto the D+/D– lines. Constant
high frequency noise usually attenuates, or increases,
temperature measurements by a linear, constant value.
The ADT7476 has temperature offset registers (0x70 and
0x72) for the Remote 1 and Remote 2 temperature channels.
By doing a one-time calibration of the system, the user can
determine the offset caused by system board noise and null
it out using the offset registers. The offset registers
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automatically add a twos complement 8-bit reading to every
temperature measurement.
Changing Bit 1 of Configuration Register 5 (0x7C)
changes the resolution and therefore, the range of the
temperature offset as either having a −63C to +127C range
with a resolution of 1C or having a −63C to +64C range
with a resolution of 0.5C. This temperature offset can be
used to compensate for linear temperature errors introduced
by noise.
Table 16. TEMPERATURE OFFSET REGISTERS
Register Description Default
0x70 Remote 1 Temperature Offset 0x00 (0C)
0x71 Local Temperature Offset 0x00 (0C)
0x72 Remote 2 Temperature Offset 0x00 (0C)
ADT7463/ADT7476 Backwards Compatible Mode
By setting Bit 0 of Configuration Register 5 (0x7C), all
temperature measurements are stored in the zone temperature
reading registers (0x25, 0x26, and 0x27) in twos complement
in the −63C to +127C range. The temperature limits must
be reprogrammed in twos complement.
If a twos complement temperature below −63C is
entered, the temperature is clamped to −63C. In this mode,
the diode fault condition remains −128C = 1000 0000,
while in the extended temperature range (−63C to +191C),
the fault condition is represented by −64C = 0000 0000.
Table 17. TEMPERATURE READING REGISTERS
Register Description Default
0x25 Remote 1 Temperature −
0x26 Local Temperature −
0x27 Remote 2 Temperature −
0x77 Extended Resolution 2 0x00
Table 18. EXTENDED RESOLUTION TEMPERATURE
MEASUREMENT REGISTER BITS
Bit Mnemonic Description
[7:6] TDM2 Remote 2 Temperature LSBs
[5:4] LTMP Local Temperature LSBs
[3:2] TDM1 Remote 1 Temperature LSBs
Temperature Limit Registers
Associated with each temperature measurement channel
are high and low limit registers. Exceeding the programmed
high or low limit causes the appropriate status bit to be set.
Exceeding either limit can also generate SMBALERT
interrupts (depending on the way the interrupt mask register
is programmed and assuming that SMBALERT is set as an
output on the appropriate pin).
Table 19. TEMPERATURE LIMIT REGISTERS
Register Description Default
0x4E Remote 1 Temperature Low Limit 0x81
0x4F Remote 1 Temperature High Limit 0x7F
0x50 Local Temperature Low Limit 0x81
0x51 Local Temperature High Limit 0x7F
0x52 Remote 2 Temperature Low Limit 0x81
0x53 Remote 2 Temperature High Limit 0x7F
Reading Temperature from the ADT7476
It is important to note that temperature can be read from
the ADT7476 as an 8-bit value (with 1C resolution) or as
a 10-bit value (with 0.25C resolution). If only 1C
resolution is required, the temperature readings can be read
back at any time and in no particular order.
If the 10-bit measurement is required, this involves a
2-register read for each measurement. Extended Resolution
Register 2 (0x77) should be read first. This causes all
temperature reading registers to be frozen until all
temperature reading registers have been read from. This
prevents an MSB reading from being updated while its two
LSBs are being read and vice versa.
Additional ADC Functions for Temperature
Measurement
A number of other functions are available on the
ADT7476 to offer the system designer increased flexibility.
Turn-off Averaging
For each temperature measurement read from a value
register, 16 readings have actually been made internally, and
the results averaged, before being placed into the value
register. Sometimes it is necessary to take a very fast
measurement. Setting Bit 4 of Configuration Register 2
(0x73) turns averaging off. The default round robin cycle
time takes 146.5 ms.
Table 20. CONVERSION TIME WITH AVERAGING
DISABLED
Channel Measurement Time (ms)
Voltage Channels 0.7
Remote Temperature 1 7
Remote Temperature 2 7
Local Temperature 1.3
When Bit 7 of Configuration Register 6 (0x10) is set, the
default round robin cycle time increases to 240 ms.
Table 21. CONVERSION TIME WITH AVERAGING
ENABLED
Channel Measurement Time (ms)
Voltage Channels 11
Remote Temperature 39
Local Temperature 12
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Single-channel ADC Conversions
Setting Bit 6 of Configuration Register 2 (0x73) places
the ADT7476 into single-channel ADC conversion mode. In
this mode, the ADT7476 can be made to read a single
temperature channel only. The appropriate ADC channel is
selected by writing to Bits [7:5] of the TACH1 minimum
high byte register (0x55).
Table 22. PROGRAMMING SINGLE-CHANNEL ADC
MODE FOR TEMPERATURES
Bits [7:5], Register 0x55 Channel Selected
101 Remote 1 Temperature
110 Local Temperature
111 Remote 2 Temperature
Table 23. CONFIGURATION REGISTER 2 (REG. 0X73)
Bit Description
[4] 1: Averaging Off
[6] 1: Single-channel Convert Mode
TACH1 Minimum High Byte (0x55)
[7:5] selects ADC channel for single-channel convert
mode.
Overtemperature Events
Overtemperature events on any of the temperature
channels can be detected and dealt with automatically in
automatic fan speed control mode. Register 0x6A to
Register 0x6C are the THERM temperature limits. When a
temperature exceeds its THERM temperature limit, all
PWM outputs run at the maximum PWM duty cycle
(Register 0x38, Register 0x39, and Register 0x3A).
This effectively runs the fans at the fastest allowed speed.
The fans run at this speed until the temperature drops
below THERM minus hysteresis. This can be disabled by
setting Bit 2, the boost bit, in Configuration Register 3
(0x78). The hysteresis value for the THERM temperature
limit is the value programmed into the hysteresis registers
(0x6D and 0x6E). The default hysteresis value is 4C.
Figure 28. THERM Temperature Limit Operation
THERM LIMIT
TEMPERATURE
FANS 100%
HYSTERESIS (C)
THERM can be disabled on specific temperature channels
using Bits [7:5] of Configuration Register 5 (0x7C).
THERM can also be disabled by:
Writing −64C to the appropriate THERM temperature
limit in Offset 64 mode.
Writing −128C to the appropriate THERM
temperature limit in twos complement mode.
Limits, Status Registers, and Interrupts
Limit Values
Associated with each measurement channel on the
ADT7476 are high and low limits. These can form the basis of
system status monitoring; a status bit can be set for any
out-of-limit condition and is detected by polling the device.
Alternatively, SMBALERT interrupts can be generated to flag
out-of-limit conditions to a processor or microcontroller.
8-bit Limits
The following is a list of 8-bit limits on the ADT7476.
Table 24. VOLTAGE LIMIT REGISTERS
Register Description Default
0x44 2.5 V Low Limit 0x00
0x45 2.5 V High Limit 0xFF
0x46 VCCP Low Limit 0x00
0x47 VCCP High Limit 0xFF
0x48 VCC Low Limit 0x00
0x49 VCC High Limit 0xFF
0x4A 5.0 V Low Limit 0x00
0x4B 5.0 V High Limit 0xFF
0x4C 12 V Low Limit 0x00
0x4D 12 V High Limit 0xFF
Table 25. TEMPERATURE LIMIT REGISTERS
Register Description Default
0x4E Remote 1 Temperature Low Limit 0x81
0x4F Remote 1 Temperature High Limit 0x7F
0x6A Remote 1 THERM Temp. Limit 0x64
0x50 Local Temperature Low Limit 0x81
0x51 Local Temperature High Limit 0x7F
0x6B Local THERM Temperature Limit 0x64
0x52 Remote 2 Temperature Low Limit 0x81
0x53 Remote 2 Temperature High Limit 0x7F
0x6C Remote 2 THERM Temp. Limit 0x64
Table 26. THERM TIMER LIMIT REGISTER
Register Description Default
0x7A THERM Timer Limit 0x00
16-bit Limits
The fan TACH measurements are 16-bit results. The fan
TACH limits are also 16 bits, consisting of a high byte and
low byte. Because fans running under speed or stalled are
normally the only conditions of interest, only high limits
exist for fan TACHs. Because the fan TACH period is
actually being measured, exceeding the limit indicates a
slow or stalled fan.
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Table 27. FAN LIMIT REGISTERS
Register Description Default
0x54 TACH1 Minimum Low Byte 0xFF
0x55 TACH1 Minimum High Byte 0xFF
0x56 TACH2 Minimum Low Byte 0xFF
0x57 TACH2 Minimum High Byte 0xFF
0x58 TACH3 Minimum Low Byte 0xFF
0x59 TACH3 Minimum High Byte 0xFF
0x5A TACH4 Minimum Low Byte 0xFF
0x5B TACH4 Minimum High Byte 0xFF
Out-of-Limit Comparisons
Once all limits have been programmed, the ADT7476 can
be enabled for monitoring. The ADT7476 measures all
voltage and temperature measurements in round robin
format and sets the appropriate status bit for out-of-limit
conditions. TACH measurements are not part of this round
robin cycle. Comparisons are done differently depending on
whether the measured value is being compared to a high or
low limit.
High Limit: > Comparison Performed
Low Limit: Comparison Performed
Voltage and temperature channels use a window
comparator for error detecting and, therefore, have high and
low limits. Fan speed measurements use only a low limit.
This fan limit is needed only in manual fan control mode.
Analog Monitoring Cycle Time
The analog monitoring cycle begins when a 1 is written to
the start bit (Bit 0) of Configuration Register 1 (0x40). The
ADC measures each analog input in turn, and, as each
measurement is completed, the result is automatically stored
in the appropriate value register. This round robin
monitoring cycle continues unless disabled by writing a 0 to
Bit 0 of Configuration Register 1.
As the ADC is normally left to free-run in this manner, the
time taken to monitor all the analog inputs is normally not
of interest, because the most recently measured value of any
input can be read out at any time.
For applications where the monitoring cycle time is
important, it can easily be calculated.
The total number of channels measured is:
Four Dedicated Supply Voltage Inputs
Supply Voltage (VCC Pin)
Local Temperature
Two Remote Temperatures
As mentioned previously, the ADC performs round robin
conversions and takes 11 ms for each voltage measurement,
12 ms for a local temperature reading, and 39 ms for each
remote temperature reading. The total monitoring cycle time
for averaged voltage and temperature monitoring is,
therefore, nominally:
(5 11))12 )(2 39)+145 ms (eq. 3)
Fan TACH measurements are made in parallel and are not
synchronized with the analog measurements in any way.
Status Registers
The results of limit comparisons are stored in Interrupt
Status Register 1 and Interrupt Status Register 2. The status
register bit for each channel reflects the status of the last
measurement and limit comparison on that channel. If a
measurement is within limits, the corresponding status register
bit is cleared to 0. If the measurement is out-of-limits, the
corresponding status register bit is set to 1.
The state of the various measurement channels can be
polled by reading the status registers over the serial bus. In
Bit 7 (OOL) of Interrupt Status Register 1 (0x41), 1 means
an out-of-limit event has been flagged in Interrupt Status
Register 2. This means the user also needs to read Interrupt
Status Register 2. Alternatively, Pin 10 or Pin 14 can be
configured as an SMBALERT output. This hard interrupt
automatically notifies the system supervisor of an
out-of-limit condition. Reading the status registers clears the
appropriate status bit as long as the error condition that
caused the interrupt has cleared. Status register bits are
sticky. Whenever a status bit is set, indicating an out-of-limit
condition, it remains set even if the event that caused it has
gone away (until read).
The only way to clear the status bit is to read the status
register after the event has gone away. Interrupt mask
registers (0x74 and 0x75) allow individual interrupt sources
to be masked from causing an SMBALERT. However, if one
of these masked interrupt sources goes out-of-limit, its
associated status bit is set in the status registers.
Table 28. INTERRUPT STATUS REGISTER 1 (0X41)
Bit Mnemonic Description
7 OOL 1 denotes a bit in Interrupt Status
Register 2 is set and Interrupt Status
Register 2 should be read.
6 R2T 1 indicates that the Remote 2
Temperature High or Low limit has been
exceeded.
5LT 1 indicates that the Local Temperature
High or Low Limit has been exceeded.
4 R1T 1 indicates that the Remote 1
Temperature High or Low Limit has
been exceeded.
3 5.0 V 1 indicates that the 5.0 V High or Low
Limit has been exceeded.
2 VCC 1 indicates that the VCC High or Low
Limit has been exceeded.
1 VCCP 1 indicates that the VCCP High or Low
Limit has been exceeded.
0 2.5 V 1 indicates that the 2.5 V High or Low
Limit has been exceeded.
If the 2.5 V input is configured as
THERM, this bit represents the status of
THERM.
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Table 29. INTERRUPT STATUS REGISTER 2 (0X42)
Bit Mnemonic Description
7 D2 1 indicates an open or short on
D2+/D2− inputs.
6 D1 1 indicates an open or short on
D1+/D1− inputs.
5 F4P 1 indicates Fan 4 has dropped below
minimum speed. Alternatively, indicates
that the THERM limit has been
exceeded, if the THERM function is
used. Alternatively, indicates the status
of GPIO6.
4 FAN3 1 indicates that Fan 3 has dropped
below minimum speed.
3 FAN2 1 indicates that Fan 2 has dropped
below minimum speed.
2 FAN1 1 indicates that Fan 1 has dropped
below minimum speed.
1 OVT 1 indicates that a THERM
overtemperature limit has been
exceeded.
0 12 V/VC 1 indicates a 12 V high or low limit has
been exceeded. If the VID code change
function is used, this bit indicates a
change in VID code on the VID0 to
VID4 inputs.
SMBALERT Interrupt Behavior
The ADT7476 can be polled for status, or an SMBALERT
interrupt can be generated for out-of-limit conditions. It is
important to note how the SMBALERT output and status
bits behave when writing interrupt handler software.
Figure 29. SMBALERT and Status Bit Behavior
HIGH LIMIT
TEMPERATURE
“STICKY”
STATUS BIT
SMBALERT
CLEARED ON READ
(TEMP BELOW LIMIT)
TEMP BACK IN LIMIT
(STATUS BIT STAYS SET)
HIGH LIMIT
TEMPERATURE
“STICKY”
STATUS BIT
SMBALERT
Figure 29 shows how the SMBALERT output and sticky
status bits behave. Once a limit is exceeded, the
corresponding status bit is set to 1. The status bit remains set
until the error condition subsides and the status register is
read. The status bits are referred to as sticky because they
remain set until read by software. This ensures that an
out-of-limit event cannot be missed if the software is
periodically polling the device.
Note that:
The SMBALERT output remains low for the entire
duration that a reading is out-of-limit and until the
status register has been read. This has implications on
how software handles the interrupt.
THERM overtemperature events are not sticky. They
reset immediately after the overtemperature condition
ceases.
Handling SMBALERT Interrupts
To prevent the system from being tied up servicing
interrupts, it is recommend to handle the SMBALERT
interrupt as follows:
1. Detect the SMBALERT assertion.
2. Enter the interrupt handler.
3. Read the status registers to identify the interrupt
source.
4. Mask the interrupt source by setting the
appropriate mask bit in the interrupt mask registers
(0x74 and 0x75).
5. Take the appropriate action for a given interrupt
source.
6. Exit the interrupt handler.
7. Periodically poll the status registers. If the
interrupt status bit has cleared, reset the
corresponding interrupt mask bit to 0. This causes
the SMBALERT output and status bits to behave
as shown in Figure 30.
Figure 30. How Masking the Interrupt Source Affects
SMBALERT Output
HIGH LIMIT
TEMPERATURE
“STICKY”
STATUS BIT
SMBALERT
CLEARED ON READ
(TEMP BELOW LIMIT)
TEMP BACK IN LIMIT
(STATUS BIT STAYS SET)
INTERRUPT
MASK BIT SET
INTERRUPT MASK BIT
CLEARED
(SMBALERT RE-ARMED)
Masking Interrupt Sources
Interrupt Mask Register 1 (0x74) and Interrupt Mask
Register 2 (0x75) allow individual interrupt sources to be
masked to prevent SMBALERT interrupts.
NOTE: Masking an interrupt source prevents only the SMBALERT
output from being asserted; the appropriate status bit is set
normally.
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Table 30. INTERRUPT MASK REGISTER 1
(REG. 0X74)
Bit Mnemonic Description
7 OOL 1 masks SMBALERT for any alert
condition flagged in Interrupt Status
Register 2.
6 R2T 1 masks SMBALERT for Remote 2
temperature.
5LT 1 masks SMBALERT for Local
temperature.
4 R1T 1 masks SMBALERT for Remote 1
temperature.
3 5.0 V 1 masks SMBALERT for the 5.0 V
channel.
2 VCC 1 masks SMBALERT for the VCC
channel.
1 VCCP 1 masks SMBALERT for the VCCP
channel.
0 2.5 V 1 masks SMBALERT for the
2.5 VIN/THERM channel.
Table 31. INTERRUPT MASK REGISTER 2
(REG. 0X75)
Bit Mnemonic Description
7 D2 1 masks SMBALERT for Diode 2 errors.
6 D1 1 masks SMBALERT for Diode 1 errors.
5 FAN4 1 masks SMBALERT for Fan 4 failure. If
the TACH4 pin is being used as the
THERM input, this bit masks
SMBALERT for a THERM event. If the
TACH4 pin is being used as GPIO6,
setting this bit masks interrupts related
to GPIO6.
4 FAN3 1 masks SMBALERT for Fan 3.
3 FAN2 1 masks SMBALERT for Fan 2.
2 FAN1 1 masks SMBALERT for Fan 1.
1 OVT 1 masks SMBALERT for
overtemperature (exceeding THERM
limits).
0 12 V/VC 1 masks SMBALERT for 12 V channel
or for a VID code change, depending on
the function used.
Enabling the SMBALERT Interrupt Output
The SMBALERT interrupt function is disabled by
default. Pin 10 or Pin 14 can be reconfigured as an
SMBALERT output to signal out-of-limit conditions.
Table 32. CONFIGURING PIN 10 AS SMBALERT
OUTPUT
Register Bit Setting
Configuration Register 3
(0x78)
[1] Pin 10 = SMBALERT
[0] Pin 10 = PWM2
Assigning THERM Functionality to a Pin
Pin 14 on the ADT7476 has four possible functions:
SMBALERT, THERM, GPIO6, and TACH4. The user
chooses the required functionality by setting Bit 0 and Bit 1
of Configuration Register 4 (0x7D).
If THERM is enabled on Bit 1, Configuration Register 3
(0x78):
Pin 22 becomes THERM.
