Dual Channel Temperature Sensor
and Over Temperature Alarm
ADT7481
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2005 Analog Devices, Inc. All rights reserved.
FEATURES
1 local and 2 remote temperature sensors
0.25°C resolution/1°C accuracy on remote channels
1°C resolution/1°C accuracy on local channel
Extended, switchable temperature measurement range 0°C
to 127°C (default) or –64°C to +191°C
2-wire SMBus serial interface with SMBus ALERT support
Programmable over/under temperature limits
Offset registers for system calibration
Up to 2 over temperature fail-safe THERM outputs
Small 10-lead MSOP package
240 μA operating current, 5 μA standby current
APPLICATIONS
Desktop and notebook computers
Industrial controllers
Smart batteries
Automotive
Embedded systems
Burn-in applications
Instrumentation
GENERAL DESCRIPTION
The ADT74811 is a 3-channel digital thermometer and under/
over temperature alarm, intended for use in PCs and thermal
management systems. It can measure its own ambient temperature
or the temperature of two remote thermal diodes. These thermal
diodes can be located in a CPU or GPU, or they can be discrete
diode connected transistors. The ambient temperature, or the
temperature of the remote thermal diode, can be accurately
measured to ±1°C. The temperature measurement range defaults
to 0°C to +127°C, compatible with ADM1032, but can be
switched to a wider measurement range from −64°C to +191°C.
The ADT7481 communicates over a 2-wire serial interface
compatible with system management bus (SMBus) standards.
The SMBus address of the ADT7481 is 0x4C. An ADT7481-1
with an SMBus address of 0x4B is also available.
An ALERT output signals when the on-chip or remote
temperature is outside the programmed limits. The THERM
output is a comparator output that allows, for example, on/off
control of a cooling fan. The ALERT output can be reconfigured
as a second THERM output if required.
FUNCTIONAL BLOCK DIAGRAM
11-BIT A-TO-D
CONVERTER
RUN/STANDBYBUSY
REMOTE 1 AND 2 TEMP
OFFSET REGISTERS
DIGITAL MUX
LIMIT COMPARATOR
STATUS REGISTERS
INTERRUPT
MASKING
CONFIGURATION
REGISTERS
REMOTE 1 AND 2 TEMP
HIGH LIMIT REGISTERS
REMOTE 1 AND 2 TEMP
LOW LIMIT REGISTERS
REMOTE 1 AND 2 TEMP
THERM LIMIT REGISTER
LOCAL TEMPERATURE
HIGH LIMIT REGISTER
LOCAL TEMPERATURE
LOW LIMIT REGISTER
LOCAL TEMPERATURE
THERM LIMIT REGISTER
ONE-SHOT
REGISTER
ADDRESS POINTER
REGISTER
CONVERSION RATE
REGISTER
REMOTE 1 AND 2 TEMP
VALUE REGISTERS
LOCAL TEMPERATURE
VALUE REGISTER
ON-CHIP TEMP
SENSOR
ANALOG
MUX
EXTERNAL DIODES OPEN-CIRCUIT
SMBUS INTERFACE
ADT7481
7
8
3
2
10 41 6 9
V
DD
GND SDATA SCLK THERM
8
D1+
D1–
D2+
D2–
05466-001
ALERT/THERM2
Figure 1.
1 Protected by U.S. Patents 5,195,827; 5,867,012, 5,982,221; 6,097,239; 6,133,753; 6,169,442, other patents pending.
ADT7481
Rev. 0 | Page 2 of 24
TABLE OF CONTENTS
Features .............................................................................................. 1
Applications....................................................................................... 1
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Specifications..................................................................................... 3
Timing Specifications .................................................................. 4
Absolute Maximum Ratings............................................................ 5
Thermal Characteristics .............................................................. 5
ESD Caution.................................................................................. 5
Pin Configuration and Function Descriptions............................. 6
Typical Performance Characteristics ............................................. 7
Theory of Operation ........................................................................ 9
Temperature Measurement Method .......................................... 9
Temperature Measurement Results.......................................... 10
Temperature Measurement Range ........................................... 10
Temperature Data Format ......................................................... 10
Registers........................................................................................... 12
Address Pointer Register ........................................................... 12
Temperature Value Registers..................................................... 12
Conversion Rate/Channel Selector Register........................... 13
Limit Registers ............................................................................ 13
Status Registers ........................................................................... 14
Offset Register ............................................................................ 14
One-Shot Register ...................................................................... 15
Consecutive ALERT Register ................................................... 15
Serial Bus Interface......................................................................... 17
Addressing the Device ............................................................... 17
ALERT Output............................................................................ 19
Masking the ALERT Output..................................................... 19
Low Power Standby Mode......................................................... 19
Sensor Fault Detection .............................................................. 19
Interrupt System......................................................................... 20
Applications Information.............................................................. 22
Noise Filtering............................................................................. 22
Factors Affecting Diode Accuracy........................................... 22
Thermal Inertia and Self-Heating............................................ 22
Layout Considerations............................................................... 22
Application Circuit..................................................................... 23
Outline Dimensions ....................................................................... 24
Ordering Guide .......................................................................... 24
REVISION HISTORY
7/05—Revision 0: Initial Version
ADT7481
Rev. 0 | Page 3 of 24
SPECIFICATIONS
TA = −40°C to +120°C, VDD = 3 V to 3.6 V, unless otherwise noted.
Table 1.
Parameter Min Typ Max Unit Test Conditions
POWER SUPPLY
Supply Voltage, VDD 3.0 3.30 3.6 V
Average Operating Supply Current, IDD 240 350 μA 0.0625 conversions/sec rate1
5 30 μA Standby mode
Undervoltage Lockout Threshold 2.55 V VDD input, disables ADC, rising edge
Power-On-Reset Threshold 1 2.5 V
TEMPERATURE-TO-DIGITAL CONVERTER
Local Sensor Accuracy2 ±1 °C 0°C ≤ TA ≤ +70°C
±1.5 0°C ≤ TA ≤ +85°C
±2.5 °C −40 ≤ TA ≤ +100°C
Resolution 1 °C
Remote Diode Sensor Accuracy2 ±1 °C 0°C ≤ TA ≤ +70°C, −55°C ≤ TD3≤ +150°C
±1.5 °C 0°C ≤ TA ≤ +85°C, −55°C ≤ TD3 ≤ +150°C
±2.5 °C −40°C ≤ TA ≤ +100°C, −55°C ≤ TD3 ≤ +150°C
Resolution 0.25 °C
Remote Sensor Source Current 233 μA High level4
14 μA Low level4
Conversion Time 73 94 ms From stop bit to conversion complete (both channels) one-shot
mode with averaging switched on
11 14 ms One-shot mode with averaging off (conversion rate = 16, 32, or
64 conversions per second)