If Pin 14 is configured as THERM on Bit 0 and Bit 1 of
Configuration Register 4 (0x7D), THERM is enabled
on this pin.
If THERM is not enabled:
Pin 22 becomes a 2.5 V measurement input.
If Pin 14 is configured as THERM, then THERM is
disabled on this pin.
Table 33. CONFIGURING PIN 14
Bit 1 Bit 0 Function
0 0 TACH4
0 1 THERM
1 0 SMBALERT
1 1 GPIO6
THERM as an Input
When THERM is configured as an input, the user can time
assertions on the THERM pin. This can be useful for
connecting to the PROCHOT output of a CPU to gauge
system performance.
When the THERM pin is driven low externally, the user
can also set up the ADT7476 to run the fans at 100%. The
fans run at 100% for the duration of time that the THERM
pin is pulled low. This is done by setting the BOOST bit
(Bit 2) in Configuration Register 3 (0x78) to 1. This works
only if the fan is already running, for example, in manual
mode, when the current duty cycle is above 0x00, or in
automatic mode when the temperature is above TMIN.
If the temperature is below TMIN or if the duty cycle in
manual mode is set to 0x00, pulling the THERM low
externally has no effect. See Figure 31 for more information.
Figure 31. Asserting THERM Low as an Input in
Automatic Fan Speed Control Mode
THERM ASSERTED TO LOW AS
AN INPUT: FANS DO NOT GO
TO 100% BECAUSE TEMPERATURE
IS BELOW TMIN
THERM ASSERTED TO LOW AS
AN INPUT: FANS GO TO 100%
BECAUSE TEMPERATURE IS
ABOVE TMIN AND FANS ARE
ALREADY RUNNING.
TMIN
THERM
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THERM Timer
The ADT7476 has an internal timer to measure THERM
assertion time. For example, the THERM input can be
connected to the PROCHOT output of a Pentium4 CPU to
measure system performance. The THERM input can also
be connected to the output of a trip-point temperature sensor.
The timer is started on the assertion of the ADT7476’s
THERM input and stopped when THERM is de-asserted.
The timer counts THERM times cumulatively; that is, the
timer resumes counting on the next THERM assertion. The
THERM timer continues to accumulate THERM assertion
times until the timer is read (where it is cleared), or until it
reaches full scale. If the counter reaches full scale, it stops
at that reading until cleared.
The 8-bit THERM timer status register (0x79) is designed
so that Bit 0 is set to 1 on the first THERM assertion. Once
the cumulative THERM assertion time has exceeded
45.52 ms, Bit 1 of the THERM timer is set and Bit 0 now
becomes the LSB of the timer with a resolution of 22.76 ms
(see Figure 32).
Figure 32. Understanding the THERM Timer
THERM ASSERTED
22.76 ms
THERM ASSERTED
45.52 ms
THERM ASSERTED
113.8 ms
(91.04 ms + 22.76 ms)
THERM
THERM
THERM
TIMER
(REG. 0x79)
THERM
TIMER
(REG. 0x79)
THERM
THERM
TIMER
(REG. 0x79)
ACCUMULATE THERM LOW
ASSERTION TIMES
ACCUMULATE THERM LOW
ASSERTION TIMES
10000000
01234567
01000000
01234567
10100000
01234567
When using the THERM timer, be aware of the following:
After a THERM timer read (0x79)
1. The contents of the timer are cleared on read.
2. The F4P bit (Bit 5) of Interrupt Status Register 2
needs to be cleared (assuming that the THERM
timer limit has been exceeded).
If the THERM timer is read during a THERM assertion, the
following occurs:
1. The contents of the timer are cleared.
2. Bit 0 of the THERM timer is set to 1, because a
THERM assertion is occurring.
3. The THERM timer increments from zero.
4. If the THERM timer limit register (0x7A) = 0x00,
the F4P bit is set.
Generating SMBALERT Interrupts from THERM Timer
Events
The ADT7476 can generate SMBALERTs when a
programmable THERM timer limit has been exceeded. This
allows the system designer to ignore brief, infrequent
THERM assertions, while capturing longer THERM timer
events. Register 0x7A is the THERM timer limit register.
This 8-bit register allows a limit from 0 sec (first THERM
assertion) to 5.825 sec to be set before an SMBALERT is
generated. The THERM timer value is compared with the
contents of the THERM timer limit register. If the THERM
timer value exceeds the THERM timer limit value, then the
F4P bit (Bit 5) of Interrupt Status Register 2 is set and an
SMBALERT is generated.
NOTE: Depending on which pins are configured as a THERM timer,
setting the F4P bit (Bit 5) of Mask Register 2 (0x75) or Bit 0
of Mask Register 1 (0x74) masks out SMBALERT; although
the F4P bit of Interrupt Status Register 2 is still set if the
THERM timer limit is exceeded.
Figure 33 is a functional block diagram of the THERM
timer, limit, and associated circuitry. Writing a value of 0x00
to the THERM timer limit register (0x7A) causes an
SMBALERT to be generated on the first THERM assertion.
A THERM timer limit value of 0x01 generates an
SMBALERT once cumulative THERM assertions exceed
45.52 ms.
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Figure 33. Functional Block Diagram of THERM Monitoring Circuitry
COMPARATOR
THERM
2.914 s
1.457 s
728.32 ms
364.16 ms
91.04 ms
45.52 ms
22.76 ms
182.08 ms
THERM LIMIT
(REG. 0x7A)
2.914 s
1.457 s
728.32 ms
364.16 ms
91.04 ms
45.52 ms
22.76 ms
182.08 ms
THERM TIMER
(REG. 0x79)
THERM TIMER CLEARED ON READ
SMBALERT
F4P BIT (BIT 5)
MASK REGISTER 2
(REG. 0x75)
CLEARED
ON READ
F4P BIT (BIT 5)
INTERRUPT STATUS
REGISTER 2
OUTIN
RESET
LATCH
1 = MASK
01234567 01234567
Configuring the Relevant THERM Behavior
1. Configure the desired pin as the THERM timer
input.
Setting Bit 1 (THERM timer enable) of
Configuration Register 3 (0x78) enables the
THERM timer monitoring functionality. This is
disabled on Pin 14 and Pin 22 by default.
Setting Bit 0 and Bit 1 (PIN14FUNC) of
Configuration Register 4 (0x7D) enables THERM
timer output functionality on Pin 22 (Bit 1 of
Configuration Register 3, THERM, must also be
set). Pin 14 can also be used as TACH4.
2. Select the desired fan behavior for THERM timer
events.
Assuming the fans are running, setting Bit 2
(BOOST bit) of Configuration Register 3 (0x78)
causes all fans to run at 100% duty cycle whenever
THERM is asserted. This allows fail-safe system
cooling. If this bit is 0, the fans run at their current
settings and are not affected by THERM events. If
the fans are not already running when THERM
is asserted, then the fans do not run to full speed.
3. Select whether THERM timer events should
generate SMBALERT interrupts.
Setting Bit 5 (F4P) of Mask Register 2 (0x75) or
Bit 0 of Mask Register 1 (0x74), depending on
which pins are configured as a THERM timer,
masks SMBALERTs when the THERM timer limit
value is exceeded. This bit should be cleared if
SMBALERTs based on THERM events are
required.
4. Select a suitable THERM limit value.
This value determines whether an SMBALERT is
generated on the first THERM assertion, or if only
a cumulative THERM assertion time limit is
exceeded. A value of 0x00 causes an SMBALERT
to be generated on the first THERM assertion.
5. Select a THERM monitoring time.
This value specifies how often OS- or BIOS-level
software checks the THERM timer. For example,
BIOS can read the THERM timer once an hour to
determine the cumulative THERM assertion time.
If, for example, the total THERM assertion time is
<22.76 ms in Hour 1, >182.08 ms in Hour 2, and
>5.825 sec in Hour 3, system performance is
degrading significantly because THERM is
asserting more frequently on an hourly basis.
Alternatively, OS or BIOS level software can
timestamp when the system is powered on. If an
SMBALERT is generated due to the THERM
timer limit being exceeded, another timestamp can
be taken. The difference in time can be calculated
for a fixed THERM timer limit time. For example,
if it takes one week for a THERM timer limit of
2.914 sec to be exceeded, and the next time it takes
only 1 hour, then a serious degradation in system
performance has occurred.
Configuring the THERM Pin as an Output
In addition to monitoring THERM as an input, the
ADT7476 can optionally drive THERM low as an output.
When PROCHOT is bidirectional, THERM can be used to
throttle the processor by asserting PROCHOT. The user can
preprogram system-critical thermal limits. If the temperature
exceeds a thermal limit by 0.25C, THERM asserts low. If the
temperature is still above the thermal limit on the next
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monitoring cycle, THERM stays low. THERM remains
asserted low until the temperature is equal to or below the
thermal limit. Because the temperature for that channel is
measured only once for every monitoring cycle, after
THERM asserts, it is guaranteed to remain low for at least one
monitoring cycle.
The THERM pin can be configured to assert low, if the
Remote 1, local, or Remote 2 THERM temperature limits
are exceeded by 0.25C. The THERM temperature limit
registers are at Register 0x6A, Register 0x6B, and Register
0x6C, respectively. Setting Bits [5:7] of Configuration
Register 5 (0x7C) enables the THERM output feature for
the Remote 1, local, and Remote 2 temperature channels,
respectively. Figure 34 shows how the THERM pin asserts
low as an output in the event of a critical overtemperature.
Figure 34. Asserting THERM as an Output, Based on
Tripping THERM Limits
THERM LIMIT
MONITORING
CYCLE
TEMP
THERM
0.255C
THERM LIMIT
An alternative method of disabling THERM is to program
the THERM temperature limit to –63C or less in Offset 64
mode, or −128C or less in twos complement mode; that is,
for THERM temperature limit values less than –63C or
–128C, respectively, THERM is disabled.
Enabling and Disabling THERM on individual Channels
THERM can be enabled/disabled for individual or
combinations of temperature channels using Bits [7:5] of
Configuration Register 5 (0x7C).
THERM Hysteresis
Setting Bit 0 of Configuration Register 7 (0x11) disables
THERM hysteresis.
If THERM hysteresis is enabled and THERM is disabled
(Bit 2 of Configuration Register 4, 0x7D), the THERM pin
does not assert low when a THERM event occurs. If
THERM hysteresis is disabled and THERM is disabled
(Bit 2 of Configuration Register 4, 0x7D) and assuming the
appropriate pin is configured as THERM), the THERM pin
asserts low when a THERM event occurs.
If THERM and THERM hysteresis are both enabled, the
THERM output asserts as expected.
THERM Operation in Manual Mode
In manual mode, THERM events do not cause fans to go
to full speed, unless Bit 3 of Configuration Register 6
(0x10) is set to 1.
Additionally, Bit 3 of Configuration Register 4 (0x7D)
can be used to select the PWM speed on a THERM event
(100% or maximum PWM).
Bit 2 in Configuration Register 4 (0x7D) can be set to
disable THERM events from affecting the fans.
Fan Drive Using PWM Control
The ADT7476 uses pulse-width modulation (PWM) to
control fan speed. This relies on varying the duty cycle (or
on/off ratio) of a square wave applied to the fan to vary the
fan speed. The external circuitry required to drive a fan using
PWM control is extremely simple. For 4-wire fans, the
PWM drive might need only a pullup resistor. In many cases,
the 4-wire fan PWM input has a built-in, pullup resistor.
The ADT7476 PWM frequency can be set to a selection
of low frequencies or a single high PWM frequency. The
low frequency options are used for 3-wire fans, while the
high frequency option is usually used with 4-wire fans.
For 3-wire fans, a single N-channel MOSFET is the only
drive device required. The specifications of the MOSFET
depend on the maximum current required by the fan being
driven and the input capacitance of the FET. Because a
10 kW (or greater) resistor must be used as a PWM pullup,
an FET with large input capacitance can cause the PWM
output to become distorted and adversely affect the fan
control range. This is a requirement only when using high
frequency PWM mode.
Typical notebook fans draw a nominal 170 mA, so SOT
devices can be used where board space is a concern. In
desktops, fans typically draw 250 mA to 300 mA each. If
you drive several fans in parallel from a single PWM output
or drive larger server fans, the MOSFET must handle the
higher current requirements. The only other stipulation is
that the MOSFET should have a gate voltage drive,
VGS < 3.3 V, for direct interfacing to the PWM output pin.
The MOSFET should also have a low on resistance to ensure
that there is not a significant voltage drop across the FET,
which would reduce the voltage applied across the fan and,
therefore, the maximum operating speed of the fan.
Figure 35 shows how to drive a 3-wire fan using PWM
control.
Figure 35. Driving a 3-wire Fan Using an N-channel
MOSFET
ADT7476
TACH
PWM Q1
NDT3055L
12 V
FAN
3.3 V
12 V12 V
10 kW
4.7 kW
10 kW
10 kW
1N4148
TACH
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Figure 35 uses a 10 kW pullup resistor for the TACH
signal. This assumes that the TACH signal is an
open-collector from the fan. In all cases, the TACH signal
from the fan must be kept below 5.5 V maximum to prevent
damaging the ADT7476.
Figure 36 shows a fan drive circuit using an NPN
transistor such as a general-purpose MMBT2222. While
these devices are inexpensive, they tend to have much lower
current handling capabilities and higher on resistance than
MOSFETs. When choosing a transistor, care should be taken
to ensure that it meets the fan’s current requirements. Ensure
that the base resistor is chosen so that the transistor is
saturated when the fan is powered on.
Because the fan drive circuitry in 4-wire fans is not
switched on or off, as with previous PWM driven/powered
fans, the internal drive circuit is always on and uses the
PWM input as a signal instead of a power supply. This
enables the internal fan drive circuit to perform better than
3-wire fans, especially for high frequency applications.
Figure 36. Driving a 3-wire Fan Using
an NPN Transistor
ADT7476
TACH
PWM Q1
MMBT2222
12 V
FAN
3.3 V
12 V12 V
10 kW
4.7 kW
470 W
10 kW
1N4148
TACH
Figure 37 shows a typical drive circuit for 4-wire fans.
Figure 37. Driving a 4-wire Fan
ADT7476
TACH
PWM
12 V, 4-WIRE FAN
12 V12 V
10 kW
4.7 kW
10 kW
TACH
VCC
TACH
PWM
3.3 V
2 kW
Driving Two Fans from PWM3
The ADT7476 has four TACH inputs available for fan
speed measurement, but only three PWM drive outputs. If a
fourth fan is being used in the system, it should be driven
from the PWM3 output in parallel with the third fan.
Figure 38 shows how to drive two fans in parallel using low
cost NPN transistors. Figure 39 shows the equivalent circuit
using a MOSFET.
Because the MOSFET can handle up to 3.5 A, users can
connect another fan directly in parallel with the first. Care
should be taken in designing drive circuits with transistors
and FETs to ensure that the PWM outputs are not required
to source current, and that they sink less than the 5 mA
maximum current specified on the data sheet.
Driving up to Three Fans from PWM3
TACH measurements for fans are synchronized to
particular PWM channels; for example, TACH1 is
synchronized to PWM1. TACH3 and TACH4 are both
synchronized to PWM3, so PWM3 can drive two fans.
Alternatively, PWM3 can be programmed to synchronize
TACH2, TACH3, and TACH4 to the PWM3 output. This
allows PWM3 to drive two or three fans. In this case, the
drive circuitry looks the same, as shown in Figure 38 and
Figure 39. The SYNC bit in Register 0x62 enables this
function.
Synchronization is not required in high frequency mode
when used with 4-wire fans.
Table 34. SYNC: ENHANCE ACOUSTICS REGISTER 1
(REG. 0X62)
Bit Mnemonic Description
[4] SYNC 1, Synchronizes TACH2, TACH3, and
TACH4 to PWM3.
Figure 38. Interfacing Two Fans in Parallel to the
PWM3 Output Using Low Cost NPN Transistors
ADT7476
PWM3 Q1
MMBT3904
3.3 V
1 kW
TACH4
2.2 kW
3.3 V TACH3
10 kW
10 kW
12 V
Q2
MMBT2222
1N4148
Q3
MMBT2222
3.3 V 3.3 V
Figure 39. Interfacing Two Fans in Parallel to the
PWM3 Output Using a Single N-channel MOSFET
ADT7476
TACH4
Q1
NDT3055L
3.3 V
10 kW
TYP
TACH3
PWM3
3.3 V
3.3 V
10 kW
TYP
10 kW
TYP
+V +V
5 V
or
12 V
FAN
TACH
1N4148
5 V
or
12 V
FAN
TACH
3.3 V
3.3 V
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Laying Out 3-wire Fans
Figure 40 shows how to lay out a common circuit
arrangement for 3-wire fans.
Figure 40. Planning for 3-wire Fans on a PCB
Q1
MMBT2222
TACH
12 V or 5 V
1N4148
PWM
R1
R2
R3
R4
3.3 V or 5 V
TACH Inputs
Pin 9, Pin 11, Pin 12, and Pin 14 (when configured as
TACH inputs) are high impedance inputs intended for fan
speed measurement.
Signal conditioning in the ADT7476 accommodates the
slow rise and fall times typical of fan tachometer outputs.
The maximum input signal range is 0 V to 5.5 V, even
though VCC is 3.3 V. In the event that these inputs are
supplied from fan outputs that exceed 0 V to 5.5 V, either
resistive attenuation of the fan signal or diode clamping
must be included to keep inputs within an acceptable range.
Figure 41 to Figure 44 show circuits for most common fan
TACH outputs.
If the fan TACH output has a resistive pullup to VCC, it can
be connected directly to the fan input, as shown in Figure 41.
Figure 41. Fan with TACH Pullup to VCC
12 V VCC
FAN SPEED
COUNTER
TACH
OUTPUT
TACH
PULLUP
4.7 kW
TYP
ADT7476
If the fan output has a resistive pullup to 12 V, or other
voltage greater than 5.5 V, the fan output can be clamped
with a Zener diode, as shown in Figure 42. The Zener diode
voltage should be chosen so that it is greater than VIH of the
TACH input but less than 5.5 V, allowing for the voltage
tolerance of the Zener. A value between 5.0 V and 5.5 V is
suitable.
Figure 42. Fan with Strong TACH Pullup to > 5.5 V,
(for Example, 12 V) Clamped with Zener Diode
12 V VCC
FAN SPEED
COUNTER
TACH
OUTPUT TACH
PULLUP
4.7 kW
TYP
ADT7476
ZD1*
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 VCC
If the fan has a strong pullup (less than 1 kW) to 12 V or
a totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 43.
Figure 43. Fan with Strong TACH. Pullup to > VCC
or Totem-Pole Output, Clamped with
Zener Diode and Resistor
5 V or12 V VCC
FAN SPEED
COUNTER
TACH
ADT7476
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 VCC
PULLUP
TYP < 1 kW
OR TOTEM-POLE
ZD1*
ZENER
TACH
OUTPUT
FAN
R1
10 kW
Alternatively, a resistive attenuator can be used, as shown
in Figure 44. R1 and R2 should be chosen such that:
(eq. 4)
2VtVPULLUP R2ń
ǒ
RPULLUP )R1 )R2
Ǔ
t5.5 V
The fan inputs have an input resistance of nominally
160 kW to ground, which should be taken into account when
calculating resistor values.
With a pullup voltage of 12 V and pullup resistor less than
1 kW, suitable values for R1 and R2 are 100 kW and 40 kW,
respectively. This gives a high input voltage of 3.42 V.
Figure 44. Fan with Strong TACH. Pullup to > VCC or
Totem-Pole Output, Attenuated with R1/R2
12 V VCC
FAN SPEED
COUNTER
TACH
ADT7476
*SEE TEXT
< 1 kW
R1* R2*
TACH
OUTPUT
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The fan counter does not count the fan TACH output
pulses directly because the fan speed could be less than
1,000 RPM, and it takes several seconds to accumulate a
reasonably large and accurate count. Instead, the period of
the fan revolution is measured by gating an on-chip 90 kHz
oscillator into the input of a 16-bit counter for N periods of
the fan TACH output (Figure 45), so the accumulated count
is actually proportional to the fan tachometer period and
inversely proportional to the fan speed.
N, the number of pulses counted, is determined by the
settings of TACH pulses per revolution register (0x7B). This
register contains two bits for each fan, allowing one, two
(default), three, or four TACH pulses to be counted.
Figure 45. Fan Speed Measurement
1
2
3
4
CLOCK
PWM
TACH
Fan Tachometer Reading Registers
The fan tachometer readings are 16-bit values consisting
of a 2-byte read from the ADT7476.
Table 35. FAN TACHOMETER READING REGISTERS
Register Description Default
0x28 TACH1 Low Byte 0x00
0x29 TACH1 High Byte 0x00
0x2A TACH2 Low Byte 0x00
0x2B TACH2 High Byte 0x00
0x2C TACH3 Low Byte 0x00
0x2D TACH3 High Byte 0x00
0x2E TACH4 Low Byte 0x00
0x2F TACH4 High Byte 0x00
Reading Fan Speed from the ADT7476
The measurement of fan speeds involves a 2-register read
for each measurement. The low byte should be read first.
This causes the high byte to be frozen until both high and
low byte registers have been read, preventing erroneous
TACH readings. The fan tachometer reading registers report
back the number of 11.11 ms period clocks (90 kHz
oscillator) gated to the fan speed counter from the rising
edge of the first fan TACH pulse to the rising edge of the
third fan TACH pulse (assuming two pulses per revolution
are being counted).
Because the device is essentially measuring the fan TACH
period, the higher the count value, the slower the fan is
actually running. A 16-bit fan tachometer reading of
0xFFFF indicates that either the fan has stalled or is running
very slowly (<100 RPM).
High Limit: > Comparison Performed
Because the actual fan TACH period is being measured,
falling below a fan TACH limit by 1 sets the appropriate
status bit and can be used to generate an SMBALERT.
Measuring fan TACH has the following caveat: When the
ADT7476 starts up, TACH measurements are locked. In
effect, an internal read of the low byte has been made for each
TACH input. The net result of this is that all TACH readings
are locked until the high byte is read from the corresponding
TACH registers. All TACH related interrupts are also ignored
until the appropriate high byte is read.
Once the corresponding high byte has been read, TACH
measurements are unlocked and interrupts are processed as
normal.
Fan TACH Limit Registers
The fan TACH limit registers are 16-bit values consisting
of two bytes.
Table 36. FAN TACH LIMIT REGISTERS
Register Description Default
0x54 TACH1 Minimum Low Byte 0xFF
0x55 TACH1 Minimum High Byte 0xFF
0x56 TACH2 Minimum Low Byte 0xFF
0x57 TACH2 Minimum High Byte 0xFF
0x58 TACH3 Minimum Low Byte 0xFF
0x59 TACH3 Minimum High Byte 0xFF
0x5A TACH4 Minimum Low Byte 0xFF
0x5B TACH4 Minimum High Byte 0xFF
Fan Speed Measurement Rate
The fan TACH readings are normally updated once every
second.
When set, the FAST bit (Bit 3) of Configuration
Register 3 (0x78) updates the fan TACH readings every
250 ms.
DC Bits
If any of the fans are not being driven by a PWM channel
but are powered directly from 5.0 V or 12 V, their associated
dc bit in Configuration Register 3 should be set. This allows
TACH readings to be taken on a continuous basis for fans
connected directly to a dc source. Once high frequency
mode is enabled in 4-wire fans, the dc bits do not need to be
set because this is automatically done internally.
Calculating Fan Speed
Assuming a fan with two pulses per revolution, and with
the ADT7476 programmed to measure two pulses per
revolution, fan speed is calculated by
Fan Speed (RPM) = (90,000 60)/Fan TACH Reading
where Fan TACH Reading is the 16-bit fan tachometer
reading.
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Example:
TACH1 High Byte (0x29) = 0x17
TACH1 Low Byte (0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TACH Reading = 0x17FF = 6143 (decimal)
RPM = (f 60)/Fan 1 TACH Reading
RPM = (90,000 60)/6143
Fan Speed = 879 RPM
TACH Pulses per Revolution
Different fan models can output either one, two, three, or
four TACH pulses per revolution. Once the number of fan
TACH pulses has been determined, it can be programmed
into the TACH Pulses per Revolution Register (0x7B) for
each fan. Alternatively, this register can be used to determine
the number of pulses per revolution output by a given fan.
By plotting fan speed measurements at 100% speed with
different pulses per revolution settings, the smoothest graph
with the lowest ripple determines the correct pulses per
revolution value.
Table 37. FAN PULSES/REVOLUTION REGISTER
(REG. 0X7B)
Bit Mnemonic Description
[1:0] FAN1 Default 2 Pulses per Revolution
[3:2] FAN2 Default 2 Pulses per Revolution
[5:4] FAN3 Default 2 Pulses per Revolution
[7:6] FAN4 Default 2 Pulses per Revolution
Table 38. FAN PULSES/REVOLUTION REGISTER
BIT VALUES
Value Description
00 1 Pulse per Revolution
01 2 Pulses per Revolution
10 3 Pulses per Revolution
11 4 Pulses per Revolution
Fan Spin-up
The ADT7476 has a unique fan spin-up function. It spins
the fan at 100% PWM duty cycle until two TACH pulses are
detected on the TACH input. Once two TACH pulses have
been detected, the PWM duty cycle goes to the expected
running value, for example, 33%. Fans have different
spin-up characteristics and take different times to overcome
inertia. The advantage of the ADT7476 is that it runs the fans
just fast enough to overcome inertia and is quieter on spin-up
than fans that are programmed to spin up for a given time.
Fan Startup Timeout
To prevent the generation of false interrupts as a fan spins
up (because it is below running speed), the ADT7476
includes a fan startup timeout function. During this time, the
ADT7476 looks for two TACH pulses. If two TACH pulses
are not detected, an interrupt is generated.
Fan startup timeout can be disabled by setting Bit 5
(FSPDIS) of Configuration Register 1 (0x40).
Table 39. PWM1 TO PWM3 CONFIGURATION
(REG. 0X5C TO 0X5E)
Bit Mnemonic Description
[2:0] SPIN These bits control the startup
timeout for PWM1 (0x5C),
PWM2 (0x5D), PWM3 (0x5E).
000 = No Startup Timeout
001 = 100 ms
010 = 250 ms (Default)
011 = 400 ms
100 = 667 ms
101 = 1 s
110 = 2 s
111 = 4 s
Disabling Fan Startup Timeout
Although fan startup makes fan spin-ups much quieter
than fixed-time spin-ups, the option exists to use fixed
spin-up times. Setting Bit 5 (FSPDIS) to 1 in Configuration
Register 1 (0x40) disables the spin-up for two TACH pulses.
Instead, the fan spins up for the fixed time as selected in
Register 0x5C to Register 0x5E.
PWM Logic State
The PWM outputs can be programmed high for 100%
duty cycle (non-inverted) or low for 100% duty cycle
(inverted).
Table 40. PWM1 TO PWM3 CONFIGURATION
(REG. 0X5C TO 0X5E) BITS
Bit Mnemonic Description
[4] INV 0 = logic high for 100% PWM duty cycle
1 = logic low for 100% PWM duty cycle
Low Frequency Mode PWM Drive Frequency
The PWM drive frequency can be adjusted for the
application. Register 0x5F to Register 0x61 configure the
PWM frequency for PWM1 to PWM3, respectively.
Table 41. PWM FREQUENCY REGISTERS
(REG. 0X5F TO 0X61)
Bit Mnemonic Description
[2:0] FREQ 000 = 11.0 Hz
001 = 14.7 Hz
010 = 22.1 Hz
011 = 29.4 Hz
100 = 35.3 Hz (Default)
101 = 44.1 Hz
110 = 58.8 Hz
111 = 88.2 Hz
High Frequency Mode PWM Drive
Setting Bit 3 of Register 0x5F, Register 0x60, and
Register 0x61 enables high frequency mode for Fan 1,
Fan 2, and Fan 3 respectively.
In high frequency mode, the PWM drive frequency is
always 22.5 kHz. When high frequency mode is enabled, the
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dc bits are automatically asserted internally and do not need
to be changed.
Fan Speed Control
The ADT7476 controls fan speed using automatic and
manual modes:
In automatic fan speed control mode, fan speed is
automatically varied with temperature and without CPU
intervention once initial parameters are set up. The
advantage is that if the system hangs, the user is
guaranteed that the system is protected from
overheating.
In manual fan speed control mode, the ADT7476
allows the duty cycle of any PWM output to be
adjusted manually. This can be useful if the user wants
to change fan speed in software or adjust PWM duty
cycle output for test purposes. Bits [7:5] of Register
0x5C to Register 0x5E (PWM Configuration) control
the behavior of each PWM output.
Table 42. PWM CONFIGURATION
(REG. 0X5C TO 0X5E) BITS
Bit Mnemonic Description
[7:5] BHVR 111 = Manual Mode
Once under manual control, each PWM output can be
manually updated by writing to Register 0x30 to Register
0x32 (PWM current duty cycle registers).
Programming the PWM Current Duty Cycle Registers
The PWM current duty cycle registers are 8-bit registers
that allow the PWM duty cycle for each output to be set
anywhere from 0% to 100% in steps of 0.39%. The value to
be programmed into the PWMMIN register is given by:
Value (decimal) = PWMMIN/0.39
Example 1:
For a PWM duty cycle of 50%,
Value (decimal) = 50/0.39 = 128 (decimal)
Value = 128 (decimal) or 0x80 (hex)
Example 2:
For a PWM duty cycle of 33%,
Value (decimal) = 33/0.39 = 85 (decimal)
Value = 85 (decimal) or 0x54 (hex)
Table 43. PWM CURRENT DUTY CYCLE REGISTERS
Register Description Default
0x30 PWM1 Current Duty Cycle 0xFF (100%)
0x31 PWM2 Current Duty Cycle 0xFF (100%)
0x32 PWM3 Current Duty Cycle 0xFF (100%)
By reading the PWMx current duty cycle registers, the
user can keep track of the current duty cycle on each PWM
output, even when the fans are running in automatic fan
speed control mode or acoustic enhancement mode.
Programming TRANGE
TRANGE defines the distance between TMIN and 100%
PWM. For the ADT7467, ADT7468 and ADT7473, TRANGE
is effectively a slope. For the ADT7475 andADT7476,
TRANGE is no longer a slope, but defines the temperature
region where the PWM output linearly ramps from PWMMIN
to 100% PWM.
Figure 46. TRANGE
TMIN
PWM = 100%
PWM = 0%
PWMMIN
PWMMAX
TRANGE
Programming the Automatic Fan Speed Control Loop
To understand the automatic fan speed control loop more
efficiently, it is recommended to use the ADT7476
evaluation board and software while reading this section.
This section provides the system designer with an
understanding of the automatic fan control loop and
provides step-by-step guidance on effectively evaluating
and selecting critical system parameters. To optimize the
system characteristics, the designer needs to give some
thought to system configuration, including the number of
fans, where they are located, and what temperatures are
being measured in the particular system.
The mechanical or thermal engineer who is tasked with
the system thermal characterization should also be involved
at the beginning of the system development process.
Manual Fan Control Overview
In unusual circumstances, it can be necessary to manually
control the speed of the fans. Because the ADT7476 has an
SMBus interface, a system can read back all necessary
voltage, fan speed, and temperature information, and use
this information to control the speed of the fans by writing
to the PWM current duty cycle register (0x30, 0x31, and
0x32) of the appropriate fan. Bits [7:5] of the PWMx
configuration registers (0x5C, 0x5D, 0x5E) are used to set
fans up for manual control.
THERM Operation in Manual Mode
In manual mode, if the temperature increases above the
programmed THERM temperature limit, the fans
automatically speed up to maximum PWM or 100% PWM,
whichever way the appropriate fan channel is configured.
Automatic Fan Control Overview
The ADT7476 can automatically control the speed of fans
based on the measured temperature. This is done
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independently of CPU intervention once initial parameters
are set up.
The ADT7476 has a local temperature sensor and two
remote temperature channels that can be connected to a CPU
on-chip thermal diode (available on Intel Pentium class
and other CPUs). These three temperature channels can be
used as the basis for automatic fan speed control to drive fans
using pulse-width modulation (PWM).
Automatic fan speed control reduces acoustic noise by
optimizing fan speed according to accurately measured
temperature. Reducing fan speed can also decrease system
current consumption. The automatic fan speed control mode
is very flexible due to the number of programmable
parameters, including TMIN and TRANGE. The TMIN and
TRANGE values for a temperature channel and, therefore, for
a given fan, are critical, because they define the thermal
characteristics of the system. The thermal validation of the
system is one of the most important steps in the design
process, so these values should be selected carefully.
Figure 47 gives a top level overview of the automatic fan
control circuitry on the ADT7476. From a systems level
perspective, up to three system temperatures can be
monitored and used to control three PWM outputs. The three
PWM outputs can be used to control up to four fans. The
ADT7476 allows the speed of four fans to be monitored.
Each temperature channel has a thermal calibration block,
allowing the designer to individually configure the thermal
characteristics of each temperature channel. For example,
designers can decide to run the CPU fan when CPU
temperature increases above 60C and a chassis fan when
the local temperature increases above 45C.
At this stage, the designer has not assigned these thermal
calibration settings to a particular fan drive (PWM) channel.
The right side of Figure 47 shows fan-specific controls. The
designer has individual control over parameters such as
minimum PWM duty cycle, fan speed failure thresholds,
and even ramp control of the PWM outputs. Automatic fan
control, then, ultimately allows graceful fan speed changes
that are less perceptible to the system user.
Figure 47. Automatic Fan Control Block Diagram
THERMAL CALIBRATION
REMOTE1
TEMP
100%
0%
TMIN TRANGE
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
LOCAL
TEMP
REMOTE2
TEMP
PWM
MIN
PWM
MIN
PWM
MIN
MUX
S
S
S
PWM
CONFIG
PWM
GENERATOR
PWM1
PWM2
PWM3
TACH1
TACH2
TACH3
TACHOMETER 1
MEASUREMENT
TACHOMETER 2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER 3
AND 4
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
CONFIG
PWM
GENERATOR
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
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Step 1 − Hardware Configuration
During system design, the motherboard sensing and
control capabilities should be addressed early in the design
stages. Decisions about how these capabilities are used
should involve the system thermal/mechanical engineer.
Ask the following questions:
1. What ADT7476 functionality is used?
PWM2 or SMBALERT?
TACH4 fan speed measurement or
overtemperature THERM function?
2.5 V voltage monitoring or overtemperature
THERM function?
12 V voltage monitoring or VID5 input?
The ADT7476 offers multifunctional pins that can
be reconfigured to suit different system
requirements and physical layouts. These
multifunction pins are software programmable.
2. How many fans are supported in system, three or
four?
This influences the choice of whether to use the
TACH4 pin or to reconfigure it for the THERM
function.
3. Is the CPU fan to be controlled using the
ADT7476, or will the CPU fan run at full speed
100% of the time?
If run at 100%, this frees up a PWM output, but
the system is louder.
4. Where will the ADT7476 be physically located in
the system?
This influences the assignment of the temperature
measurement channels to particular system
thermal zones. For example, locating the
ADT7476 close to the VRM controller circuitry
allows the VRM temperature to be monitored
using the local temperature channel.
Figure 48. Hardware Configuration Example
THERMAL CALIBRATION
REMOTE1 =
AMBIENT TEMP
100%
0%
TMIN TRANGE
PWM
MIN
MUX
S
TACHOMETER1
MEASUREMENT
S
S
PWM
MIN
PWM
MIN
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
LOCAL =
VRM TEMP
REMOTE2 =
CPU TEMP
TACHOMETER2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER3
AND 4
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
PWM1
PWM2
PWM3
TACH3
TACH2
TACH1
CPU FAN SINK
FRONT CHASSIS
REAR CHASSIS
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
CONFIG
PWM
GENERATOR
PWM
CONFIG
PWM
GENERATOR
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Recommended Implementation 1
Configuring the ADT7476 as shown in Figure 49
provides the system designer with the following features:
Six VID inputs (VID0, VID1, VID2, VID3, VID4, and
VID6) for VRM10 support.
Two PWM outputs for fan control of up to three fans.
The front and rear chassis fans are connected in
parallel.
Three TACH fan speed measurement inputs.
VCC measured internally through Pin 4.
CPU core voltage measurement (VCORE).
2.5 V measurement input used to monitor CPU current
(connected to VCOMP output of ADP316x VRM
controller). This is used to determine CPU power
consumption.
5.0 V measurement input.
VRM temperature using local temperature sensor.
CPU temperature measured using the Remote 1
temperature channel.
Ambient temperature measured through the Remote 2
temperature channel.
If not using VID5, it can be reconfigured as the 12 V
monitoring input.
Bidirectional THERM pin allows the monitoring of
PROCHOT output from an Intel P4 processor, for
example, or can be used as an overtemperature THERM
output.
SMBALERT system interrupt output.
Figure 49. Recommended Implementation 1
CPU FAN
CPU
FRONT
CHASSIS
FAN TACH2
ADT7476
PWM3
REAR
CHASSIS
FAN
AMBIENT
TEMPERATURE
TACH3
D1+
D1−
GND
PWM1
TACH1
D2+
D2−
ICH
SDA
SCL
THERM
SMBALERT
PROCHOT
VCC
+5VIN
+12VIN/VID5
VID[0:4]/VID[0:5]
5(VRM9)/6(VRM10)
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Recommended Implementation 2
Configuring the ADT7476 as shown in Figure 50
provides the system designer with the following features:
Six VID inputs (VID0, VID1, VID2, VID3, VID4, and
VID6) for VRM10 support.