OPEN-DRAIN DIGITAL OUTPUTS: THERM,
ALERT/THERM2
Output Low Voltage, VOL 0.4 V IOUT = −6.0 mA
High Level Output Leakage Current, IOH 0.1 1 μA VOUT = VDD
SMBus INTERFACE4, 5
Logic Input High Voltage, VIH 2.1 V
SCLK, SDATA
Logic Input Low Voltage, VIL 0.8 V
SCLK, SDATA
Hysteresis 500 mV
SDA Output Low Voltage, VOL 0.4 V IOUT = −6.0 mA
Logic Input Current, IIH, IIL −1 +1 μA
SMBus Input Capacitance, SCLK, SDATA 5 pF
SMBus Clock Frequency 400 kHz
SMBus Timeout6 25 32 ms User programmable
SCLK Falling Edge to SDATA Valid Time 1 μs Master clocking in data
1 See Table 10 for information on other conversion rates.
2 Averaging enabled.
3 Guaranteed by characterization, not production tested.
4 Guaranteed by design, not production tested.
5 See Timing Specifications section for more information.
6 Disabled by default. See the Serial Bus Interface section for details to enable it.
ADT7481
Rev. 0 | Page 4 of 24
TIMING SPECIFICATIONS
Table 2. SMBus Timing Specifications1
Parameter Limit at TMIN, TMAX Unit Description
fSCLK 400 kHz max
tLOW 4.7 μs min Clock low period, between 10% points
tHIGH 4 μs min Clock high period, between 90% points
tR1 μs max Clock/data rise time
tF300 ns max Clock/data fall time
tSU; STA 4.7 μs min Start condition setup time
tHD; STA24 μs min Start condition hold time
tSU; DATT
3250 ns min Data setup time
tSU; STO4 4 μs min Stop condition setup time
tBUF 4.7 μs min Bus free time between stop and start conditions
1 Guaranteed by design, not production tested.
2 Time from 10% of SDATA to 90% of SCLK.
3 Time for 10% or 90% of SDATA to 10% of SCLK.
4 Time for 90% of SCLK to 10% of SDATA.
05466-002
SCLK
SDATA
t
R
t
F
t
LOW
t
HD;DAT
t
HD;STA
t
HIGH
t
SU;DAT
STOP START STOPSTART
t
SU;STA
t
SU;STO
t
HD;STA
t
BUF
Figure 2. Serial Bus Timing
ADT7481
Rev. 0 | Page 5 of 24
ABSOLUTE MAXIMUM RATINGS
Table 3.
Parameter Rating
Positive Supply Voltage (VDD) to GND −0.3 V to +3.6 V
D+ −0.3 V to VDD + 0.3 V
D− to GND −0.3 V to +0.6 V
SCLK, SDATA, ALERT, THERM −0.3 V to +3.6 V
Input Current, SDATA, THERM −1 mA to +50 mA
Input Current, D− ±1 mA
ESD Rating, All Pins (Human Body Model) 1500 V
Maximum Junction Temperature (TJMAX) 150°C
Storage Temperature Range −65°C to +150°C
IR Reflow Peak Temperature 220°C
IR Reflow Peak Temperature for Pb-Free 260°C
Lead Temperature (Soldering 10 sec) 300°C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
THERMAL CHARACTERISTICS
10-lead MSOP package:
θJA = 142°C/W.
θJC = 43.74°C/W.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
ADT7481
Rev. 0 | Page 6 of 24
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
ADT7481
TOP VIEW
(Not to Scale)
V
DD 1
D1+
2
D1–
3
4
GND
5
SCLK
SDATA
D2+
D2–
10
9
8
7
6
ALERT/THERM2
THERM
05466-003
Figure 3. Pin Configuration
Table 4. Pin Function Descriptions
Pin No. Mnemonic Description
1 VDD Positive Supply, 3 V to 3.6 V.
2 D1+ Positive Connection to the Remote 1 Temperature Sensor.
3 D1− Negative Connection to the Remote 1 Temperature Sensor.
4 THERM Open-Drain Output. Requires pull-up resistor. Signals overtemperature events, could be used to turn a fan
on/off, or throttle a CPU clock.
5 GND Supply Ground Connection.
6 D2− Negative Connection to the Remote 2 Temperature Sensor.
7 D2+ Positive Connection to the Remote 2 Temperature Sensor.
8 ALERT/THERM2 Open-Drain Logic Output. Used as interrupt or SMBALERT. This may also be configured as a second THERM
output. Requires pull-up resistor.
9 SDATA Logic Input/Output, SMBus Serial Data. Open-Drain Output. Requires pull-up resistor.
10 SCLK Logic Input, SMBus Serial Clock. Requires pull-up resistor.
ADT7481
Rev. 0 | Page 7 of 24
TYPICAL PERFORMANCE CHARACTERISTICS
3.5
–1.0
–50 150
05466-019
TEMPERATURE (°C)
TEMPERATURE ERROR
3.0
2.5
2.0
1.5
1.0
0.5
0
–0.5
0 50 100
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 15
DEV 16
MEAN
HIGH 4Σ
LOW 4Σ
Figure 4. Local Temperature Error vs. Temperature
3.5
–1.0
–50 150
05466-020
TEMPERATURE ERROR
3.0
2.5
2.0
1.5
1.0
0.5
0
–0.5
0 50 100
TEMPERATURE (°C)
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 15
DEV 16
HIGH 4Σ
LOW 4Σ
Figure 5. Remote 1 Temperature Error vs. Temperature
3.5
–1.0
–50 150
05466-021
TEMPERATURE ERROR
3.0
2.5
2.0
1.5
1.0
0.5
0
–0.5
0 50 100
TEMPERATURE (°C)
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 15
DEV 16
MEAN
HIGH 4Σ
LOW 4Σ
Figure 6. Remote 2 Temperature Error vs. Temperature
10
–251100
05466-022
LEAKAGE RESISTANCE (MΩ)
TEMPERATURE ERROR (°C)
5
0
–5
–10
–15
–20
10
D+ TO GND
D+ TO V
CC
Figure 7. Temperature Error vs. D+/D− Leakage Resistance
0
–18025
05466-023
CAPACITANCE (nF)
TEMPERATURE ERROR (°C)
–2
–4
–6
–8
–10
–12
–14
–16
5 101520
DEV 3
DEV 2
DEV 4
Figure 8. Temperature Error vs. D+/D− Capacitance
1000
0
0.01 100
05466-024
CONVERTION RATE (Hz)
I
DD
(μA)
900
800
700
600
500
400
300
200
100
0.1 1 10
DEV 4BC
DEV 3BC
DEV 2BC
Figure 9. Operating Supply Current vs. Conversion Rate
ADT7481
Rev. 0 | Page 8 of 24
422
408
3.0 3.6
05466-025
V
DD
(V)
I
DD
(μA)
420
418
416
414
412
410
3.1 3.2 3.3 3.4 3.5
DEV 3BC
DEV 2BC
DEV 4BC
Figure 10. Operating Supply Current vs. Voltage
4.4
3.0
3.0 3.6
05466-026
V
DD
(V)
I
DD
(μA)
4.2
4.0
3.8
3.6
3.4
3.2
3.1 3.2 3.3 3.4 3.5
DEV 3
DEV 2
DEV 4
Figure 11. Standby Supply Current vs. Voltage
35
011000
05466-027
FSCL (kHz)
I
STBY
(μA)
30
25
20
15
10
5
10 100
DEV 2BC
DEV 3BC
DEV 4BC
Figure 12. Standby Supply Current vs. SCLK Frequency
0
5
10
15
25
100 200 300 400 500 600
NOISE FREQUENCY (MHz)
TEMPERATURE ERROR (°C)
20mV
20
0
100mV
50mV
05466-028
Figure 13. Temperature Error vs. Common-Mode Noise Frequency
–10
0
20
40
80
100 200 300 400 500 600
NOISE FREQUENCY (MHz)
TEMPERATURE ERROR (°C)
20mV
60
0
100mV
10
30
70
50
50mV
05466-029
Figure 14. Temperature Error vs. Differential Mode Noise Frequency
ADT7481
Rev. 0 | Page 9 of 24
THEORY OF OPERATION
The ADT7481 is a local and dual remote temperature sensor
and over/under temperature alarm. When the ADT7481 is
operating normally, the on-board ADC operates in a free-
running mode. The analog input multiplexer alternately selects
either the on-chip temperature sensor to measure its local
temperature, or either of the remote temperature sensors. The
ADC digitizes these signals and the results are stored in the
local, Remote 1, and Remote 2 temperature value registers.
The local and remote measurement results are compared with
the corresponding high, low, and THERM temperature limits,
stored in on-chip registers. Out-of-limit comparisons generate
flags that are stored in the status register. A result that exceeds
the high temperature limit, the low temperature limit, or remote
diode open circuit will cause the ALERT output to assert low.
Exceeding THERM temperature limits causes the THERM
output to assert low. The ALERT output can be reprogrammed
as a second THERM output.
The limit registers can be programmed, and the device controlled
and configured via the serial SMBus. The contents of any register
can also be read back via the SMBus.
Control and configuration functions consist of switching the
device between normal operation and standby mode, selecting
the temperature measurement scale, masking or enabling the
ALERT output, switching Pin 8 between ALERT and THERM2,
and selecting the conversion rate.
TEMPERATURE MEASUREMENT METHOD
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, measuring the base-
emitter voltage (VBE) of a transistor operated at constant current.
This technique requires calibration to null the effect of the
absolute value of VBE, which varies from device to device.
The technique used in the ADT7481 measures the change in
VBE when the device is operated at two different currents.
Figure 15 shows the input signal conditioning used to measure
the output of a remote temperature sensor. This figure shows
the remote sensor as a substrate transistor, but it could equally
be a discrete transistor. If a discrete transistor is used, the
collector is not grounded and is linked to the base. To prevent
ground noise 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. C1
may optionally be added as a noise filter with a recommended
maximum value of 1,000 pF.
To me asure ΔVBE, the operating current through the sensor is
switched among two related currents. The currents through the
temperature diode are switched between I, and N × I, giving
ΔVBE. The temperature can then be calculated using the ΔVBE
measurement.
The resulting ΔVBE waveforms pass through a 65 kHz low-pass
filter to remove noise and then to a chopper-stabilized amplifier.
This amplifies and rectifies the waveform to produce a dc
voltage proportional to ΔVBE. The ADC digitizes this voltage
producing a temperature measurement. To reduce the effects of
noise, digital filtering is performed by averaging the results of
16 measurement cycles for low conversion rates. At rates of 16,
32, and 64 conversions/second, no digital averaging takes place.
Signal conditioning and measurement of the local temperature
sensor is performed in the same manner.
LPF
V
DD
I
BIAS
N
×
II
BIAS
DIODE
D+
D–
C1
1
1
CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000pF MAX.
REMOTE
SENSING
TRANSISTO
R
f
C
= 65kHz
V
OUT+
V
OUT–
TO ADC
05466-010
Figure 15. Input Signal Conditioning
ADT7481
Rev. 0 | Page 10 of 24
TEMPERATURE MEASUREMENT RESULTS
The results of the local and remote temperature measurements
are stored in the local and remote temperature value registers
and are compared with limits programmed into the local and
remote high and low limit registers.
The local temperature measurement is an 8-bit measurement
with 1°C resolution. The remote temperature measurements are
10-bit measurements, with the 8 MSBs stored in one register
and the 2 LSBs stored in another register. Table 5 is a list of the
temperature measurement registers.
Table 5. Register Address for the Temperature Values
Temperature
Channel
Register Address,
MSBs
Register Address,
LSBs
Local 0x00 N/A
Remote 1 0x01 0x10 (2 MSBs)
Remote 2 0x30 0x33 (2 MSBs)
If Bit 3 of the Configuration 1 register is set to 1, then the
Remote 2 temperature values can be read from the following
register addresses:
Remote 2, MSBs = 0x01
Remote 2, LSBs = 0x10
The above is true only when Bit 3 of the Configuration 1 register
is set. To read the Remote 1 temperatures, this bit needs to be
switched back to 0.
Only the two MSBs in the remote temperature low byte are
used. This gives the remote temperature measurement a
resolution of 0.25°C. Table 6 shows the data format for the
remote temperature low byte.
Table 6. Extended Temperature Resolution
(Remote Temperature Low Byte)
Extended Resolution Remote Temperature Low Byte
0.00°C 0 000 0000
0.25°C 0 100 0000
0.50°C 1 000 0000
0.75°C 1 100 0000
When reading the full remote temperature value, including both
the high and low byte, the two registers should be read LSB first
and then the MSB. This is because reading the LSB will cause the
MSB to be locked until it is read. This is to guarantee that the two
values read are derived from the same temperature measurement.
The MSB register updates only after it has been read. The MSB
will not lock if a SMBus repeat start is used between reading the
two registers. There needs to be a stop between reading the LSB
and MSB.
If the LSB register is read but not the MSB register, then fail-safe
protection is provided by the THERM and ALERT signals
which update with the latest temperature measurements rather
than the register values.
TEMPERATURE MEASUREMENT RANGE
The temperature measurement range for both local and remote
measurements is, by default, 0°C to +127°C. However, the
ADT7481 can be operated using an extended temperature range.
The temperature range in the extended mode is −64°C to +191°C.
The user can switch between these two temperature ranges by
setting or clearing Bit 2 in the Configuration 1 register. A valid
result is available in the next measurement cycle after changing
the temperature range.
Bit 2 Configuration Register 2 = 0 = 0°C to +127°C = default
Bit 2 Configuration Register 2 = 1 = −64°C to +191°C
In extended temperature mode, the upper and lower temperatures
that can be measured by the ADT7481 are limited by the remote
diode selection. While the temperature registers can have values
from −64°C to +191°C, most temperature sensing diodes have a
maximum temperature range of −55°C to +150°C.
Note that while both local and remote temperature measurements
can be made while the part is in extended temperature mode,
the ADT7481 should not be exposed to temperatures greater
than those specified in the Absolute section. Furthermore, the
device is only guaranteed to operate as specified at ambient
temperatures from −40°C to +120°C.
TEMPERATURE DATA FORMAT
The ADT7481 has two temperature data formats. When the
temperature measurement range is from 0°C to +127°C
(default), the temperature data format is binary for both local
and remote temperature results. See the Temperature Measurement
Range section for information on how to switch between the
two data formats.
When the measurement range is in extended mode, an offset
binary data format is used for both local and remote results.