Three PWM outputs for fan control of up to three fans.
All three fans can be individually controlled.
Three TACH fan speed measurement inputs.
VCC measured internally through Pin 4.
CPU core voltage measurement (VCORE).
2.5 V measurement input used to monitor CPU current
(connected to VCOMP output of ADP316x VRM
controller). This is used to determine CPU power
consumption.
5.0 V measurement input.
VRM temperature using local temperature sensor.
CPU temperature measured using the Remote 1
temperature channel.
Ambient temperature measured through the Remote 2
temperature channel.
If not using VID5, it can be reconfigured as the 12 V
monitoring input.
Bidirectional THERM pin allows the monitoring of
PROCHOT output/input from an Intel P4 processor,
for example, or can be used as an overtemperature
THERM output.
Figure 50. Recommended Implementation 2
CPU FAN
CPU
FRONT
CHASSIS
FAN TACH2
ADT7476
PWM3
REAR
CHASSIS
FAN
AMBIENT
TEMPERATURE
TACH3
D1+
D1−
GND
PWM1
TACH1
D2+
D2−
ICH
SDA
SCL
THERM
PROCHOT
VCC
+5VIN
+12VIN/VID5
VID[0:4]/VID[0:5]
5(VRM9)/6(VRM10)
PWM2
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Step 2 − Configuring the Mux
After the system hardware configuration is determined,
the fans can be assigned to particular temperature channels.
Not only can fans be assigned to individual channels, but the
behavior of the fans is also configurable. For example, fans
can be run under automatic fan control, manually (under
software control), or at the fastest speed calculated by
multiple temperature channels. The mux is the bridge
between temperature measurement channels and the three
PWM outputs.
Bits [7:5] (BHVR) of Register 0x5C, Register 0x5D, and
Register 0x5E (PWM configuration registers) control the
behavior of the fans connected to the PWM1, PWM2, and
PWM3 outputs. The values selected for these bits determine
how the mux connects a temperature measurement channel
to a PWM output.
Automatic Fan Control Mux Options
[7:5] (BHVR), Register 0x5C, Register 0x5D, Register
0x5E.
000 = Remote 1 temperature controls PWMx
001 = Local temperature controls PWMx
010 = Remote 2 temperature controls PWMx
101 = Fastest speed calculated by local and Remote 2
temperature controls PWMx
110 = Fastest speed calculated by all three
temperature channels controls PWMx
The fastest speed calculated options pertain to controlling
one PWM output based on multiple temperature channels.
The thermal characteristics of the three temperature zones
can be set to drive a single fan. An example would be the fan
turning on when Remote 1 temperature exceeds 60C or if
the local temperature exceeds 45C.
Other Mux Options
[7:5] (BHVR), Register 0x5C, Register 0x5D, Register 0x5E.
011 = PWMx runs full speed
100 = PWMx disabled (default)
111 = Manual mode. PWMx is running under
software control. In this mode, PWM current
duty cycle registers (0x30 to 0x32) are writable
and control the PWM outputs.
Figure 51. Assigning Temperature Channels to Fan Channels
THERMAL CALIBRATION
REMOTE1 =
AMBIENT TEMP
100%
0%
TMIN TRANGE
PWM
MIN
MUX
S
TACHOMETER1
MEASUREMENT
S
S
PWM
MIN
PWM
MIN
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
LOCAL =
VRM TEMP
REMOTE2 =
CPU TEMP
TACHOMETER2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER3
AND 4
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
PWM1
PWM2
PWM3
TACH3
TACH2
TACH1
CPU FAN SINK
FRONT CHASSIS
REAR CHASSIS
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
CONFIG
PWM
GENERATOR
PWM
CONFIG
PWM
GENERATOR
MUX
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Mux Configuration Example
This is an example of how to configure the mux in a
system using the ADT7476 to control three fans. The CPU
fan sink is controlled by PWM1, the front chassis fan is
controlled by PWM2, and the rear chassis fan is controlled
by PWM3. The mux is configured for the following fan
control behavior:
PWM1 (CPU fan sink) is controlled by the fastest speed
calculated by the local (VRM temperature) and
Remote 2 (processor) temperature. In this case, the
CPU fan sink is also being used to cool the VRM.
PWM2 (front chassis fan) is controlled by the Remote 1
temperature (ambient).
PWM3 (rear chassis fan) is controlled by the Remote 1
temperature (ambient).
Example Mux Settings
[7:5] (BHVR), PWM1 Configuration Register (0x5C).
101 = Fastest speed calculated by local and Remote 2
temperature controls PWM1
[7:5] (BHVR), PWM2 Configuration Register (0x5D).
000 = Remote 1 temperature controls PWM2
[7:5] (BHVR), PWM3 Configuration Register (0x5E).
000 = Remote 1 temperature controls PWM3
These settings configure the mux, as shown in Figure 52.
Figure 52. Mux Configuration Example
THERMAL CALIBRATION
REMOTE1 =
AMBIENT TEMP
100%
0%
TMIN TRANGE
PWM
MIN
MUX
S
TACHOMETER1
MEASUREMENT
S
S
PWM
MIN
PWM
MIN
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
LOCAL =
VRM TEMP
REMOTE2 =
CPU TEMP
TACHOMETER2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER3
AND 4
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
PWM1
PWM2
PWM3
TACH3
TACH2
TACH1 CPU FAN SINK
FRONT CHASSIS
REAR CHASSIS
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
CONFIG
PWM
GENERATOR
PWM
CONFIG
PWM
GENERATOR
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Step 3 − TMIN Settings for Thermal Calibration
Channels
TMIN is the temperature at which the fans start to turn on
under automatic fan control. The speed at which the fan runs
at TMIN is programmed later. The TMIN values chosen are
temperature channel specific, for example, 25C for
ambient channel, 30C for VRM temperature, and 40C for
processor temperature.
TMIN is an 8-bit value, either twos complement or Offset
64, which can be programmed in 1C increments. A TMIN
register is associated with each temperature measurement
channel: Remote 1, local, and Remote 2 temperature. Once
the TMIN value is exceeded, the fan turns on and runs at the
minimum PWM duty cycle. The fan turns off once the
temperature has dropped below TMIN − THYST.
To overcome fan inertia, the fan is spun up until two valid
TACH rising edges are counted. See the Fan Startup
Timeout section for more details. In some cases, primarily
for psycho-acoustic reasons, it is desirable that the fan never
switch off below TMIN. Setting Bits [7:5] of Enhance
Acoustics Register 1 (0x62) keeps the fans running at the
PWM minimum duty cycle if the temperature should fall
below TMIN.
Table 44. TMIN REGISTERS
Register Description Default
0x67 Remote 1 Temperature TMIN 0x5A (90C)
0x68 Local Temperature TMIN 0x5A (90C)
0x69 Remote 2 Temperature TMIN 0x5A (90C)
Enhance Acoustics Register 1 (0x62)
Bit 7 (MIN3) = 0, PWM3 is off (0% PWM duty cycle) when
temperature is below TMIN – THYST.
Bit 7 (MIN3) = 1, PWM3 runs at PWM3 minimum duty
cycle below TMIN – THYST.
Bit 6 (MIN2) = 0, PWM2 is off (0% PWM duty cycle) when
temperature is below TMIN – THYST.
Bit 6 (MIN2) = 1, PWM2 runs at PWM2 minimum duty
cycle below TMIN – THYST.
Bit 5 (MIN1) = 0, PWM1 is off (0% PWM duty cycle) when
temperature is below TMIN – THYST.
Bit 5 (MIN1) = 1, PWM1 runs at PWM1 minimum duty
cycle below TMIN – THYST.
Figure 53. Understanding the TMIN Parameter
THERMAL CALIBRATION
REMOTE1 =
AMBIENT TEMP
100%
0%
TMIN TRANGE
PWM
MIN
MUX
S
TACHOMETER1
MEASUREMENT
S
S
PWM
MIN
PWM
MIN
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
LOCAL =
VRM TEMP
REMOTE2 =
CPU TEMP
TACHOMETER2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER3
AND 4
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
PWM1
PWM2
PWM3
TACH3
TACH2
TACH1
CPU FAN SINK
FRONT CHASSIS
REAR CHASSIS
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
CONFIG
PWM
GENERATOR
PWM
CONFIG
PWM
GENERATOR
TMIN
100%
0%
PWM DUTY CYCLE
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Step 4 − PWMMIN for Each PWM (Fan) Output
PWMMIN is the minimum PWM duty cycle at which each
fan in the system runs. It is also the start speed for each fan
under automatic fan control once the temperature rises
above TMIN. For maximum system acoustic benefit,
PWMMIN should be as low as possible. Depending on the
fan used, the PWMMIN setting is usually in the 20% to 33%
duty cycle range. This value can be found through fan
validation.
Figure 54. PWMMIN Determines Minimum
PWM Duty Cycle
TMIN
100%
0%
PWM DUTY CYCLE
TEMPERATURE
PWMMIN
More than one PWM output can be controlled from a
single temperature measurement channel. For example,
Remote 1 temperature can control PWM1 and PWM2
outputs. If two different fans are used on PWM1 and PWM2,
the fan characteristics can be set up differently. As a result,
Fan 1 driven by PWM1 can have a different PWMMIN value
than that of Fan 2 connected to PWM2. Figure 55 illustrates
this as PWM1MIN (front fan), which is turned on at a
minimum duty cycle of 20%, while PWM2MIN (rear fan)
turns on at a minimum of 40% duty cycle. Note: Both fans
turn on at exactly the same temperature, defined by TMIN.
Figure 55. Operating Two Different Fans from
a Single Temperature Channel
TMIN
100%
0%
TEMPERATURE
PWM1MIN
PWM2MIN PWM1
PWM2
PWM DUTY CYCLE
Programming the PWM Minimum Duty Cycle Registers
The PWM minimum duty cycle registers are 8-bit
registers that allow the minimum PWM duty cycle for each
output to be configured anywhere from 0% to 100%. This
allows the minimum PWM duty cycle to be set in steps of
0.39%.
The value to be programmed into the PWMMIN register is
given by:
Value (decimal) = PWMMIN/0.39
Example 1:
For a minimum PWM duty cycle of 50%,
Value (decimal) = 50/0.39 = 128 (decimal)
Value = 128 (decimal) or 80 (hex)
Example 2:
For a minimum PWM duty cycle of 33%,
Value (decimal) = 33/0.39 = 85 (decimal)
Value = 85 (decimal) l or 54 (hex)
Table 45. PWM MINIMUM DUTY CYCLE REGISTERS
Register Description Default
0x64 PWM1 Minimum Duty Cycle 0x80 (50%)
0x65 PWM2 Minimum Duty Cycle 0x80 (50%)
0x66 PWM3 Minimum Duty Cycle 0x80 (50%)
Note on Fan Speed and PWM Duty Cycle
The PWM duty cycle does not directly correlate to fan
speed in RPM. Running a fan at 33% PWM duty cycle does
not equate to running the fan at 33% speed. Driving a fan at
33% PWM duty cycle actually runs the fan at closer to 50%
of its full speed. This is because fan speed in %RPM
generally relates to the square root of PWM duty cycle.
Given a PWM square wave as the drive signal, fan speed in
RPM approximates to:
% fanspeed +PWM Duty Cycle 10
Ǹ(eq. 5)
Step 5 − PWMMAX for PWM (Fan) Outputs
PWMMAX is the maximum duty cycle that each fan in the
system runs at under the automatic fan speed control loop.
For maximum system acoustic benefit, PWMMAX should be
as low as possible but should be capable of maintaining the
processor temperature limit at an acceptable level. If the
THERM temperature limit is exceeded, the fans are still
boosted to 100% for fail-safe cooling.
There is a PWMMAX limit for each fan channel. The
default value of this register is 0xFF and has no effect unless
it is programmed.
Figure 56. PWMMAX Determines Maximum PWM Duty
Cycle Below the THERM Temperature Limit
TMIN
100%
0%
TEMPERATURE
PWMMIN
PWMMAX
PWM DUTY CYCLE
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Programming the PWM Maximum Duty Cycle
Registers
The PWM maximum duty cycle registers are 8-bit
registers that allow the maximum PWM duty cycle for each
output to be configured anywhere from 0% to 100%. This
allows the maximum PWM duty cycle to be set in steps of
0.39%.
The value to be programmed into the PWM maximum
duty cycle register is given by:
Value (decimal) = PWMMAX/0.39
Example 1:
For a maximum PWM duty cycle of 50%,
Value (decimal) – 50/0.39 = 128 (decimal)
Value = 128 (decimal) or 80 (hex)
Example 2:
For a minimum PWM duty cycle of 75%,
Value (decimal) = 75/0.39 = 85 (decimal)
Value = 192 (decimal) or C0 (hex)
Table 46. PWM MAXIMUM DUTY CYCLE REGISTERS
Register Description Default
0x38 PWM1 Maximum Duty Cycle 0xFF (100%)
0x39 PWM2 Maximum Duty Cycle 0xFF (100%)
0x3A PWM3 Maximum Duty Cycle 0xFF (100%)
Step 6 − TRANGE for Temperature Channels
TRANGE is the range of temperature over which automatic
fan control occurs once the programmed TMIN temperature
has been exceeded. TRANGE is the temperature range between
PWMMIN and 100% PWM where the fan speed changes
linearly. Otherwise stated, it is the line drawn between the
TMIN/PWMMIN and the (TMIN +T
RANGE)/PWM100%
intersection points.
Figure 57. TRANGE Parameter Affects Cooling Slope
TMIN
100%
0%
TEMPERATURE
PWMMIN
PWM DUTY CYCLE
TRANGE
The TRANGE is determined by the following procedure:
1. Determine the maximum operating temperature for
that channel (for example, 70C).
2. Determine experimentally the fan speed (PWM
duty cycle value) that does not exceed the
temperature at the worst-case operating points. For
example, 70C is reached when the fans are
running at 50% PWM duty cycle.
3. Determine the slope of the required control loop to
meet these requirements.
4. Using the ADT7476 evaluation software, you can
graphically program and visualize this
functionality.
As PWMMIN is changed, the automatic fan control slope
changes.
Figure 58. Adjusting PWMMIN Changes the Automatic
Fan Control Slope
TMIN
100%
0%
30C
PWM DUTY CYCLE
33%
50%
As TRANGE is changed, the slope changes. As TRANGE
gets smaller, the fans reach 100% speed with a smaller
temperature change.
Figure 59. Increasing TRANGE Changes the AFC Slope
TMIN
100%
0%
30C
PWM DUTY CYCLE
10%
40C
45C
54C
TMIN−HYST
Figure 60. Changing PWMMAX Does Not Change the
AFC Slope
TRANGE
100%
0%
PWM DUTY CYCLE
10%
TMIN−HYST
MAX
PWM
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Selecting TRANGE
The TRANGE value can be selected for each temperature
channel: Remote 1, local, and Remote 2 temperature.
Bits [7:4] (TRANGE) of Register 0x5F to Register 0x61
define the TRANGE value for each temperature channel.
Table 47. SELECTING A TRANGE VALUE
Bits [7:4] (Note 1) TRANGE (5C)
0000 2
0001 2.5
0010 3.33
0011 4
0100 5
0101 6.67
0110 8
0111 10
1000 13.33
1001 16
1010 20
1011 26.67
1100 32 (Default)
1101 40
1110 53.33
1111 80
1. Register 0x5F configures Remote 1 TRANGE; Register 0x60
configures local TRANGE; Register 0x61 configures Remote 2
TRANGE.
Actual Changes in PWM Output
(Advanced Acoustics Settings)
While the automatic fan control algorithm describes the
general response of the PWM output, it is also necessary to
note that the enhance acoustics registers (0x62 and 0x63)
can be used to set/clamp the maximum rate of change of
PWM output for a given temperature zone. This means that
if TRANGE is programmed with an AFC slope that is quite
steep, a relatively small change in temperature could cause
a large change in PWM output and possibly an audible
change in fan speed, which can be noticeable/ bothersome
to end users.
Decreasing the speed the PWM output changes by
programming the smoothing on the appropriate temperature
channels (Register 0x62 and Register 0x63) changes how
fast the fan speed increases/decreases in the event of a
temperature spike. The PWM duty cycle increases slowly
until the PWM duty cycle reaches the appropriate duty cycle
as defined by the AFC curve.
Figure 61 shows PWM duty cycle vs. temperature for
each TRANGE setting. The lower graph shows how each
TRANGE setting affects fan speed vs. temperature. As can be
seen from the graph, the effect on fan speed is nonlinear.
Figure 61. TRANGE vs. Actual Fan Speed
(Not PWM Drive) Profile
25C
805C
53.35C
405C
325C
26.65C
205C
165C
13.35C
105C
85C
6.675C
55C
45C
3.335C
2.55C
25C
805C
53.35C
405C
325C
26.65C
205C
165C
13.35C
105C
85C
6.675C
55C
45C
3.335C
2.55C
TEMPERATURE ABOVE TMIN
0
PWM DUTY CYCLE (%)
020 40 60 80 100 120
10
20
30
40
50
60
70
80
90
100
20
30
40
50
60
70
80
TEMPERATURE ABOVE TMIN
FAN SPEED (% OF MAX)
020 40 60 80 100 120
10
90
100
0
The graphs in Figure 61 assume that the fan starts from
0% PWM duty cycle. Clearly, the minimum PWM duty
cycle, PWMMIN, needs to be factored in to see how the loop
actually performs in the system. Figure 62 shows how
TRANGE is affected when the PWMMIN value is set to 20%.
It can be seen that the fan actually runs at about 45% fan
speed when the temperature exceeds TMIN.
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Figure 62. TRANGE and % Fan Speed Slopes with
PWMMIN = 20%
25C
805C
53.35C
405C
325C
26.65C
205C
165C
13.35C
105C
85C
6.675C
55C
45C
3.335C
2.55C
25C
805C
53.35C
405C
325C
26.65C
205C
165C
13.35C
105C
85C
6.675C
55C
45C
3.335C
2.55C
TEMPERATURE ABOVE TMIN
0
PWM DUTY CYCLE (%)
020 40 60 80 100 120
10
20
30
40
50
60
70
80
90
100
TEMPERATURE ABOVE TMIN
0
FAN SPEED (% OF MAX)
020 40 60 80 100 120
10
20
30
40
50
60
70
80
90
100
Example: Determining TRANGE for Each Temperature
Channel
The following example shows how the different TMIN and
TRANGE settings can be applied to three different thermal
zones. In this example, the following TRANGE values apply:
TRANGE = 80C for ambient temperature
TRANGE = 53.33C for CPU temperature
TRANGE = 40C for VRM temperature
This example uses the mux configuration described in
Step 2 − Configuring the Mux with the ADT7476 connected
as shown in Figure 52. Both CPU temperature and VRM
temperature drive the CPU fan connected to PWM1.