Temperature values in the offset binary data format are offset by
+64. Examples of temperatures in both data formats are shown
in Table 7.
ADT7481
Rev. 0 | Page 11 of 24
Table 7. Temperature Data Format (Local and Remote Temperature High Byte)
Temperature Binary Offset Binary1
−55°C 0 000 00002 0 000 1001
0°C 0 000 0000 0 100 0000
+1°C 0 000 0001 0 100 0001
+10°C 0 000 1010 0 100 1010
+25°C 0 001 1001 0 101 1001
+50°C 0 011 0010 0 111 0010
+75°C 0 100 1011 1 000 1011
+100°C 0 110 0100 1 010 0100
+125°C 0 111 1101 1 011 1101
+127°C 0 111 1111 1 011 1111
+150°C 0 111 11113 1 101 0110
1 Offset binary scale temperature values are offset by +64.
2 Binary scale temperature measurement returns 0 for all temperatures <0°C.
3 Binary scale temperature measurement returns 127 for all temperatures >127°C.
The user may switch between measurement ranges at any time.
Switching the range will also switch the data format. The next
temperature result following the switching will be reported back
to the register in the new format. However, the contents of the
limit registers will not change. It is up to the user to ensure that
when the data format changes, the limit registers are repro-
grammed as necessary. More information on this can be found
in the Limit Registers section.
ADT7481
Rev. 0 | Page 12 of 24
REGISTERS
The registers in the ADT7481 are eight bits wide. These
registers are used to store the results of remote and local
temperature measurements, high and low temperature limits,
and to configure and control the device. A description of these
registers follows.
ADDRESS POINTER REGISTER
The address pointer register does not have, nor does it require,
an address because the first byte of every write operation is
automatically written to this register. The data in this first byte
always contains the address of another register on the ADT7481,
which is stored in the address pointer register. It is to this register
address that the second byte of a write operation is written to, or
to which a subsequent read operation is performed.
The power-on default value of the address pointer register is
0x00, so if a read operation is performed immediately after
power-on, without first writing to the address pointer, the value
of the local temperature will be returned since its register
address is 0x00.
TEMPERATURE VALUE REGISTERS
The ADT7481 has five registers to store the results of local and
remote temperature measurements. These registers can only be
written to by the ADC and read by the user over the SMBus.
• The local temperature value register is at Address 0x00.
• The Remote 1 temperature value high byte register is at
Address 0x01, with the Remote 1 low byte register at
Address 0×10.
• The Remote 2 temperature value high byte register is at
Address 0x30, with the Remote 2 low byte register at
Address 0x33.
• The Remote 2 temperature values can also be read from
Address 0x01 for the high byte, and Address 0x10 for the low
byte if Bit 3 of Configuration Register 1 is set to 1.
• To read the Remote 1 temperature values, set Bit 3 of
Configuration Register 1 to 0.
• The power-on default value for all five registers is 0x00.
Table 8: Configuration 1 Register
(Read Address 0x03, Write Address 0x09)
Bit Mnemonic Function
7 Mask Setting this bit to 1 masks all ALERTs on the ALERT pin. Default = 0 = ALERT enabled. This applies only if Pin 8 is
configured as ALERT, otherwise it has no effect.
6 Mon/STBY Setting this bit to 1 places the ADT7481 in standby mode, that is, it suspends all temperature measurements (ADC).
The SMBus remains active and values can be written to, and read from, the registers. However THERM and ALERT are
not active in standby mode, and their states in standby mode are not reliable. Default = 0 = temperature monitoring
enabled.
5 AL/TH This bit selects the function of Pin 8. Default = 0 = ALERT. Setting this bit to 1 configures Pin 8 as the THERM2 pin.
4 Reserved Reserved for future use.
3 Remote 1/2 Setting this bit to 1 enables the user to read the Remote 2 values from the Remote 1 registers. When default = 0,
Remote 1 temperature values and limits are read from these registers.
2 Temp Range Setting this bit to 1 enables the extended temperature measurement range of −64°C to +191°C. When using the
default = 0, the temperature range is 0°C to +127°C.
1 Mask R1 Setting this bit to 1 masks ALERTs due to the Remote 1 temperature exceeding a programmed limit. Default = 0.
0 Mask R2 Setting this bit to 1 masks ALERTs due to the Remote 2 temperature exceeding a programmed limit. Default = 0.
Table 9. Configuration 2 Register (Address 0x24)
Bit Mnemonic Function
7 Lock Bit Setting this bit to 1 locks all lockable registers to their current values. This prevents tampering with settings until
the device is powered down. Default = 0.
<6:0> Res Reserved for future use.
ADT7481
Rev. 0 | Page 13 of 24
CONVERSION RATE/CHANNEL
SELECTOR REGISTER
The conversion rate/channel selector register for reads is at
Address 0x04, and at Address 0x0A for writes. The four LSBs
of this register are used to program the conversion times from
15.5 ms (Code 0x0A) to 16 seconds (Code 0x00). To program
the ADT7481 to perform continuous measurements, set the
conversion rate register to 0x0B. For example, a conversion rate
of eight conversions/second means that beginning at 125 ms
intervals, the device performs a conversion on the local and the
remote temperature channels.
This register can be written to, and read back from, the SMBus.
The default value of this register is 0x08, giving a rate of 16
conversions per second. Using slower conversion times greatly
reduces the device power consumption.
Bit 7 in this register can be used to disable averaging of the
temperature measurements. All temperature channels are
measured by default. It is possible to configure the ADT7481 to
measure the temperature of one channel only. This can be
configured using Bit 4 and Bit 5 (see Table 10).
Table 10. Conversion Rate/Channel Selector Register (Read Address 0x04, Write Address 0x0A)
Bit Mnemonic Function
7 Averaging Setting this bit to 1 disables averaging of the temperature measurements at the slower conversion rates
(averaging cannot take place at the three faster rates, so setting this bit has no effect). When default = 0,
averaging is enabled.
6 Reserved Reserved for future use. Do not write to this bit.
<5:4> Channel
Selector
These bits are used to select the temperature measurement channels:
00 = round robin = default = all channels measured
01 = local temperature only measured
10 = Remote 1 temperature only measured
11 = Remote 2 temperature only measured
<3:0> Conversion
Rates
These bits set how often the ADT7481 measures each temperature channel. Conversion rates are as follows:
Conversions/sec Time (seconds)
0000 = 0.0625 16
0001 = 0.125 8
0010 = 0.25 4
0011 = 0.5 2
0100 = 1 1
0101 = 2 500 m
0110 = 4 250 m
0111 = 8 125 m
1000 = 16 = default 62.5 m
1001 = 32 31.25 m
1010 = 64 15.5 m
1011 = continuous measurements 73 m (averaging enabled)
LIMIT REGISTERS
The ADT7481 has three limits for each temperature channel:
high, low, and THERM temperature limits for local, Remote 1,
and Remote 2 temperature measurements. The remote tempera-
ture high and low limits span two registers each to contain an
upper and lower byte for each limit. There is also a THERM
hysteresis register. All limit registers can be written to, and read
back from, the SMBus. See Table 15 for details of the limit register
addresses and power-on default values.
When Pin 8 is configured as an ALERT output, the high limit
registers perform a > comparison while the low limit registers
perform a ≤ comparison. For example, if the high limit register
is programmed with 80°C, then measuring 81°C will result in
an out-of-limit condition, setting a flag in the status register.
If the low limit register is programmed with 0°C, measuring
0°C or lower will result in an out-of-limit condition.
Exceeding either the local or remote THERM limit asserts
THERM low. When Pin 8 is configured as THERM2, exceeding
either the local or remote high limit asserts THERM2 low. A
default hysteresis value of 10°C is provided that applies to both
THERM channels. This hysteresis value may be reprogrammed.
It is important to remember that the temperature limits data
format is the same as the temperature measurement data for-
mat. So if the temperature measurement uses the default binary
scale, then the temperature limits also use the binary scale. If
the temperature measurement scale is switched, however, the
temperature limits do not automatically switch. The user must
reprogram the limit registers to the desired value in the correct
data format. For example, if the remote low limit is set at 10°C
and the default binary scale is being used, the limit register
value should be 0000 1010b. If the scale is switched to offset
binary, the value in the low temperature limit register should be
reprogrammed to be 0100 1010b.
ADT7481
Rev. 0 | Page 14 of 24
STATUS REGISTERS
The status registers are read-only registers, at Address 0x02
(Status Register 1) and Address 0x23 (Status Register 2). They
contain status information for the ADT7481.
Table 11. Status Register 1 Bit Assignments
Bit Mnemonic Function ALERT
7 BUSY 1 when ADC converting No
6 LHIGH11 when local high temperature
limit tripped
Yes
5 LLOW11 when local low temperature limit
tripped
Yes
4 R1HIGH11 when Remote 1 high temperature
limit tripped
Yes
3 R1LOW11 when Remote 1 low temperature
limit tripped
Yes
2 D1 OPEN11 when Remote 1 sensor open
circuit
Yes
1 R1THRM1 1 when Remote1 THERM limit
tripped
No
0 LTHRM1 1 when local THERM limit tripped No
1 These flags stay high until the status register is read, or they are reset by
POR.
Table 12. Status Register 2 Bit Assignments
Bit Mnemonic Function ALERT
7 Res Reserved for Future Use No
6 Res Reserved for Future Use No
5 Res Reserved for Future Use No
4 R2HIGH11 When Remote 2 High
Temperature Limit Tripped Yes
3 R2LOW11 When Remote 2 Low Temperature
Limit Tripped Yes
2 D2 OPEN11 When Remote 2 Sensor Open
Circuit Yes
1 R2THRM1 1 When Remote2 THERM Limit
Tripped No
0 ALERT 1 When ALERT Condition Exists No
1 These flags stay high until the status register is read, or they are reset by
POR.
The eight flags that can generate an ALERT are NOR’d together.
When any flag is high, the ALERT interrupt latch is set and the
ALERT output goes low (provided that the flag(s) is/are not
masked out).
Reading the Status 1 register will clear the five flags (Bit 6
through Bit 2) in Status Register 1, provided the error
conditions that caused the flags to be set have gone away.
Reading the Status 2 register will clear the three flags (Bit 4
through Bit 2) in Status Register 2, provided the error
conditions that caused the flags to be set have gone away. A flag
bit can only be reset if the corresponding value register contains
an in-limit measurement, or if the sensor is good.
The ALERT interrupt latch is not reset by reading the status
register. It will be reset when the ALERT output has been
serviced by the master reading the device address, provided the
error condition has gone away and the status register flag bits
have been reset.
When Flag 1 and/or Flag 0 of Status Register 1, or Flag 1 of
Status Register 2 are set, the THERM output goes low to
indicate that the temperature measurements are outside the
programmed limits. The THERM output does not need to be
reset, unlike the ALERT output. Once the measurements are
within the limits, the corresponding status register bits are reset
automatically, and the THERM output goes high. The user may
add hysteresis by programming Register 0x21. The THERM
output will be reset only when the temperature falls below the
THERM limit minus hysteresis.
When Pin 8 is configured as THERM2, only the high
temperature limits are relevant. If Flag 6 and Flag 4 of Status
Register 1, or Flag 4 of Status Register 2 are set, the THERM2
output goes low to indicate that the temperature measurements
are outside the programmed limits. Flag 5 and Flag 3 of Status
Register 1, and Flag 3 of Status Register 2 have no effect on
THERM2. The behavior of THERM2 is otherwise the same as
THERM.
Bit 0 of Status Register 2 gets set whenever the ALERT output is
asserted low. Thus, the user need only read Status Register 2 to
determine if the ADT7481 is responsible for the ALERT. This
bit gets reset when the ALERT output gets reset. If the ALERT
output is masked, then this bit is not set.
OFFSET REGISTER
Offset errors may be introduced into the remote temperature
measurement by clock noise or by the thermal diode being
located away from the hot spot. To achieve the specified
accuracy on this channel, these offsets must be removed.
The offset values are stored as 10-bit, twos complement values.
• The Remote 1 offset MSBs are stored in Register 0x11 and the
LSBs are stored in Register 0x12 (low byte, left justified).
• The Remote 2 offset MSBs are stored in Register 0x34 and the
LSBs are stored in Register 0x35 (low byte, left justified). The
Remote 2 offset can be written to, or read from, the Remote 1
offset registers if Bit 3 of the Configuration 1 register is set to
1. This bit should be set to 0 (default) to read the Remote 1
offset values.
Only the upper two bits of the LSB registers are used. The MSB
of the MSB offset register is the sign bit. The minimum offset
that can be programmed is −128°C, and the maximum is
+127.75°C. The value in the offset register is added to, or
subtracted from, the measured value of the remote temperature.
The offset register powers up with a default value of 0°C and
will have no effect unless the user writes a different value to it.
ADT7481
Rev. 0 | Page 15 of 24
Table 13. Sample Offset Register Codes
Offset Value 0x11/0x34 0x12/0x35
−128°C 1000 0000 00 00 0000
−4°C 1111 1100 00 00 0000
−1°C 1111 1111 00 000000
−0.25°C 1111 1111 10 00 0000
0°C 0000 0000 00 00 0000
+0.25°C 0000 0000 01 00 0000
+1°C 0000 0001 00 00 0000
+4°C 0000 0100 00 00 0000
+127.75°C 0111 1111 11 00 0000
ONE-SHOT REGISTER
The one-shot register is used to initiate a conversion and
comparison cycle when the ADT7481 is in standby mode, after
which the device returns to standby. Writing to the one-shot
register address (0x0F) causes the ADT7481 to perform a
conversion and comparison on both the local and the remote
temperature channels. This is not a data register as such, and it
is the write operation to Address 0x0F that causes the one-shot
conversion. The data written to this address is irrelevant and is
not stored. However the ALERT and THERM outputs are not
operational in one-shot mode and should not be used.
CONSECUTIVE ALERT REGISTER
The value written to this register determines how many out-of-
limit measurements must occur before an ALERT is generated.
The default value is that one out-of-limit measurement gener-
ates an ALERT. The maximum value that can be chosen is 4.