Ambient temperature drives the front chassis fan and rear
chassis fan connected to PWM2 and PWM3. The front
chassis fan is configured to run at PWMMIN = 20%. The rear
chassis fan is configured to run at PWMMIN = 30%. The
CPU fan is configured to run at PWMMIN = 10%.
Note: The control range for 4-wire fans is much wider than
that of 3-wire fans. In many cases, 4-wire fans can start with
a PWM drive of as little as 20% or less. In extreme cases,
some 3-wire fans cannot run unless a PWM drive of 60% or
more is applied.
Figure 63. TRANGE and % Fan Speed Slopes for VRM,
Ambient, and CPU Temperature Channels
TEMPERATURE ABOVE TMIN
0
PWM DUTY CYCLE (%)
0
10 20 30 40 50 60 70 80 90 100
10
20
30
40
50
60
70
80
90
100 VRM TEMP.
CPU TEMPERATURE
AMBIENT TEMPERATURE
TEMPERATURE ABOVE TMIN
0
FAN SPEED (% MAX RPM)
0
10 20 30 40 50 60 70 80 90 100
10
20
30
40
50
60
70
80
90
100 VRM TEMP.
CPU TEMPERATURE
AMBIENT TEMPERATURE
Step 7 − TTHERM for Temperature Channels
TTHERM is the absolute maximum temperature allowed
on a temperature channel. Above this temperature, a
component such as the CPU or VRM can operate beyond its
safe operating limit. When the temperature measured
exceeds TTHERM, all fans are driven at 100% PWM duty
cycle (full speed) to provide critical system cooling.
The fans remain running at 100% until the temperature
drops below TTHERM minus hysteresis, where hysteresis is
the number programmed into the hysteresis registers (0x6D
and 0x6E). The default hysteresis value is 4C.
The TTHERM limit should be considered the maximum
worst-case operating temperature of the system. Because
exceeding any TTHERM limit runs all fans at 100%, it has
very negative acoustic effects. Ultimately, this limit should
be set up as a fail-safe, and users should ensure that it is not
exceeded under normal system operating conditions.
Note: TTHERM limits are nonmaskable and affect the fan
speed no matter how automatic fan control settings are
configured. This allows some flexibility, because a TRANGE
value can be selected based on its slope, while a hard limit
(such as 70C), can be programmed as TMAX (the
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temperature at which the fan reaches full speed) by setting
TTHERM to that limit (for example, 70C).
Table 48. THERM LIMIT REGISTERS
Register Description Default
0x6A Remote 1 THERM Limit 0x64 (100C)
0x6B Local THERM Limit 0x64 (100C)
0x6C Remote 2 THERM Limit 0x64 (100C)
THERM Hysteresis
THERM hysteresis on a particular channel is configured
via the hysteresis settings (Register 0x6D and
Register 0x6E). For example, setting hysteresis on the
Remote 1 channel also sets the hysteresis on Remote 1
THERM.
Hysteresis Registers
Register 0x6D, Remote 1, Local Temperature Hysteresis
[7:4], Remote 1 temperature hysteresis (4C default).
[3:0], Local temperature hysteresis (4C default).
Register 0x6E, Remote 2 Temperature Hysteresis
[7:4], Remote 2 temperature hysteresis (4C default).
Because each hysteresis setting is four bits, hysteresis
values are programmable from 1C to 15C. It is not
recommended to program hysteresis values to 0C, because
this disables hysteresis. In effect, this causes the fans to cycle
(during a THERM event) between normal speed and 100%
speed, or, while operating close to TMIN, between normal
speed and off, creating unsettling acoustic noise.
Figure 64. How TTHERM Relates to Automatic Fan Control
THERMAL CALIBRATION
REMOTE1 =
AMBIENT TEMP
100%
0%
TMIN TRANGE
PWM
MIN
MUX
S
TACHOMETER1
MEASUREMENT
S
S
PWM
MIN
PWM
MIN
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
LOCAL =
VRM TEMP
REMOTE2 =
CPU TEMP
TACHOMETER2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER3
AND 4
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
PWM1
PWM2
PWM3
TACH3
TACH2
TACH1
CPU FAN SINK
FRONT CHASSIS
REAR CHASSIS
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
CONFIG
PWM
GENERATOR
PWM
CONFIG
PWM
GENERATOR
TMIN
100%
0%
PWM DUTY CYCLE
TTHERM
TRANGE
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Step 8 − THYST for Temperature Channels
THYST is the amount of extra cooling a fan provides after
the temperature measured has dropped back below TMIN
before the fan turns off. The premise for temperature
hysteresis (THYST) is that without it, the fan would merely
chatter, or cycle on and off regularly, whenever the
temperature hovers around the TMIN setting.
The THYST value chosen determines the amount of time
needed for the system to cool down or heat up as the fan is
turning on and off. Values of hysteresis are programmable in
the range 1C to 15C. Larger values of THYST prevent the
fans from chattering on and off. The THYST default value is
set at 4C.
The THYST setting applies not only to the temperature
hysteresis for fan on/off, but the same setting is used for the
TTHERM hysteresis value, described in Step 6 − TRANGE for
Temperature Channels. Therefore, programming Register
0x6D and Register 0x6E sets the hysteresis for both fan
on/off and the THERM function.
In some applications, it is required that fans not turn off
below TMIN but remain running at PWMMIN. Bits [7:5] of
Enhance Acoustics Register 1 (0x62) allow the fans to be
turned off or to be kept spinning below TMIN. If the fans are
always on, the THYST value has no effect on the fan when the
temperature drops below TMIN.
THERM Hysteresis
Any hysteresis programmed via Register 0x6D and
Register 0x6E also applies hysteresis on the appropriate
THERM channel.
Figure 65. The THYST Value Applies to Fan On/Off Hysteresis and THERM Hysteresis
THERMAL CALIBRATION
REMOTE1 =
AMBIENT TEMP
100%
0%
TMIN TRANGE
PWM
MIN
MUX
S
TACHOMETER1
MEASUREMENT
S
S
PWM
MIN
PWM
MIN
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
THERMAL CALIBRATION
100%
0%
TMIN TRANGE
LOCAL =
VRM TEMP
REMOTE2 =
CPU TEMP
TACHOMETER2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER3
AND 4
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
PWM1
PWM2
PWM3
TACH3
TACH2
TACH1
CPU FAN SINK
FRONT CHASSIS
REAR CHASSIS
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
CONFIG
PWM
GENERATOR
PWM
CONFIG
PWM
GENERATOR
TMIN
100%
0%
PWM DUTY CYCLE
TTHERM
TRANGE
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Enhance Acoustics Register 1 (0x62)
Bit 7 (MIN3) = 0, PWM3 is off (0% PWM duty cycle) when
temperature is below TMIN − THYST.
Bit 7 (MIN3) = 1, PWM3 runs at PWM3 minimum duty cycle
below TMIN − THYST.
Bit 6 (MIN2) = 0, PWM2 is off (0% PWM duty cycle) when
temperature is below TMIN − THYST.
Bit 6 (MIN2) = 1, PWM2 runs at PWM2 minimum duty cycle
below TMIN − THYST.
Bit 5 (MIN1) = 0, PWM1 is off (0% PWM duty cycle) when
temperature is below TMIN − THYST.
Bit 5 (MIN1) = 1, PWM1 runs at PWM1 minimum duty cycle
below TMIN − THYST.
Configuration Register 6 (0x10)
[0] SLOW = 1, slows the ramp rate for PWM changes
associated with the Remote 1 temperature channel by a factor
of 4.
[1] SLOW = 1, slows the ramp rate for PWM changes
associated with the local temperature channel by a factor of 4.
[2] SLOW = 1, slows the ramp rate for PWM changes
associated with the Remote 2 temperature channel by a factor
of 4.
[7] ExtraSlow = 1, slows the ramp rate for all fans by a factor
of 39.2%.
The following sections list the ramp-up times when
enhanced acoustics is enabled for each temperature channel.
Enhance Acoustics Register 1 (0x62)
[2:0] ACOU selects the ramp rate for PWM outputs
associated with the Remote Temperature 1 input.
000 = 37.5 sec
001 = 18.8 sec
010 = 12.5 sec
011 = 7.5 sec
100 = 4.7 sec
101 = 3.1 sec
110 = 1.6 sec
111 = 0.8 sec
Enhance Acoustics Register 2 (0x63)
[2:0] ACOU3 selects the ramp rate for PWM outputs
associated with the local temperature channel.
000 = 37.5 sec
001 = 18.8 sec
010 = 12.5 sec
011 = 7.5 sec
100 = 4.7 sec
101 = 3.1 sec
110 = 1.6 sec
111 = 0.8 sec
[6:4] ACOU2 selects the ramp rate for PWM outputs
associated with the Remote Temperature 2 input.
000 = 37.5 sec
001 = 18.8 sec
010 = 12.5 sec
011 = 7.5 sec
100 = 4.7 sec
101 = 3.1 sec
110 = 1.6 sec
111 = 0.8 sec
When Bit 7 of Configuration Register 6 (0x10) = 1, the
above ramp rates change to the values below.
000 = 52.2 sec
001 = 26.1 sec
010 = 17.4 sec
011 = 10.4 sec
100 = 6.5 sec
101 = 4.4 sec
110 = 2.2 sec
111 = 1.1 sec
Setting the appropriate slow bit [2:0] of Configuration
Register 6 (0x10) slows the ramp rate further by a factor of 4.
Fan Presence Detect
This feature is used to determine if a 4-wire fan is directly
connected to a PWM output. This feature does not work for
3-wire fans. To detect whether a 4-wire fan is connected
directly to a PWM output, the following must be performed
in this order:
1. Drive the appropriate PWM outputs to 100%
duty cycle.
2. Set Bit 0 of Configuration Register 2 (0x73).
3. Wait 5 ms.
4. Program fans to run at a different speed if necessary.
5. Read the state of Bits [3:1] of Configuration
Register 2 (0x73). The state of these bits reflects
whether a 4-wire fan is directly connected to the
PWM output.
As the detection time only takes 5 ms, programming the
PWM outputs to 100% and then back to its normal speed is
not noticeable in most cases.
How Fan Presence Detect Works
4-wire fans typically have an internal pull up to 4.75 V
10%, which typically sources 5 mA. While the detection
cycle is on, an internal current sink is turned on, which sinks
current from the fan’s internal pullup. By driving some of the
current from the fan’s internal pullup (~100 mA) the logic
buffer switches to a defined logic state. If this state is high,
a fan is present; if the state is low, no fan is present.
Note: The PWM input voltage should be clamped to 3.3 V.
This ensures the PWM output is not pulled to a voltage
higher than the maximum allowable voltage on that pin
(5.5 V).
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Fan Sync
When two ADT7476s are used in a system, it is possible
to synchronize them so that one PWM channel from each
device can be effectively OR’ed together to create a PWM
output that reflects the maximum speed of the two OR’ed
PWMs. This OR’ed PWM can in turn be used to drive a
chassis fan.
Standby Mode
The ADT7476 has been specifically designed to respond
to the STBY supply. In computers that support S3 and S5
states, the core voltage of the processor is lowered in these
states. When monitoring THERM, the THERM timer
should be disabled during these states.
When the VCCP voltage drops below the VCCP low limit,
the following occurs:
1. Status Bit 1 (VCCP) in Interrupt Status Register 1
is set.
2. SMBALERT is generated, if enabled.
3. THERM monitoring is disabled. The THERM
timer should hold its value prior to the S3 or S5
state.
Once the core voltage, VCCP
, goes above the VCCP low
limit, everything is re-enabled and the system resumes
normal operation.
XNOR Tree Test Mode
The ADT7476 includes an XNOR tree test mode. This
mode is useful for in-circuit test equipment at board-level
testing. By applying stimulus to the pins included in the
XNOR tree, it is possible to detect opens, or shorts, on the
system board.
The XNOR tree test is invoked by setting Bit 0 (XEN) of
the XNOR Tree Test Enable Register (0x6F).
Figure 66 shows the signals that are exercised in the
XNOR tree test mode.
Figure 66. XNOR Tree Test
TACH1
TACH2
TACH3
TACH4
PWM2
PWM3 PWM1/XTO
VID4
VID3
VID2
VID1
VID0
Power-On Default
When the ADT7476 is powered up, monitoring is off by
default and the PWM outputs go to 100%. All necessary
registers then need to be configured via the SMBus for the
appropriate functions to operate.
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Register Tables
Table 49. ADT7476 REGISTERS
Addr R/W Desc Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
De-
fault
Lock -
able
0x10 R/W Configuration
Register 6
Extra
Slow
VCCP
Low
Master
En
SlaveEn THERM
in
Manual
SlowFan
Remote
1
SlowFan
Local
SlowFan
Remote
1
0x00 Yes
0x11 R/W Configuration
Register 7
RES RES RES RES RES RES RES Dis
THERM
Hys
0x00 Yes
0x20 R2.5 V
Measurement
9 8 7 6 5 4 3 2 0x00 −
0x21 R VCCP
Measurement
9 8 7 6 5 4 3 2 0x00 −
0x22 R VCC
Measurement
9 8 7 6 5 4 3 2 0x00 −
0x23 R5.0 V
Measurement
9 8 7 6 5 4 3 2 0x00 −
0x24 R12 V
Measurement
9 8 7 6 5 4 3 2 0x00 −
0x25 RRemote 1
Temperature
9 8 7 6 5 4 3 2 0x80 −
0x26 RLocal
Temperature
9 8 7 6 5 4 3 2 0x80 −
0x27 RRemote 2
Temperature
9 8 7 6 5 4 3 2 0x80 −
0x28 RTACH1 Low
Byte
7 6 5 4 3 2 1 0 0x00 −
0x29 RTACH1 High
Byte
15 14 13 12 11 10 9 8 0x00 −
0x2A RTACH2 Low
Byte
7 6 5 4 3 2 1 0 0x00 −
0x2B RTACH2 High
Byte
15 14 13 12 11 10 9 8 0x00 −
0x2C RTACH3 Low
Byte
7 6 5 4 3 2 1 0 0x00 −
0x2D RTACH3 High
Byte
15 14 13 12 11 10 9 8 0x00 −
0x2E RTACH4 Low
Byte
7 6 5 4 3 2 1 0 0x00 −
0x2F RTACH4 High
Byte
15 14 13 12 11 10 9 8 0x00 −
0x30 R/W PWM1 Current
Duty Cycle
7 6 5 4 3 2 1 0 0xFF −
0x31 R/W PWM2 Current
Duty Cycle
7 6 5 4 3 2 1 0 0xFF −
0x32 R/W PWM3 Current
Duty Cycle
7 6 5 4 3 2 1 0 0xFF −
0x38 R/W PWM1 Max
Duty Cycle
7 6 5 4 3 2 1 0 0xFF Yes
0x39 R/W PWM2 Max
Duty Cycle
7 6 5 4 3 2 1 0 0xFF Yes
0x3A R/W PWM3 Max
Duty Cycle
7 6 5 4 3 2 1 0 0xFF Yes
0x3D RDevice ID
Register
7 6 5 4 3 2 1 0 0x76 −
0x3E RCompany ID
Number
7 6 5 4 3 2 1 0 0x41 −
0x3F RRevision ID 7 6 5 4 3 2 1 0 0x6B −
0x40 R/W Configuration
Register 1
RES TODIS FSPDIS Vx1 FSPD RDY LOCK STRT 0x04 Yes
0x41 RInterrupt Status
Register 1
OOL R2T LT R1T 5.0 V VCC VCCP 2.5 V/
THERM
0x00 −
0x42 RInterrupt Status
Register 2
D2 D1 F4P FAN3 FAN2 FAN1 OVT 12 V/VC 0x00 −
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Table 49. ADT7476 REGISTERS (continued)
Addr
Lock -
able
De-
fault
Bit 0Bit 1Bit 2Bit 3Bit 4Bit 5Bit 6Bit 7DescR/W
0x43 R/W VID/GPIO VIDSEL THLD VID 5 VID4/
GPIO4
VID3/
GPIO3
VID2/
GPIO2
VID1/
GPIO1
VID 0/
GPIO 0
0x1F −
0x44 R/W 2.5 V Low Limit 7 6 5 4 3 2 1 0 0x00 −
0x45 R/W 2.5 V High Limit 7 6 5 4 3 2 1 0 0xFF −
0x46 R/W VCCP Low Limit 7 6 5 4 3 2 1 0 0x00 −
0x47 R/W VCCP High Limit 7 6 5 4 3 2 1 0 0xFF −
0x48 R/W VCC Low Limit 7 6 5 4 3 2 1 0 0x00 −
0x49 R/W VCC High Limit 7 6 5 4 3 2 1 0 0xFF −
0x4A R/W 5.0 V Low Limit 7 6 5 4 3 2 1 0 0x00 −
0x4B R/W 5.0 V High Limit 7 6 5 4 3 2 1 0 0xFF −
0x4C R/W 12 V Low Limit 7 6 5 4 3 2 1 0 0x00 −
0x4D R/W 12 V High Limit 7 6 5 4 3 2 1 0 0xFF −
0x4E R/W Remote 1 Temp
Low Limit
7 6 5 4 3 2 1 0 0x81 −
0x4F R/W Remote 1 Temp
High Limit
7 6 5 4 3 2 1 0 0x7F −
0x50 R/W Local Temp Low
Limit
7 6 5 4 3 2 1 0 0x81 −
0x51 R/W Local Temp High
Limit
7 6 5 4 3 2 1 0 0x7F −
0x52 R/W Remote 2 Temp
Low Limit
7 6 5 4 3 2 1 0 0x81 −
0x53 R/W Remote 2 Temp
High Limit
7 6 5 4 3 2 1 0 0x7F −
0x54 R/W TACH1 Min Low
Byte
7 6 5 4 3 2 1 0 0xFF −
0x55 R/W TACH1 Min High
Byte
15 14 13 12 11 10 9 8 0xFF −
0x56 R/W TACH2 Min Low
Byte
7 6 5 4 3 2 1 0 0xFF −
0x57 R/W TACH2 Min High
Byte
15 14 13 12 11 10 9 8 0xFF −
0x58 R/W TACH3 Min Low
Byte
7 6 5 4 3 2 1 0 0xFF −
0x59 R/W TACH3 Min High
Byte
15 14 13 12 11 10 9 8 0xFF −
0x5A R/W TACH4 Min Low
Byte
7 6 5 4 3 2 1 0 0xFF −
0x5B R/W TACH4 Min High
Byte
15 14 13 12 11 10 9 8 0xFF −
0x5C R/W PWM1
Configuration
BHVR BHVR BHVR INV RES SPIN SPIN SPIN 0x62 Yes
0x5D R/W PWM2
Configuration
BHVR BHVR BHVR INV RES SPIN SPIN SPIN 0x62 Yes
0x5E R/W PWM3
Configuration
BHVR BHVR BHVR INV RES SPIN SPIN SPIN 0x62 Yes
0x5F R/W Remote 1
TRANGE/PWM1
Frequency
RANGE RANGE RANGE RANGE HF/LF FREQ FREQ FREQ 0XC4 Yes
0x60 R/W Local
TRANGE/PWM2
Frequency
RANGE RANGE RANGE RANGE HF/LF FREQ FREQ FREQ 0XC4 Yes
0x61 R/W Remote 2
TRANGE/PWM3
Frequency
RANGE RANGE RANGE RANGE HF/LF FREQ FREQ FREQ 0XC4 Yes
0x62 R/W Enhance
Acoustics
Register 1
MIN3 MIN2 MIN1 SYNC EN1 ACOU ACOU ACOU 0X00 Yes
0x63 R/W Enhance