The purpose of this register is to allow the user to perform
some filtering of the output. This is particularly useful at the
fastest three conversion rates, where no averaging takes place.
This register is at Address 0x22. This register has other functions
that are listed in Table 14.
Table 14. Consecutive ALERT Register Bit
Bit Name Description
7 SCL Timeout Set to 1, enables the SMBus SCL
timeout bit. Default = 0 =
timeout disabled. See the Serial
Bus Interface section for more
information.
6 SDA Timeout Set to 1 to enable the SMBus
SDA Timeout Bit. Default = 0 =
Timeout disabled. See the Serial
Bus Interface section for more
information.
5 Mask Local Setting this bit to 1 masks
ALERTs due to the local
temperature exceeding a
programmed limit. Default = 0.
4 Res Reserved for future use.
<3:0> Consecutive
ALERT
These bits set the number of
consecutive out-of-limit
measurements that have to
occur before an ALERT is
generated.
000x = 1
001x = 2
011x = 3
111x = 4
ADT7481
Rev. 0 | Page 16 of 24
Table 15. List of Registers
Read Address
(Hex)
Write Address
(Hex) Mnemonic Power-On Default Comment Lock
N/A N/A Address Pointer Undefined No
00 N/A Local Temperature Value 0000 0000 (0x00) No
01 N/A Remote 1 Temperature Value High Byte 0000 0000 (0x00) Bit 3 Conf Reg = 0 No
01 N/A Remote 2 Temperature Value High Byte 0000 0000 (0x00) Bit 3 Conf Reg = 1 No
02 N/A Status Register 1 Undefined No
03 09 Configuration Register 1 0000 0000 (0x00) Yes
04 0A Conversion Rate/Channel Selector 0000 0111 (0x07) Yes
05 0B Local Temperature High Limit 0101 0101 (0x55) (85°C) Yes
06 0C Local Temperature Low Limit 0000 0000 (0x00) (0°C) Yes
07 0D Remote 1 Temp High Limit High Byte 0101 0101 (0x55) (85°C) Bit 3 Conf Reg = 0 Yes
07 0D Remote 2 Temp High Limit High Byte 0101 0101 (0x55) (85°C) Bit 3 Conf Reg = 1 Yes
08 0E Remote 1 Temp Low Limit High Byte 0000 0000 (0x00) (0°C) Bit 3 Conf Reg = 0 Yes
08 0E Remote 2 Temp Low Limit High Byte 0000 0000 (0x00) (0°C) Bit 3 Conf Reg = 1 Yes
N/A 0F1One-Shot N/A
10 N/A Remote 1 Temperature Value Low Byte 0000 0000 Bit 3 Conf Reg = 0 No
10 N/A Remote 2 Temperature Value Low Byte 0000 0000 Bit 3 Conf Reg = 1 No
11 11 Remote 1 Temperature Offset High Byte 0000 0000 Bit 3 Conf Reg = 0 Yes
11 11 Remote 2 Temperature Offset High Byte 0000 0000 Bit 3 Conf Reg = 1 Yes
12 12 Remote 1 Temperature Offset Low Byte 0000 0000 Bit 3 Conf Reg = 0 Yes
12 12 Remote 2 Temperature Offset Low Byte 0000 0000 Bit 3 Conf Reg = 1 Yes
13 13 Remote 1 Temp High Limit Low Byte 0000 0000 Bit 3 Conf Reg = 0 Yes
13 13 Remote 2 Temp High Limit Low Byte 0000 0000 Bit 3 Conf Reg = 1 Yes
14 14 Remote 1 Temp Low Limit Low Byte 0000 0000 Bit 3 Conf Reg = 0 Yes
14 14 Remote 2 Temp Low Limit Low Byte 0000 0000 Bit 3 Conf Reg = 1 Yes
19 19 Remote 1 THERM Limit 0101 0101 (0x55) (85°C) Bit 3 Conf Reg = 0 Yes
19 19 Remote 2 THERM Limit 0101 0101 (0x55) (85°C) Bit 3 Conf Reg = 1 Yes
20 20 Local THERM Limit 0101 0101 (0x55) (85°C) Yes
21 21 THERM Hysteresis 0000 1010 (0x0A) (10°C) Yes
22 22 Consecutive ALERT 0000 0001 (0x01) Yes
23 N/A Status Register 2 0000 0000 (0x00) No
24 24 Configuration 2 Register 0000 0000 (0x00) Yes
30 N/A Remote 2 Temperature Value High Byte 0000 0000 (0x00) No
31 31 Remote 2 Temp High Limit High Byte 0101 0101 (0x55) (85°C) Yes
32 32 Remote 2 Temp Low Limit High Byte 0000 0000 (0x00) (0°C) Yes
33 N/A Remote 2 Temperature Value Low Byte 0000 0000 (0x00) No
34 34 Remote 2 Temperature Offset High Byte 0000 0000 (0x00) Yes
35 35 Remote 2 Temperature Offset Low Byte 0000 0000 (0x00) Yes
36 36 Remote 2 Temp High Limit Low Byte 0000 0000 (0x00) (0°C) Yes
37 37 Remote 2 Temp Low Limit Low Byte 0000 0000 (0x00) (0°C) Yes
39 39 Remote 2 THERM Limit 0101 0101 (0x55) (85°C) Yes
3D N/A Manufacturer ID 0100 0001 (0x41) N/A
3E N/A Device ID 1000 0001 (0x81)
1 Writing to Address 0F causes the ADT7481 to perform a single measurement. It is not a data register as such, and it does not matter what data is written to it.
ADT7481
Rev. 0 | Page 17 of 24
SERIAL BUS INTERFACE
Control of the ADT7481 is achieved via the serial bus. The
ADT7481 is connected to this bus as a slave device under the
control of a master device.
The ADT7481 has an SMBus timeout feature. When this is
enabled, the SMBus will typically timeout after 25 ms of no
activity. However, this feature is not enabled by default. Set Bit 7
(SCL timeout bit) of the consecutive alert register (Address 0x22)
to enable the SCL timeout. Set Bit 6 (SDA timeout bit) of the
consecutive alert register (Address 0x22) to enable the SDA
timeout.
The ADT7481 supports packet error checking (PEC) and its use
is optional. It is triggered by supplying the extra clock for the
PEC byte. The PEC byte is calculated using CRC-8. The frame
check sequence (FCS) conforms to CRC-8 by the polynomial
()
1
128 +++= xxxxC
Consult the SMBus 1.1 specification for more information
(www.smbus.org).
ADDRESSING THE DEVICE
In general, every SMBus device has a 7-bit device address,
except for some devices that have extended, 10-bit addresses.
When the master device sends a device address over the bus,
the slave device with that address responds. The ADT7481 is
available with one device address, 0x4C (1001 100b). An
ADT7481-1 is also available. The only difference between the
ADT7481 and the ADT7481-1 is the SMBus address. The
ADT7481-1 has a fixed SMBus address of 0x4B (1001 011b).
The addresses mentioned in this datasheet are 7-bit addresses.
The R/W bit needs to be added to arrive at an 8-bit address.