Acoustics
Register 2
EN2 ACOU2 ACOU2 ACOU2 EN3 ACOU3 ACOU3 ACOU3 0X00 Yes
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Table 49. ADT7476 REGISTERS (continued)
Addr
Lock -
able
De-
fault
Bit 0Bit 1Bit 2Bit 3Bit 4Bit 5Bit 6Bit 7DescR/W
0x64 R/W PWM1 Min Duty
Cycle
7 6 5 4 3 2 1 0 0X80 Yes
0x65 R/W PWM2 Min Duty
Cycle
7 6 5 4 3 2 1 0 0X80 Yes
0x66 R/W PWM3 Min Duty
Cycle
7 6 5 4 3 2 1 0 0X80 Yes
0x67 R/W Remote 1 Temp
TMIN
7 6 5 4 3 2 1 0 0X5A Yes
0x68 R/W Local Temp
TMIN
7 6 5 4 3 2 1 0 0X5A Yes
0x69 R/W Remote 2 Temp
TMIN
7 6 5 4 3 2 1 0 0X5A Yes
0x6A R/W Remote 1
THERM Limit
7 6 5 4 3 2 1 0 0X64 Yes
0x6B R/W Local THERM
Limit
7 6 5 4 3 2 1 0 0X64 Yes
0x6C R/W Remote 2
THERM Limit
7 6 5 4 3 2 1 0 0X64 Yes
0x6D R/W Remote 1 and
Local Temp/TMIN
Hysteresis
HYSR1 HYSR1 HYSR1 HYSR1 HYSL HYSL HYSL HYSL 0X44 Yes
0x6E R/W Remote 2
Tem p/ T MIN
Hysteresis
HYSR2 HYSR2 HYSR2 HYRS RES RES RES RES 0X40 Yes
0x6F R/W XNOR Tree Test
Enable
RES RES RES RES RES RES RES XEN 0X00 Yes
0x70 R/W Remote 1 Temp
Offset
7 6 5 4 3 2 1 0 0X00 Yes
0x71 R/W Local Temp
Offset
7 6 5 4 3 2 1 0 0X00 Yes
0x72 R/W Remote 2 Temp
Offset
7 6 5 4 3 2 1 0 0X00 Yes
0x73 R/W Configuration
Register 2
RES CONV ATTN AVG Fan3
Detect
Fan2
Detect
Fan1
Detect
Fan
PresDT
0X00 Yes
0x74 R/W Interrupt Mask
Register 1
OOL R2T LT R1T 5.0 V VCC VCCP 2.5 V/
THERM
0X00 −
0x75 R/W Interrupt Mask
Register 2
D2 D1 F4P FAN3 FAN2 FAN1 OVT 12 V/VC 0X00 −
0x76 R/W Extended
Resolution
Register 1
5.0 V 5.0 V VCC VCC VCCP VCCP 2.5 V 2.5 V 0X00 −
0x77 R/W Extended
Resolution
Register 2
TDM2 TDM2 LTMP LTMP TDM1 TDM1 12 V 12 V 0X00 −
0x78 R/W Configuration
Register 3
DC4 DC3 DC2 DC1 FAST BOOST THERM/
2.5 V
ALERT 0x00 Yes
0x79 RTHERM Timer
Status
TMR TMR TMR TMR TMR TMR TMR ASRT/T
MRO
0x00 −
0x7A R/W THERM Timer
Limit
LIMT LIMT LIMT LIMT LIMT LIMT LIMT LIMT 0x00 −
0x7B R/W TACH Pulses
per Revolution
FAN4 FAN4 FAN3 FAN3 FAN2 FAN2 FAN1 FAN1 0x55 −
0x7C R/W Configuration
Register 5
R2
THERM
Local
THERM
R1
THERM
VID/
GPIO
GPIO6P GPIO6D Te mp
Offset
2sC 0x01 Yes
0x7D R/W Configuration
Register 4
BpAtt
12 V
BpAtt
5.0 V
BpAtt
VCCP
BpAtt
2.5 V
Max
Speed
on
THERM
THERM
Disable
PIN14
FUNC
PIN14
FUNC
0x00 Yes
0x7E RTes t 1 DO NOT WRITE TO THESE REGISTERS 0x00 Yes
0x7F RTes t 2 DO NOT WRITE TO THESE REGISTERS 0x00 Yes
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Table 50. REGISTER 0X10 − CONFIGURATION REGISTER 6 (POWER-ON DEFAULT = 0X00) (Note 1 and 2)
Bit No. Mnemonic R/W Description
[0] SlowFan
Remote 1
R/W When this bit is set, Fan 1 smoothing times are multiplied x4 for Remote 1 temperature
channel (as defined in Register 0x62).
[1] SlowFan
Local
R/W When this bit is set, Fan 2 smoothing times are multiplied x4 for local temperature channel
(as defined in Register 0x63).
[2] SlowFan
Remote 2
R/W When this bit is set, Fan 3 smoothing times are multiplied x4 for Remote 2 temperature
channel (as defined in Register 0x63).
[3] THERM in
Manual
R/W When this bit is set, THERM is enabled in manual mode. (Note 1)
[4] SlaveEn R/W Setting this bit configures the ADT7476 as a slave for use in fan sync mode.
[5] MasterEn R/W Setting this bit configures the ADT7476 as a master for use in fan sync mode.
[6] VCCP Low R/W VCCPLow = 1. When the power is supplied from 3.3 V STANDBY and the core voltage (VCCP)
drops below its VCCP low limit value (Register 0x46), the following occurs:
Status Bit 1 in Interrupt Status Register 1 is set.
SMBALERT is generated, if enabled.
PROCHOT monitoring is disabled.
Everything is re-enabled once VCCP increases above the VCCP low limit.When VCCP increases
above the low limit:
PROCHOT monitoring is enabled.
Fans return to their programmed state after a spin-up cycle.
[7] ExtraSlow R/W When this bit is set, all fan smoothing times are increased by a further 39.2%
1. A THERM event always overrides any fan setting (even when fans are disabled).
2. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any subsequent attempts to write to this register fail.
Table 51. REGISTER 0X11 − CONFIGURATION REGISTER 7 (POWER-ON DEFAULT = 0X00) (Note 1)
Bit No. Mnemonic R/W Description
[0] DisTHERM
Hys
Read/Write Setting this bit to 1 disables THERM hysteresis.
[7:1] Reserved N/A Reserved. Do not write to these bits.
1. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any subsequent attempts to write to this register fail.
Table 52. VOLTAGE READING REGISTERS (POWER-ON DEFAULT = 0X00) (Note 1)
Register Address R/W Description
0x20 Read-only Reflects the voltage measurement at the 2.5 V input on Pin 22 (8 MSBs of reading).
0x21 Read-only Reflects the voltage measurement (Note 2) at the VCCP input on Pin 23 (8 MSBs of reading).
0x22 Read-only Reflects the voltage measurement (Note 3) at the VCC input on Pin 4 (8 MSBs of reading).
0x23 Read-only Reflects the voltage measurement at the 5.0 V input on Pin 20 (8 MSBs of reading).
0x24 Read-only Reflects the voltage measurement at the 12 V input on Pin 21 (8 MSBs of reading).
1. If the extended resolution bits of these readings are also being read, the extended resolution registers (Register 0x76, Register 0x77) must
be read first. Once the extended resolution registers have been read, the associated MSB reading registers are frozen until read. Both the
extended resolution registers and the MSB registers are frozen.
2. If VCCPLow (Bit 7 of 0x40) is set, VCCP can control the sleep state of the ADT7476.
3. VCC (Pin 4) is the supply voltage for the ADT7476.
Table 53. TEMPERATURE READING REGISTERS (POWER-ON DEFAULT = 0X80) (Note 1. 2 and 3)
Register Address R/W Description
0x25 Read-only Remote 1 temperature reading (Note 3 and 4) (8 MSBs of reading).
0x26 Read-only Local temperature reading (8 MSBs of reading).
0x27 Read-only Remote 2 temperature reading (Note 3 and 4) (8 MSBs of reading).
1. If the extended resolution bits of these readings are also being read, the extended resolution registers (Register 0x76, Register 0x77) must
be read first. Once the extended resolution registers have been read, all associated MSB reading registers are frozen until read. Both the
extended resolution registers and the MSB registers are frozen.
2. These temperature readings can be in twos complement or Offset 64 format; this interpretation is determined by Bit 0 of Configuration
Register 5 (0x7C).
3. In twos complement mode, a temperature reading of −128C (0x80) indicates a diode fault (open or short) on that channel.
4. In Offset 64 mode, a temperature reading of −64C (0x00) indicates a diode fault (open or short) on that channel.
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Table 54. FAN TACHOMETER READING REGISTERS (POWER-ON DEFAULT = 0X00) (Note 1)
Register Address R/W Description
0x28 Read-only TACH1 Low Byte
0x29 Read-only TACH1 High Byte
0x2A Read-only TACH2 Low Byte
0x2B Read-only TACH2 High Byte
0x2C Read-only TACH3 Low Byte
0x2D Read-only TACH3 High Byte
0x2E Read-only TACH4 Low Byte
0x2F Read-only TACH4 High Byte
1. These registers count the number of 11.11 ms periods (based on an internal 90 kHz clock) that occur between a number of consecutive fan TACH
pulses (default = 2). The number of TACH pulses used to count can be changed using the TACH Pulses per Revolution register (Register 0x7B).
This allows the fan speed to be accurately measured. Because a valid fan tachometer reading requires that two bytes be read, the low byte must
be read first. Both the low and high bytes are then frozen until read. At power-on, these registers contain 0x0000 until the first valid fan TACH
measurement is read into these registers. This prevents false interrupts from occurring while the fans are spinning up. A count of 0xFFFF
indicates that a fan is one of the following: stalled or blocked (object jamming the fan), failed (internal circuitry destroyed), or not populated. (The
ADT7476 expects to see a fan connected to each TACH. If a fan is not connected to that TACH, its TACH minimum high and low bytes should
be set to 0xFFFF.) An alternate function, for example, is TACH4 reconfigured as the THERM pin.
Table 55. CURRENT PWM DUTY CYCLE REGISTERS (POWER-ON DEFAULT = 0XFF) (Note 1)
Register Address R/W Description
0x30 R/W PWM1 Current Duty Cycle (0% to 100% Duty Cycle = 0x00 to 0xFF)
0x31 R/W PWM2 Current Duty Cycle (0% to 100% Duty Cycle = 0x00 to 0xFF)
0x32 R/W PWM3 Current Duty Cycle (0% to 100% Duty Cycle = 0x00 to 0xFF)
1. These registers reflect the PWM duty cycle driving each fan at any given time. When in automatic fan speed control mode, the ADT7476
reports the PWM duty cycles back through these registers. The PWM duty cycle values vary according to temperature in automatic fan speed
control mode. During fan startup, these registers report back 0x00. In manual mode, the PWM duty cycle outputs can be set to any duty cycle
value by writing to these registers.
Table 56. PWM MAXIMUM DUTY CYCLE (POWER-ON DEFAULT = 0XFF) (Note 1 and 2)
Register Address R/W Description
0x38 R/W Maximum Duty Cycle for PWM1 Output, Default = 100% (0xFF)
0x39 R/W Maximum Duty Cycle for PWM2 Output, Default = 100% (0xFF)
0x3A R/W Maximum Duty Cycle for PWM3 Output, Default = 100% (0xFF)
1. These registers set the maximum PWM duty cycle of the PWM output.
2. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any subsequent attempts to write to this register fail.
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Table 57. REGISTER 0X40 − CONFIGURATION REGISTER 1 (POWER-ON DEFAULT = 0X04)
Bit No. Mnemonic R/W Description
[0] STRT
(Notes 1, 2)
R/W Logic 1 enables monitoring and PWM control outputs based on the limit settings
programmed.
Logic 0 disables monitoring and PWM control is based on the default powerup limit settings.
Note that the limit values programmed are preserved even if a Logic 0 is written to this bit
and the default settings are enabled. This bit does not become locked once Bit 1 (LOCK bit)
has been set.
[1] LOCK Write Once Logic 1 locks all limit values to their current settings. Once this bit is set, all lockable registers
become read-only and cannot be modified until the ADT7476 is powered down and powered
up again. This prevents rogue programs such as viruses from modifying critical system limit
settings. (Lockable.)
[2] RDY Read-only This bit is set to 1 by the ADT7476 to indicate that the device is fully powered-up and ready
to begin system monitoring.
[3] FSPD R/W When set to 1, this bit runs all fans at max speed as programmed in the max PWM current
duty cycle registers (0x30 to 0x32). Power-on default = 0. This bit is not locked at any time.
[4] Vx1 R/W BIOS should set this bit to a 1 when the ADT7476 is configured to measure current from an
ADOPT VRM controller and to measure the CPU’s core voltage. This bit allows monitoring
software to display CPU watts usage. (Lockable.)
[5] FSPDIS R/W Logic 1 disables fan spin-up for two TACH pulses. Instead, the PWM outputs go high for the
entire fan spin-up timeout selected.
[6] TODIS R/W When this bit is set to 1, the SMBus timeout feature is enabled. In this state, if at any point
during an SMBus transaction involving the ADT7476 activity ceases for more than 35 ms, the
ADT7476 assumes the bus is locked and releases the bus. This allows the ADT7476 to be
used with SMBus controllers that cannot handle SMBus timeouts. (Lockable.)
[7] Reserved N/A Reserved. Do not write to this bit.
1. Bit 0 (STRT) of Configuration Register 1 (0x40) remains writable after lock bit is set.
2. When monitoring (STRT) is disabled, PWM outputs always go to 100% for thermal protection.
Table 58. REGISTER 0X41 − INTERRUPT STATUS REGISTER 1 (POWER-ON DEFAULT = 0X00)
Bit No. Mnemonic R/W Description
[0] 2.5 V/
THERM
Read-only 2.5 V = 1 indicates that the 2.5 V high or low limit has been exceeded. This bit is cleared on a
read of the status register only if the error condition has subsided. If Pin 22 is configured as
THERM, this bit is asserted when the timer limit has been exceeded.
[1] VCCP Read-only VCCP = 1 indicates that the VCCP high or low limit has been exceeded. This bit is cleared on
a read of the status register only if the error condition has subsided.
[2] VCC Read-only VCC = 1 indicates that the VCC high or low limit has been exceeded. This bit is cleared on a
read of the status register only if the error condition has subsided.
[3] 5.0 V Read-only A 1 indicates that the 5.0 V high or low limit has been exceeded. This bit is cleared on a read
of the status register only if the error condition has subsided.
[4] R1T Read-only R1T = 1 indicates that the Remote 1 low or high temperature has been exceeded. This bit is
cleared on a read of the status register only if the error condition has subsided.
[5] LT Read-only LT = 1 indicates that the local low or high temperature has been exceeded. This bit is cleared
on a read of the status register only if the error condition has subsided.
[6] R2T Read-only R2T = 1 indicates that the Remote 2 low or high temperature has been exceeded. This bit is
cleared on a read of the status register only if the error condition has subsided.
[7] OOL Read-only OOL = 1 indicates that an out-of-limit event has been latched in Interrupt Status Register 2.
This bit is a logical OR of all status bits in Interrupt Status Register 2. Software can test this
bit in isolation to determine whether any of the voltage, temperature, or fan speed readings
represented by Interrupt Status Register 2 are out-of-limit, which eliminates the need to read
Interrupt Status Register 2 during every interrupt or polling cycle.
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Table 59. REGISTER 0X42 − INTERRUPT STATUS REGISTER 2 (POWER-ON DEFAULT = 0X00)
Bit No. Mnemonic R/W Description
[0] 12 V/VC Read-only A 1 indicates that the 12 V high or low limit has been exceeded. This bit is cleared on a read
of the status register only if the error condition has subsided. If Pin 21 is configured as VID5,
this bit is the VID change bit. This bit is set when the levels on VID0 to VID5 are different
than they were 11 ms previously. This pin can be used to generate an SMBALERT whenever
the VID code changes.
[1] OVT Read-only OVT = 1 indicates that one of the THERM overtemperature limits has been exceeded. This
bit is cleared on a read of the status register when the temperature drops below
THERM −THYST
.
[2] FAN1 Read-only FAN1 = 1 indicates that Fan 1 has dropped below minimum speed or has stalled. This bit is
not set when the PWM1 output is off.
[3] FAN2 Read-only FAN2 = 1 indicates that Fan 2 has dropped below minimum speed or has stalled. This bit is
not set when the PWM2 output is off.
[4] FAN3 Read-only FAN3 = 1 indicates that Fan 3 has dropped below minimum speed or has stalled. This bit is
not set when the PWM3 output is off.
[5] F4P Read-only When Pin 14 is programmed as a TACH4 input, F4P = 1 indicates that Fan 4 has dropped
below minimum speed or has stalled. This bit is not set when the PWM3 output is off.
R/W When Pin 14 is programmed as the GPIO6 output, writing to this bit determines the logic
output of GPIO6. When GPIO6 is programmed as an input, this bit reflects the value read by
GPIO6.
Read-only If Pin 14 is configured as the THERM timer input for THERM monitoring, then this bit is set
when the THERM assertion time exceeds the limit programmed in the THERM timer limit
register (0x7A).
[6] D1 Read-only D1 = 1 indicates either an open or short circuit on the Thermal Diode 1 inputs.
[7] D2 Read-only D2 = 1 indicates either an open or short circuit on the Thermal Diode 2 inputs.