Other than the different SMBus addresses, the ADT7481 and
the ADT7481-1 are functionally identical.
The serial bus protocol operates as follows:
The master initiates data transfer by establishing a start
condition, defined as a high-to-low transition on the serial data
line (SDATA) while the serial clock line (SCLK) 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 determines the
direction of the data transfer, that is, whether data will be
written to, or read from, the slave device. The peripheral with
the address corresponding 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 remain idle while the selected device waits
for data to be read from or written to it. If the R/W bit is 0, the
master writes to the slave device. If the R/W bit is 1, the master
reads from the slave device.
Data is sent over the serial bus in a sequence 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, since a low-to-high transition when the clock is
high may be interpreted as a stop signal. The number of data
bytes that can be transmitted over the serial bus in a single read
or write operation is limited only by what the master and slave
devices can handle.
When all data bytes have been read or written, stop conditions
are established. In write mode, the master will pull the data line
high during the tenth clock pulse to assert a stop condition. In
read mode, the master device will override 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 will then take the data line low during the low period
before the tenth clock pulse, then high during the tenth clock
pulse to assert a stop condition.
Any number of bytes of data may be transferred over the serial
bus in one operation, but 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 case of the ADT7481, write
operations contain either one or two bytes, while read
operations contain one byte.
To write data to one of the device data registers or to read data
from it, the address pointer register must be set so that the
correct data register is addressed. The first byte of a write
operation always contains a valid address that is stored in the
address pointer register. If data is to be written to the device, the
write operation contains a second data byte that is written to the
register selected by the address pointer register.
This procedure is illustrated in Figure 16. The device address is
sent over the bus followed by R/W set to 0 and 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.
ADT7481
Rev. 0 | Page 18 of 24
D7 D6 D5 D4 D3 D2 D1 D0
FRAME 3
DATA
BYTE
ACK. BY
ADT7481 STOP BY
MASTER
···
···
SDA (CONTINUED)
SCL (CONTINUED)
FRAME 1 DATA
SERIAL BUS ADDRESS BYTE
ACK. BY
ADT7481
SDA
SCL
START BY
MASTER FRAME 2
ADDRESS POINTER REGISTER BYTE
D7 D6 D5 D4 D3 D2 D1 D0
10011 0 1 R/W
ACK. BY
ADT7481
05466-011
1 19 9
1 9
Figure 16. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
FRAME 1 DATA
SERIAL BUS ADDRESS BYTE
ACK. BY
ADT7481
SDA
SCL
START BY
MASTER FRAME 2
ADDRESS POINTER REGISTER BYTE
D7 D6 D5 D4 D3 D2 D1 D01 0 0 1 1 0 1 R/W
ACK. BY
ADT7481
05466-012
1 19 9
STOP BY
MASTER
Figure 17. Writing to the Address Pointer Register Only
FRAME 1 DATA
SERIAL BUS ADDRESS BYTE
ACK. BY
ADT7481
SDA
SCL
START BY
MASTER FRAME 2
ADDRESS POINTER REGISTER BYTE
D7 D6 D5 D4 D3 D2 D1 D0100110 1R/W
ACK. BY
ADT7481
05466-013
1 19 9
STOP BY
MASTER
Figure 18. Reading from a Previously Selected Register
When reading data from a register there are two possible
scenarios:
• If the address pointer register value of the ADT7481 is
unknown or not the desired value, it is necessary to set it to
the correct value before data can be read from the desired
data register. This is done by performing a write to the
ADT7481 as before, but only the data byte containing the
register read address is sent, as data is not to be written to the
register. This is shown in Figure 17.
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 (shown in Figure 18).
• If the address pointer register is already at the desired
address, data can be read from the corresponding data
register without first writing to the address pointer register,
and the bus transaction shown in Figure 17 can be omitted.
NOTES
It is possible to read a data byte from a data register without
first writing to the address pointer register. However, if the
address pointer register is already at the correct value, it is not
possible to write data to a register without writing to the
address pointer register. This is because the first data byte of a
write is always written to the address pointer register.
ADT7481
Rev. 0 | Page 19 of 24
Remember that some of the ADT7481 registers have different
addresses for read and write operations. The write address of a
register must be written to the address pointer if data is to be
written to that register, but it may not be possible to read data
from that address. The read address of a register must be
written to the address pointer before data can be read from that
register.
ALERT OUTPUT
Pin 8 can be configured as an ALERT output. The ALERT out-
put goes low whenever an out-of-limit measurement is detected,
or if the remote temperature sensor is open circuit. It is an open-
drain output and requires a pull-up. Several ALERT outputs can
be wire-OR’ed together, so that the common line will go low if
one or more of the ALERT outputs goes low.
The ALERT output can be used as an interrupt signal to a
processor, or it may be used as an SMBALERT. Slave devices on
the SMBus cannot normally signal to the bus master that they
want to talk, but the SMBALERT function allows them to do so.
One or more ALERT outputs can be connected to a common
SMBALERT line connected to the master. When the SMBALERT
line is pulled low by one of the devices, the following procedure
occurs as illustrated in Figure 19.
05466-014
ALERT RESPONSE
ADDRESS
MASTER SENDS
ARA AND READ
COMMAND DEVICE SENDS
ITS ADDRESS
RDSTART ACK DEVICE
ADDRESS NO
ACK STOP
MASTER
RECEIVES
SMBALERT
Figure 19. Use of SMBALERT
1. SMBALERT is pulled low.
2. 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 with a low ALERT output responds to the alert
response address, and the master reads the address from the
responding device. An LSB of 1 is added because the device
address is comprised of seven bits. The address of the device
is now known and it can be interrogated in the usual way.
4. If more than one device has a low ALERT output, the one
with the lowest device address will have priority, in accor-
dance with normal SMBus arbitration.
5. Once the ADT7481 has responded to the alert response
address, it will reset its ALERT output, provided that the
error condition that caused the ALERT no longer exists. If
the SMBALERT line remains low, the master sends the ARA
again, and so on until all devices with low ALERT outputs
respond.
MASKING THE ALERT OUTPUT
The ALERT output can be masked for local, Remote 1, Remote 2
or all three channels. This is done by setting the appropriate
mask bits in either the Configuration 1 register (read address =
0x03, write address = 0x09) or in the consecutive ALERT
register (address = 0x22)
To mask ALERTs due to local temperature, set Bit 5 of the
consecutive ALERT register to 1. Default = 0.
To mask ALERTs due to Remote 1 temperature, set Bit 1 of the
Configuration 1 register to 1. Default = 0.
To mask ALERTs due to Remote 2 temperature, set Bit 0 of the
Configuration 1 register to 1. Default = 0.
To mask ALERTs due to any channel, set Bit 7 of the
Configuration 1 register to 1. Default = 0.
LOW POWER STANDBY MODE
The ADT7481 can be put into low power standby mode by
setting Bit 6 (Mon/STBY bit) of the Configuration 1 register
(Read Address 0x03, Write Address 0x09) to 1. The ADT7481
operates normally when Bit 6 is 0. When Bit 6 is 1, the ADC is
inhibited, and any conversion in progress is terminated without
writing the result to the corresponding value register.