Table 60. REGISTER 0X43 − VID/GPIO REGISTER (POWER-ON DEFAULT = 0X1F)
Bit No. Mnemonic R/W Description
[4:0] VID[4:0]/
GPIO[4:0]
R/W The VID[4:0] inputs from the CPU indicate the expected processor core voltage. On
powerup, these bits reflect the state of the VID pins, even if monitoring is not enabled. When
Bit 4 of Configuration Register 5 (0x7C) = 1, these bits become general-purpose outputs. The
state of these bits then reflects the state of the appropriate GPIO pin.
[5] VID5 R/W Reads VID5 from the CPU when Bit 7 = 1. If Bit 7 = 0, the VID5 bit always reads back 0
(power-on default).
[6] THLD R/W Selects the input switching threshold for the VID inputs.
THLD = 0 selects a threshold of 1 V (VOL < 0.8 V, VIH > 1.7 V).
THLD = 1 lowers the switching threshold to 0.6 V (VOL < 0.4 V, VIH > 0.8 V).
[7] VIDSEL R/W VIDSEL = 0 configures Pin 21 as the 12 V measurement input (default).
Table 61. VOLTAGE LIMIT REGISTERS (Note 1)
Register Address R/W Description (Note 2) Power-On Default
0x44 R/W 2.5 V Low Limit 0x00
0x45 R/W 2.5 V High Limit 0xFF
0x46 R/W VCCP Low Limit 0x00
0x47 R/W VCCP High Limit 0xFF
0x48 R/W VCC Low Limit 0x00
0x49 R/W VCC High Limit 0xFF
0x4A R/W 5.0 V Low Limit 0x00
0x4B R/W 5.0 V High Limit 0xFF
0x4C R/W 12 V Low Limit 0x00
0x4D R/W 12 V High Limit 0xFF
1. Setting the Configuration Register 1 Lock bit has no effect on these registers.
2. High limits: An interrupt is generated when a value exceeds its high limit (> comparison). Low limits: An interrupt is generated when a value
is equal to or below its low limit (comparison).
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Table 62. TEMPERATURE LIMIT REGISTERS (Note 1)
Register Address R/W Description (Note 2) Power-On Default
0x4E R/W Remote 1 Temperature Low Limit 0x81
0x4F R/W Remote 1 Temperature High Limit 0x7F
0x50 R/W Local Temperature Low Limit 0x81
0x51 R/W Local Temperature High Limit 0x7F
0x52 R/W Remote 2 Temperature Low Limit 0x81
0x53 R/W Remote 2 Temperature High Limit 0x7F
1. Exceeding any of these temperature limits by 1C causes the appropriate status bit to be set in the interrupt status register. Setting the
Configuration Register 1 Lock bit has no effect on these registers.
2. High limits: An interrupt is generated when a value exceeds its high limit (> comparison). Low limits: An interrupt is generated when a value
is equal to or below its low limit (comparison).
Table 63. FAN TACH LIMIT REGISTERS (Note 1)
Register Address R/W Description Power-On Default
0x54 R/W TACH1 Minimum Low Byte 0xFF
0x55 R/W TACH1 Minimum High Byte/Single-channel ADC Channel Select 0xFF
0x56 R/W TACH2 Minimum Low Byte 0xFF
0x57 R/W TACH2 Minimum High Byte 0xFF
0x58 R/W TACH3 Minimum Low Byte 0xFF
0x59 R/W TACH3 Minimum High Byte 0xFF
0x5A R/W TACH4 Minimum Low Byte 0xFF
0x5B R/W TACH4 Minimum High Byte 0xFF
1. Exceeding any of the TACH limit registers by 1 indicates that the fan is running too slowly or has stalled. The appropriate status bit is set
in Interrupt Status Register 2 to indicate the fan failure. Setting the Configuration Register 1 Lock bit has no effect on these registers.
Table 64. REGISTER 0X55 − TACH1 MINIMUM HIGH BYTE (POWER-ON DEFAULT = 0XFF)
Bit No. Mnemonic R/W Description
[4:0] Reserved Read-only When Bit 6 of Configuration 2 Register (0x73) is set (single-channel ADC mode), these bits
are reserved. Otherwise, these bits represent Bits [4:0] of the TACH1 minimum high byte.
[7:5] SCADC R/W When Bit 6 of Configuration 2 Register (0x73) is set (single-channel ADC mode), these bits
are used to select the only channel from which the ADC will take measurements. Otherwise,
these bits represent Bits [7:5] of the TACH1 minimum high byte.
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Table 65. PWM CONFIGURATION REGISTERS (Note 1)
Register Address R/W Description Power-On Default
0x5C R/W PWM1 Configuration 0x62
0x5D R/W PWM2 Configuration 0x62
0x5E R/W PWM3 Configuration 0x62
Bit No. Name R/W Description
[2:0] SPIN R/W These bits control the startup timeout for PWMx. The PWM output stays high until two valid
TACH rising edges are seen from the fan. If there is not a valid TACH signal during the fan
TACH measurement directly after the fan startup timeout period, the TACH measurement
reads 0xFFFF and Interrupt Status Register 2 reflects the fan fault. If the TACH minimum
high and low bytes contain 0xFFFF or 0x0000, the Interrupt Status Register 2 bit is not set,
even if the fan has not started.
000 = No Startup Timeout
001 = 100 ms
010 = 250 ms (Default)
011 = 400 ms
100 = 667 ms
101 = 1 sec
110 = 2 sec
111 = 4 sec
[3] RES N/A Reserved. Do not write to this bit.
[4] INV R/W This bit inverts the PWM output. The default is 0, which corresponds to a logic high output for
100% duty cycle. Setting this bit to 1 inverts the PWM output, so 100% duty cycle
corresponds to a logic low output.
[7:5] BHVR R/W These bits assign each fan to a particular temperature sensor for localized cooling.
000 = Remote 1 temperature controls PWMx (automatic fan control mode).
001 = Local temperature controls PWMx (automatic fan control mode).
010 = Remote 2 temperature controls PWMx (automatic fan control mode).
011 = PWMx runs full speed (default).
100 = PWMx disabled.
101 = Fastest speed calculated by local and Remote 2 temperature controls PWMx.
110 = Fastest speed calculated by all three temperature channel controls PWMx.
111 = Manual mode. PWM current duty cycle registers (0x30 to 0x32) become writable.
1. These registers become read-only when the Configuration Register 1 Lock bit is set to 1. Any subsequent attempts to write to these registers fail.
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Table 66. TRANGE/PWM FREQUENCY REGISTERS (Note 1)
Register Address R/W Description Power-On Default
0x5F R/W Remote 1 TRANGE/PWM1 Frequency 0xC4
0x60 R/W Local TRANGE/PWM2 Frequency 0xC4
0x61 R/W Remote 2 TRANGE/PWM3 Frequency 0xC4
Bit No. Name R/W Description
[2:0] FREQ R/W These bits control the PWMx frequency (only apply when PWM channel is in low frequency
mode).
000 = 11.0 Hz
001 = 14.7 Hz
010 = 22.1 Hz
011 = 29.4 Hz
100 = 35.3 Hz (Default)
101 = 44.1 Hz
110 = 58.8 Hz
111 = 88.2 Hz
[3] HF/LR R/W HF/LF = 1, High frequency PWM mode is enabled for PWMx.
HF/LF = 0, Low frequency PWM mode is enabled for PWMx.
[7:4] RANGE R/W These bits determine the PWM duty cycle vs. the temperature range for automatic fan
control.
0000 = 2C
0001 = 2.5C
0010 = 3.33C
0011 = 4C
0100 = 5C
0101 = 6.67C
0110 = 8C
0111 = 10C
1000 = 13.33C
1001 = 16C
1010 = 20C
1011 = 26.67C
1100 = 32C (Default)
1101 = 40C
1110 = 53.33C
1111 = 80C
1. These registers become read-only when the Configuration Register 1 Lock bit is set. Any further attempts to write to these registers have
no effect.
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Table 67. REGISTER 0X62 − ENHANCED ACOUSTICS REGISTER 1 (POWER-ON DEFAULT = 0X00) (Note 1)
Bit No. Mnemonic R/W Description
[2:0] ACOU
(Note 2)
R/W Assuming that PWMx is associated with the Remote 1 temperature channel, these bits define
the maximum rate of change of the PWMx output for Remote 1 temperature related changes.
Instead of the fan speed jumping instantaneously to its newly determined speed, it ramps
gracefully at the rate determined by these bits. This feature ultimately enhances the acoustics of
the fan.
When Bit 7 of Configuration Register 6 (0x10) is 0
Time Slot Increase Time for 0% to 100%
000 = 1 37.5 sec
001 = 2 18.8 sec
010 = 3 12.5 sec
011 = 4 7.5 sec
100 = 8 4.7 sec
101 = 12 3.1 sec
110 = 24 1.6 sec
111 = 48 0.8 sec
When Bit 7 of Configuration Register 6 (0x10) is 1
Time Slot Increase Time for 0% to 100%
000 = 1 52.2 sec
001 = 2 26.1 sec
010 = 3 17.4 sec
011 = 4 10.4 sec
100 = 8 6.5 sec
101 = 12 4.4 sec
110 = 24 2.2 sec
111 = 48 1.1 sec
[3] EN1 R/W When this bit is 1, smoothing is enabled on Remote 1 temperature channel.
[4] SYNC R/W SYNC = 1 synchronizes fan speed measurements on TACH2, TACH3, and TACH4 to PWM3.
This allows up to three fans to be driven from PWM3 output and their speeds to be measured.
SYNC = 0 synchronizes only TACH3 and TACH4 to PWM3 output.
[5] MIN1 R/W When the ADT7476 is in automatic fan control mode, this bit defines whether PWM1 is off
(0% duty cycle) or at PWM1 minimum duty cycle when the controlling temperature is below
its TMIN − hysteresis value.
0 = 0% duty cycle below TMIN – hysteresis.
1 = PWM1 minimum duty cycle below TMIN – hysteresis.
[6] MIN2 R/W When the ADT7476 is in automatic fan speed control mode, this bit defines whether PWM2 is
off (0% duty cycle) or at PWM2 minimum duty cycle when the controlling temperature is
below its TMIN − hysteresis value.
0 = 0% duty cycle below TMIN – hysteresis.
1 = PWM2 minimum duty cycle below TMIN – hysteresis.
[7] MIN3 R/W When the ADT7476 is in automatic fan speed control mode, this bit defines whether PWM3 is
off (0% duty cycle) or at PWM3 minimum duty cycle when the controlling temperature is
below its TMIN – hysteresis value.
0 = 0% duty cycle below TMIN – hysteresis.
1 = PWM3 minimum duty cycle below TMIN – hysteresis.
1. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to this register have no effect.
2. Setting the relevant bit of Configuration Register 6 (0x10, [2:0]) further decreases these ramp rates by a factor of 4.
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Table 68. REGISTER 0X63 − ENHANCED ACOUSTICS REGISTER 2 (POWER-ON DEFAULT = 0X00) (Note 1)
Bit No. Mnemonic R/W Description
[2:0] ACOU3 R/W Assuming that PWMx is associated with the local temperature channel, these bits define the
maximum rate of change of the PWMx output for local temperature related changes. Instead of
the fan speed jumping instantaneously to its newly determined speed, it ramps gracefully at the
rate determined by these bits. This feature ultimately enhances the acoustics of the fan.
When Bit 7 of Configuration Register 6 (0x10) is 0
Time Slot Increase Time for 0% to 100%
000 = 1 37.5 sec
001 = 2 18.8 sec
010 = 3 12.5 sec
011 = 4 7.5 sec
100 = 8 4.7 sec
101 = 12 3.1 sec
110 = 24 1.6 sec
111 = 48 0.8 sec
When Bit 7 of Configuration Register 6 (0x10) is 1
Time Slot Increase Time for 0% to 100%
000 = 1 52.2 sec
001 = 2 26.1 sec
010 = 3 17.4 sec
011 = 4 10.4 sec
100 = 8 6.5 sec
101 = 12 4.4 sec
110 = 24 2.2 sec
111 = 48 1.1 sec
[3] EN3 R/W When this bit is 1, smoothing is enabled on the local temperature channel.
[6:4] ACOU2 R/W Assuming that PWMx is associated with the Remote 2 temperature channel, these bits define
the maximum rate of change of the PWMx output for Remote 2 Temperature related changes.
Instead of the fan speed jumping instantaneously to its newly determined speed, it ramps
gracefully at the rate determined by these bits. This feature ultimately enhances the acoustics of
the fan.
When Bit 7 of Configuration Register 6 (0x10) is 0
Time Slot Increase Time for 0% to 100%
000 = 1 37.5 sec
001 = 2 18.8 sec
010 = 3 12.5 sec
011 = 4 7.5 sec
100 = 8 4.7 sec
101 = 12 3.1 sec
110 = 24 1.6 sec
111 = 48 0.8 sec
When Bit 7 of Configuration Register 6 (0x10) is 1
Time Slot Increase Time for 0% to 100%
000 = 1 52.2 sec
001 = 2 26.1 sec
010 = 3 17.4 sec
011 = 4 10.4 sec
100 = 8 6.5 sec
101 = 12 4.4 sec
110 = 24 2.2 sec
111 = 48 1.1 sec
[7] EN2 R/W When this bit is 1, smoothing is enabled on the Remote 2 temperature channel.
1. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to this register have no effect.
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Table 69. PWM MINIMUM DUTY CYCLE REGISTERS (Note 1)
Register Address R/W Description Power-On Default
0x64 R/W PWM1 Minimum Duty Cycle 0x80 (50% Duty Cycle)
0x65 R/W PWM2 Minimum Duty Cycle 0x80 (50% Duty Cycle)
0x66 R/W PWM3 Minimum Duty Cycle 0x80 (50% Duty Cycle)
Bit No. Name R/W Description
[7:0] PWM Duty
Cycle
R/W These bits define the PWMMIN duty cycle for PWMx.
0x00 = 0% Duty Cycle (Fan Off)
0x40 = 25% Duty Cycle
0x80 = 50% Duty Cycle
0xFF = 100% Duty Cycle (Fan Full Speed)
1. These registers become read-only when the ADT7476 is in automatic fan control mode.
Table 70. TMIN REGISTERS (Note 1 and 2)
Register Address R/W Description Power-On Default
0x67 R/W Remote 1 Temperature TMIN 0x5A (90C)
0x68 R/W Local Temperature TMIN 0x5A (90C)
0x69 R/W Remote 2 Temperature TMIN 0x5A (90C)
1. These are the TMIN registers for each temperature channel. When the temperature measured exceeds TMIN, the appropriate fan runs at
minimum speed and increases with temperature according to TRANGE.
2. These registers become read-only when the Configuration Register 1 Lock bit is set. Any further attempts to write to these registers have no
effect.
Table 71. THERM LIMIT REGISTERS (Note 1 and 2)
Register Address R/W Description Power-On Default
0x6A R/W Remote 1 THERM Temperature Limit 0x64 (100C)
0x6B R/W Local THERM Temperature Limit 0x64 (100C)
0x6C R/W Remote 2 THERM Temperature Limit 0x64 (100C)
1. If any temperature measured exceeds its THERM limit, all PWM outputs drive their fans at 100% duty cycle. This is a fail-safe mechanism
incorporated to cool the system in the event of a critical overtemperature. It also ensures some level of cooling in the event that software
or hardware locks up. If set to 0x80, this feature is disabled. The PWM output remains at 100% until the temperature drops below
THERM limit −hysteresis. If the THERM pin is programmed as an output, exceeding these limits by 0.25C can cause the THERM pin to
assert low as an output.
2. These registers become read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to these registers have
no effect.
Table 72. TEMPERATURE/TMIN HYSTERESIS REGISTERS (Note 1 and 2)
Register Address R/W Description Power-On Default
0x6D R/W Remote 1 and local temperature hysteresis. 0x44
[3:0] HYSL Local temperature hysteresis. 0C to 15C of hysteresis can be
applied to the local temperature AFC control loops.
[7:4] HYSR1 Remote 1 temperature hysteresis. 0C to 15C of hysteresis can
be applied to the Remote 1 temperature AFC control loops.
0x6E R/W Remote 2 temperature hysteresis. 0x40
0x6E HYSR2 Local temperature hysteresis. 0C to 15C of hysteresis can be
applied to the local temperature AFC control loops.
1. Each 4-bit value controls the amount of temperature hysteresis applied to a particular temperature channel. Once the temperature for that
channel falls below its TMIN value, the fan remains running at PWMMIN duty cycle until the temperature = TMIN – hysteresis. Up to 15C of
hysteresis can be assigned to any temperature channel. The hysteresis value chosen also applies to that temperature channel if its THERM
limit is exceeded. The PWM output being controlled goes to 100% if the THERM limit is exceeded and remains at 100% until the temperature
drops below THERM – hysteresis. For acoustic reasons, it is recommended that the hysteresis value not be programmed less than 4C.
Setting the hysteresis value lower than 4C causes the fan to switch on and off regularly when the temperature is close to TMIN.
2. These registers become read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to these registers have
no effect.
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Table 73. XNOR TREE TEST ENABLE (Note 1)
Register Address R/W Description Power-On Default
0x6F R/W XNOR tree test enable register. 0x00
[0] XEN If the XEN bit is set to 1, the device enters the XNOR tree test mode.
Clearing the bit removes the device from the XNOR tree test mode.
[7:1] Reserved Unused. Do not write to these bits.
1. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to this register have no effect.
Table 74. REMOTE 1 TEMPERATURE OFFSET (Note 1)
Register Address R/W Description Power-On Default
0x70 R/W Remote 1 temperature offset. 0x00
[7:0] R/W Allows a temperature offset to be automatically applied to the Remote
Temperature 1 channel measurement. Bit 1 of Configuration
Register 5 (0x7C) determines the range and resolution of this register.
1. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to this register have no effect.
Table 75. LOCAL TEMPERATURE OFFSET (Note 1)
Register Address R/W Description Power-On Default
0x71 R/W Local temperature offset. 0x00
[7:0] R/W Allows a temperature offset to be automatically applied to the local
temperature measurement. Bit 1 of Configuration Register 5 (0x7C)
determines the range and resolution of this register.
1. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to this register have no effect.
Table 76. REMOTE 2 TEMPERATURE OFFSET (Note 1)
Register Address R/W Description Power-On Default
0x72 R/W Remote 2 temperature offset. 0x00
[7:0] R/W Allows a temperature offset to be automatically applied to the Remote
Temperature 2 channel measurement. Bit 1 of Configuration
Register 5 (0x7C) determines the range and resolution of this register.
1. This register becomes read-only when the Configuration Register 1 lock bit is set to 1. Any further attempts to write to this register have no effect.
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Table 77. REGISTER 0X73 − CONFIGURATION REGISTER 2 (POWER-ON DEFAULT = 0X00) (Note 1)
Bit No. Mnemonic R/W Description
[0] FanPresDT R/W When FanPresenceDT = 1, the state of Bits [3:1] of 0x73 reflects the presence of a 4-wire fan
on the appropriate TACH channel.
[1] Fan1Detect Read-only Fan1Detect = 1 indicates that a 4-wire fan is connected to the TACH1 input.
[2] Fan2Detect Read-only Fan2Detect = 1 indicates that a 4-wire fan is connected to the TACH2 input.
[3] Fan3Detect Read-only Fan3Detect = 1 indicates that a 4-wire fan is connected to the TACH3 input.
[4] AVG R/W AVG = 1 indicates that averaging on the temperature and voltage measurements is turned
off. This allows measurements on each channel to be made much faster (x16).
[5] ATTN R/W ATTN = 1 indicates that the ADT7476 removes the attenuators from the +2.5 VIN, VCCP
,
+5.0 VIN, and +12 VIN inputs. These inputs can be used for other functions such as connecting
up external sensors. It is also possible to remove attenuators from individual channels using
Bits [7:4] of Configuration Register 4 (0x7D).
[6] CONV R/W CONV = 1 indicates that the ADT7476 is put into a single-channel ADC conversion mode. In
this mode, the ADT7476 can be made to read continuously from one input only, for example,
Remote 1 temperature. The appropriate ADC channel is selected by writing to Bits [7:5] of
TACH1 minimum high byte register (0x55).
Bits [7:5], Register 0x55
000 2.5 V
001 VCCP
010 VCC (3.3 V)
011 5.0 V
100 12 V
101 Remote 1 Temperature
110 Local Temperature
111 Remote 2 Temperature
[7] Res This bit is reserved and should not be changed.
1. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to this register have no effect.
Table 78. REGISTER 0X74 − INTERRUPT MASK REGISTER 1 (POWER-ON DEFAULT [7:0] = 0X00)
Bit No. Mnemonic R/W Description
[0] 2.5 V/
THERM
R/W 2.5 V/THERM = 1 masks SMBALERT for out-of-limit conditions on the 2.5 V/THERM timer
channel.
[1] VCCP R/W VCCP = 1 masks SMBALERT for out-of-limit conditions on the VCCP channel.
[2] VCC R/W VCC = 1 masks SMBALERT for out-of-limit conditions on the VCC channel.
[3] 5.0 V R/W 5.0 V = 1 masks SMBALERT for out-of-limit conditions on the 5.0 V channel.
[4] R1T R/W R1T = 1 masks SMBALERT for out-of-limit conditions on the Remote 1 temperature channel.
[5] LT R/W LT = 1 masks SMBALERT for out-of-limit conditions on the local temperature channel.
[6] R2T R/W R2T = 1 masks SMBALERT for out-of-limit conditions on the Remote 2 temperature channel.
[7] OOL R/W OOL = 0 when one or more alerts are generated in Interrupt Status Register 2, assuming all
the mask bits in the Interrupt Mask Register 2 (0x75) = 1, SMBALERT is still asserted.
OOL = 1 when one or more alerts are generated in Interrupt Status Register 2, assuming all
the mask bits in the Interrupt Mask Register 2 (0x75) = 1, SMBALERT is not asserted.
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Table 79. REGISTER 0X75 − INTERRUPT MASK REGISTER 2 (POWER-ON DEFAULT [7:0] = 0X00)
Bit No. Mnemonic R/W Description
[0] 12 V/VC R/W When Pin 21 is configured as a 12 V input, 12 V/VC = 1 masks SMBALERT for out-of-limit
conditions on the 12 V channel. When Pin 21 is programmed as VID5, this bit masks an
SMBALERT, if the VID5 VID code bit changes.
[1] OVT R/W OVT = 1 masks SMBALERT for overtemperature THERM conditions.
[2] FAN1 R/W FAN1 = 1 masks SMBALERT for a Fan 1 fault.
[3] FAN2 R/W FAN2 = 1 masks SMBALERT for a Fan 2 fault.
[4] FAN3 R/W FAN3 = 1 masks SMBALERT for a Fan 3 fault.
[5] F4P R/W If Pin 14 is configured as TACH4, F4P = 1 masks SMBALERT for a Fan 4 fault. If Pin 14 is
configured as THERM, F4P = 1 masks SMBALERT for an exceeded THERM timer limit. If Pin 14
is configured as GPIO, F4P = 1 masks SMBALERT when GPIO is an input and GPIO is asserted.
[6] D1 R/W D1 = 1 masks SMBALERT for a diode open or short on a Remote 1 channel.
[7] D2 R/W D2 = 1 masks SMBALERT for a diode open or short on a Remote 2 channel.
Table 80. REGISTER 0X76 − EXTENDED RESOLUTION REGISTER 1 (POWER-ON DEFAULT [7:0] = 0X00) (Note 1)
Bit No. Mnemonic R/W Description
[1:0] 2.5 V Read-only 2.5 V LSBs. Holds the 2 LSBs of the 10-bit 2.5 V measurement.
[3:2] VCCP Read-only VCCP LSBs. Holds the 2 LSBs of the 10-bit VCCP measurement.
[5:4] VCC Read-only VCC LSBs. Holds the 2 LSBs of the 10-bit VCC measurement.
[7:6] 5.0 V Read-only 5.0 V LSBs. Holds the 2 LSBs of the 10-bit 5.0 V measurement.
1. If this register is read, this register and the registers holding the MSB of each reading are frozen until read.
Table 81. REGISTER 0X77 − EXTENDED RESOLUTION REGISTER 2 (POWER-ON DEFAULT [7:0] = 0X00) (Note 1)
Bit No. Mnemonic R/W Description
[1:0] 12 V Read-only 12 V LSBs. Holds the 2 LSBs of the 10-bit 12 V measurement.
[3:2] TDM1 Read-only Remote 1 temperature LSBs. Holds the 2 LSBs of the 10-bit Remote 1 temperature
measurement.
[5:4] LTMP Read-only Local temperature LSBs. Holds the 2 LSBs of the 10-bit local temperature measurement.
[7:6] TDM2 Read-only Remote 2 temperature LSBs. Holds the 2 LSBs of the 10-bit Remote 2 temperature
measurement.
1. If this register is read, this register and the registers holding the MSB of each reading are frozen until read.
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Table 82. REGISTER 0X78 − CONFIGURATION REGISTER 3 (POWER-ON DEFAULT = 0X00) (Note 1)
Bit No. Mnemonic R/W Description
[0] ALERT R/W ALERT = 1, Pin 10 (PWM2/SMBALERT) is configured as an SMBALERT interrupt output to
indicate out-of-limit error conditions.
ALERT = 0, Pin 10 (PWM2/SMBALERT) is configured as the PWM2 output.
[1] THERM/
2.5V
R/W THERM = 1 enables THERM functionality on Pin 22 and Pin 14, if Pin 14 is configured as
THERM, determined by Bits 0 and 1 (PIN14FUNC) of Configuration Register 4. When THERM
is asserted, if the fans are running and the BOOST bit is set, then the fans run at full speed.
Alternatively, THERM can be programmed so that a timer is triggered to time how long THERM
has been asserted.
THERM = 0 enables 2.5 V measurement on Pin 22 and disables THERM. If Bits [5:7] of
Configuration Register 5 are set, THERM is bidirectional. If they are 0, THERM is a timer input
only.
Pin 14 FUNC THERM/2.5 V Pin 22 Pin 14
00
01
10
11
00
01
10
11
0
0
0
0
1
1
1
1
+2.5 VIN
+2.5 VIN
+2.5 VIN
+2.5 VIN
THERM
+2.5 VIN
THERM
THERM
TACH4
THERM
SMBALERT
GPIO6
TACH4
THERM
SMBALERT
GPIO6
[2] BOOST R/W When THERM is an input and BOOST = 1, assertion of THERM causes all fans to run at the
maximum programmed duty cycle for fail-safe cooling.
[3] FAST R/W FAST = 1 enables fast TACH measurements on all channels. This increases the TACH
measurement rate from once per second to once every 250 ms (4x).
[4] DC1 R/W DC1 = 1 enables TACH measurements to be continuously made on TACH1. Fans must be
driven by dc. Setting this bit prevents pulse stretching because it is not required for dc driven
motors.
[5] DC2 R/W DC2 = 1 enables TACH measurements to be continuously made on TACH2. Fans must be
driven by dc. Setting this bit prevents pulse stretching because it is not required for dc driven
motors.
[6] DC3 R/W DC3 = 1 enables TACH measurements to be continuously made on TACH3. Fans must be
driven by dc. Setting this bit prevents pulse stretching because it is not required for dc driven
motors.
[7] DC4 R/W DC4 = 1 enables TACH measurements to be continuously made on TACH4. Fans must be
driven by dc. Setting this bit prevents pulse stretching because it is not required for dc driven
motors.
1. This register become read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to to this register have no
effect.
Table 83. REGISTER 0X79 − THERM TIMER STATUS REGISTER (POWER-ON DEFAULT = 0X00)
Bit No. Mnemonic R/W Description
[7:1] TMR Read-only Times how long THERM input is asserted. These seven bits read 0 until the THERM
assertion time exceeds 45.52 ms.
[0] ASRT/
TMR0
Read-only This bit is set high on the assertion of the THERM input and is cleared on read. If the THERM
assertion time exceeds 45.52 ms, this bit is set and becomes the LSB of the 8-bit TMR
reading. This allows THERM assertion times from 45.52 ms to 5.82 sec to be reported back
with a resolution of 22.76 ms.
Table 84. REGISTER 0X7A − THERM TIMER LIMIT REGISTER (POWER-ON DEFAULT = 0X00)
Bit No. Mnemonic R/W Description
[7:0] LIMT R/W Sets maximum THERM assertion length allowed before an interrupt is generated. This is an
8-bit limit with a resolution of 22.76 ms allowing THERM assertion limits of 45.52 ms to
5.82 sec to be programmed. If the THERM assertion time exceeds this limit, Bit 5 (F4P) of
Interrupt Status Register 2 (0x42) is set. If the limit value is 0x00, an interrupt is generated
immediately on the assertion of the THERM input.
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Table 85. REGISTER 0X7B − TACH PULSES PER REVOLUTION REGISTER (POWER-ON DEFAULT = 0X55)
Bit No. Mnemonic R/W Description
[1:0] FAN1 R/W Sets number of pulses to be counted when measuring Fan 1 speed. Can be used to
determine fan pulses per revolution for unknown fan type.
Pulses Counted
00 = 1
01 = 2 (Default)
10 = 3
11 = 4
[3:2] FAN2 R/W Sets number of pulses to be counted when measuring Fan 2 speed. Can be used to
determine fan pulses per revolution for unknown fan type.
Pulses Counted
00 = 1
01 = 2 (Default)
10 = 3
11 = 4
[5:4] FAN3 R/W Sets number of pulses to be counted when measuring Fan 3 speed. Can be used to
determine fan pulses per revolution for unknown fan type.
Pulses Counted
00 = 1
01 = 2 (Default)
10 = 3
11 = 4
[7:6] FAN4 R/W Sets number of pulses to be counted when measuring Fan 4 speed. Can be used to
determine fan pulses per revolution for unknown fan type.
Pulses Counted
00 = 1
01 = 2 (Default)
10 = 3
11 = 4
Table 86. REGISTER 0X7C − CONFIGURATION REGISTER 5 (POWER-ON DEFAULT = 0X01) (Note 1)
Bit No. Mnemonic R/W Description
[0] 2sC R/W 2sC = 1 sets the temperature range to the twos complement temperature range.
2sC = 0 changes the temperature range to the Offset 64 temperature range. When this bit is
changed, the ADT7476 interprets all relevant temperature register values as defined by this bit.
[1] Temp Offset R/W TempOffset = 0 sets offset range to −63C to +64C with 0.5C resolution.
TempOffset = 1 sets offset range to −63C to +127C with 1C resolution. These settings
apply to Remote 1, Local, and Remote2 temperature offset registers (0x70, 0x71, and 0x72).
[2] GPIO6D R/W GPIO6 direction. When GPIO6 function is enabled, this determines whether GPIO6 is an
input (0) or an output (1).
[3] GPIO6P R/W GPIO6 polarity. When the GPIO6 function is enabled and is programmed as an output, this
bit determines whether the GPIO6 is active low (0) or high (1).
[4] VID/GPIO R/W VID/GPIO = 0 enables VID functionality on Pin 5, Pin 6, Pin 7, Pin 8, and Pin 19.
VID/GPIO = 1 enables GPIO functionality on Pin 5, Pin 6, Pin 7, Pin 8, and Pin 19.
[5] R1 THERM R/W R1 THERM = 1 enables THERM temperature limit functionality for Remote 1 temperature
channel; that is, THERM is bidirectional.
R1 THERM = 0 indicates THERM is a timer input only. THERM can also be disabled on any
channel by:
Writing −64C to the appropriate THERM temperature limit in Offset 64 mode.
Writing −128C to the appropriate THERM temperature limit in twos complement mode.
[6] Local
THERM
R/W Local THERM = 1 enables THERM temperature limit functionality for local temperature
channel; that is, THERM is bidirectional.
Local THERM = 0 indicates THERM is a timer input only. THERM can also be disabled on
any channel by:
Writing −64C to the appropriate THERM temperature limit in Offset 64 mode.
Writing −128C to the appropriate THERM temperature limit in twos complement mode.
[7] R2 THERM R/W R2 THERM = 1 enables THERM temperature limit functionality for Remote 2 temperature
channel; that is, THERM is bidirectional.
R2 THERM = 0 indicates THERM is a timer input only.THERM can also be disabled on any
channel by:
Writing −64C to the appropriate THERM temperature limit in Offset 64 mode.
Writing −128C to the appropriate THERM temperature limit in twos complement mode.
1. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to this register have no effect.
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Table 87. REGISTER 0X7D − CONFIGURATION REGISTER 4 (POWER-ON DEFAULT = 0X00) (Note 1)
Bit No. Mnemonic R/W Description
[1:0] PIN14FUNC R/W These bits set the functionality of Pin 14.
00 = TACH4 (Default)
01 = THERM
10 = SMBALERT
11 = GPIO
[2] THERM
Disable
R/W THERM Disable = 0 enables THERM overtemperature output assuming THERM is correctly
configured (Registers 0x78, 0x7C, and 0x7D).
THERM Disable = 1 disables THERM overtemperature output on all channels.
THERM can also be disabled on any channel by:
Writing −64C to the appropriate THERM temperature limit in Offset 64 mode.
Writing −128C to the appropriate THERM temperature limit in twos complement mode.
[3] MaxSpeed
THERM
R/W MaxSpeed on THERM = 0 indicates that fans go to full speed when THERM temperature
limit is exceeded.
MaxSpeed on THERM = 1 indicates that fans go to max speed (0x38, 0x39, 0x3A) when
THERM temperature limit is exceeded.
[4] BpAtt 2.5 V R/W Bypass 2.5 V attenuator. When set, the measurement scale for this channel changes from
0 V (0x00) to 2.25 V (0xFF).
[5] BpAtt VCCP R/W Bypass VCCP attenuator. When set, the measurement scale for this channel changes from
0 V (0x00) to 2.25 V (0xFF).
[6] BpAtt 5.0 V R/W Bypass 5.0 V attenuator. When set, the measurement scale for this channel changes from
0 V (0x00) to 2.25 V (0xFF).
[7] BpAtt 12 V R/W Bypass 12 V attenuator. When set, the measurement scale for this channel changes
from 0 V (0x00) to 2.25 V (0xFF).
1. This register becomes read-only when the Configuration Register 1 Lock bit is set to 1. Any further attempts to write to this register have no effect.
Table 88. REGISTER 0X7E − MANUFACTURER’S TEST REGISTER 1 (POWER-ON DEFAULT = 0X00)
Bit No. Mnemonic R/W Description
[7:0] Reserved Read-only Manufacturer’s test register. These bits are reserved for manufacturer’s test purposes and
should not be written to under normal operation.
Table 89. REGISTER 0X7F − MANUFACTURER’S TEST REGISTER 2 (POWER-ON DEFAULT = 0X00)
Bit No. Mnemonic R/W Description
[7:0] Reserved Read-only Manufacturer’s test register. These bits are reserved for manufacturer’s test purposes and
should not be written to under normal operation.
Table 90. ORDERING INFORMATION
Device Order Number* Package Type Package Option Shipping†
ADT7476ARQZ−REEL 24-lead QSOP RQ−24 2,500 Tape & Reel
†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging
Specifications Brochure, BRD8011/D.
*The “Z” suffix indicates Pb-Free packages.
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PACKAGE DIMENSIONS
QSOP24 NB
CASE 492B−01
ISSUE A
*For additional information on our Pb-Free strategy and soldering
details, please download the ON Semiconductor Soldering and
Mounting Techniques Reference Manual, SOLDERRM/D.
SOLDERING FOOTPRINT*
E
M
0.25 C
A1
A
C
DETAIL A
DETAIL A
h x 45 _
DIM MAXMIN
MILLIMETERS
A1.35 1.75
b0.20 0.30
L0.40 1.27
e0.635 BSC
h0.22 0.50
C0.19 0.25
A1 0.10 0.25
M0 8
NOTES:
1. DIMENSIONING AND TOLERANCING PER ASME
Y14.5M, 1994.
2. CONTROLLING DIMENSION: MILLIMETERS.
3. DIMENSION b DOES NOT INCLUDE DAMBAR
PROTRUSION.
4. DIMENSION D DOES NOT INCLUDE MOLD FLASH,
PROTRUSIONS, OR GATE BURRS. MOLD FLASH,
PROTRUSIONS, OR GATE BURRS SHALL NOT EX
CEED 0.15 PER SIDE. DIMENSION E1 DOES NOT
INCLUDE INTERLEAD FLASH OR PROTRUSION. IN
TERLEAD FLASH OR PROTRUSION SHALL NOT EX
CEED 0.15 PER SIDE. D AND E1 ARE DETERMINED
AT DATUM H.
5. DATUMS A AND B ARE DETERMINED AT DATUM H.
__
b
L
D
D
24X
SEATING
PLANE
0.10 C
E1
A
A-B D
0.20 C
e
112
24 13
24X CM
D8.65 BSC
E6.00 BSC
E1 3.90 BSC
L2 0.25 BSC
D
0.25 C D
B
0.20 C D
2X
2X
2X 12 TIPS
0.10 C H
L2
GAUGE
PLANE
C
6.40
24X
0.42 24X
1.12
0.635
DIMENSIONS: MILLIMETERS
24
PITCH
13
112
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ADT7476/D
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