The SMBus is still enabled in low power standby mode. Power
consumption in this standby mode is reduced to a typical of
5 μA if there is no SMBus activity, or up to 30 μA if there are
clock and data signals on the bus.
When the device is in standby mode, it is still possible to initiate
a one-shot conversion of both channels by writing to the one-
shot register (Address 0x0F), after which the device will return
to standby. It does not matter what is written to the one-shot
register, all data written to it is ignored. It is also possible to
write new values to the limit register while in standby mode.
ALERT and THERM are not available in standby mode and,
therefore, should not be used because the state of these pins is
unreliable.
SENSOR FAULT DETECTION
The ADT7481 has internal sensor fault detection circuitry at its
D+ input. This circuit can detect situations where a remote
diode is not connected, or is incorrectly connected, to the
ADT7481. If the voltage at D+ exceeds VDD − 1 V (typical), it
signifies an open circuit between D+ and D−, and consequently,
trips the simple voltage comparator. The output of this compar-
ator is checked when a conversion is initiated. Bit 2 (D1 open
flag) of the Status Register 1 (Address 0x02) is set if a fault is
detected on the Remote 1 channel. Bit 2 (D2 open flag) of the
Status Register 2 (Address 0x23) is set if a fault is detected on
the Remote 2 channel. If the ALERT pin is enabled, setting this
flag will cause ALERT to assert low.
ADT7481
Rev. 0 | Page 20 of 24
If a remote sensor is not used with the ADT7481, then the D+
and D− inputs of the ADT7481 need to be tied together to
prevent the open flag from being continuously set.
Most temperature sensing diodes have an operating temperature
range of −55°C to +150°C. Above 150°C, they lose their
semiconductor characteristics and approximate conductors
instead. This results in a diode short, setting the open flag. The
remote diode in this case no longer gives an accurate temperature
measurement. A read of the temperature result register will give
the last good temperature measurement. The user should be
aware that while the diode fault is triggered, the temperature
measurement on the remote channels is likely to be inaccurate.
INTERRUPT SYSTEM
The ADT7481 has two interrupt outputs, ALERT and THERM.
Both outputs have different functions and behavior. ALERT is
maskable and responds to violations of software-programmed
temperature limits or an open-circuit fault on the remote diode.
THERM is intended as a fail-safe interrupt output that cannot
be masked.
If the Remote 1, Remote 2, or local temperature exceeds the
programmed high temperature limits, or equals or exceeds the
low temperature limits, the ALERT output is asserted low. An
open-circuit fault on the remote diode also causes ALERT to
assert. ALERT is reset when serviced by a master reading its
device address, provided the error condition has gone away, and
the status register has been reset.
The THERM output asserts low if the Remote 1, Remote 2, or
local temperature exceeds the programmed THERM limits. The
THERM temperature limits should normally be equal to or
greater than the high temperature limits. THERM is automatically
reset when the temperature falls back within the (THERM −
hysteresis) limit. The local and remote THERM limits are set by
default to 85°C. A hysteresis value can be programmed, in
which case THERM will reset when the temperature falls to the
limit value minus the hysteresis value. This applies to both local
and remote measurement channels. The power-on hysteresis
default value is 10°C, but this may be reprogrammed to any
value after power-up.
The hysteresis loop on the THERM outputs is useful when
THERM is used for on/off control of a fan. The user’s system can
be set up so that when THERM asserts, a fan can be switched on
to cool the system. When THERM goes high again, the fan can
be switched off. Programming a hysteresis value protects from
fan jitter, a condition wherein the temperature hovers around
the THERM limit, and the fan is constantly being switched on
and off.
Table 16. THERM Hysteresis
THERM Hysteresis Binary Representation
0°C 0 000 0000
1°C 0 000 0001
10°C 0 000 1010
Figure 20 shows how the THERM and ALERT outputs operate.
A user may wish to use the ALERT output as a SMBALERT to
signal to the host via the SMBus that the temperature has risen.
The user could use the THERM output to turn on a fan to cool
the system, if the temperature continues to increase. This
method would ensure that there is a fail-safe mechanism to cool
the system, without the need for host intervention.
05466-015
1
32
4
THERM LIMIT
HIGH TEMP LIMIT
THERM LIMIT-HYSTERESIS
RESET BY MASTER
ALERT
THERM
100°C
T
EMPERATUR
E
90°C
80°C
70°C
60°C
50°C
40°C
Figure 20. Operation of the ALERT and THERM Interrupts
• If the measured temperature exceeds the high temperature
limit, the ALERT output will assert low.
• If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low. This can be
used to throttle the CPU clock or switch on a fan.
• The THERM output de-asserts (goes high) when the tempera-
ture falls to THERM limit minus hysteresis. In Figure 20, the
default hysteresis value of 10°C is shown.
• The ALERT output de-asserts only when the temperature has
fallen below the high temperature limit, and the master has
read the device address and cleared the status register.
Pin 8 on the ADT7481 can be configured as either an ALERT
output or as an additional THERM output. THERM2 will assert
low when the temperature exceeds the programmed local and/or
remote high temperature limits. It is reset in the same manner
as THERM, and it is not maskable. The programmed hysteresis
value also applies to THERM2.
ADT7481
Rev. 0 | Page 21 of 24
Figure 21 shows how THERM and THERM2 might operate
together to implement two methods of cooling the system. In
this example, the THERM2 limits are set lower than the
THERM limits. The THERM2 output could be used to turn on
a fan. If the temperature continues to rise and exceeds the
THERM limits, the THERM output could provide additional
cooling by throttling the CPU.
THERM2 LIMIT
THERM LIMIT
05466-016
THERM2 1
3
2
4
TEMPERATURE
THERM
90°C
80°C
70°C
60°C
50°C
40°C
30°C
Figure 21. Operation of the THERM and THERM2 Interrupts
When the THERM2 limit is exceeded, the THERM2 signal
asserts low.
• If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low.
• The THERM output de-asserts (goes high) when the tem-
perature falls to THERM limit minus hysteresis. In Figure 21,
there is no hysteresis value shown.
• As the system cools further, and the temperature falls below
the THERM2 limit, the THERM2 signal resets. Again, no
hysteresis value is shown for THERM2.
The temperature measurement could be either the local or the
remote temperature measurement.
ADT7481
Rev. 0 | Page 22 of 24
APPLICATIONS INFORMATION
NOISE FILTERING
For temperature sensors operating in noisy environments, previ-
ous practice was to place a capacitor across the D+ and D− pins
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.
FACTORS AFFECTING DIODE ACCURACY
Remote Sensing Diode
The ADT7481 is designed to work with substrate transistors
built into processors or with discrete transistors. Substrate
transistors will generally be PNP types with the collector
connected to the substrate. Discrete types can be either a PNP
or an NPN transistor connected as a diode (base shorted to
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 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 de-
viation of the thermal diode from ideal behavior. The ADT7481
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 where nf does not equal 1.008. Consult the
processor data sheet for the nf values.
(
)
(
)
TKelvinnT f+×=Δ 15.273008.1/008.1–
To factor this in, the user can write the ΔT value to the offset
register. It will then automatically be added to, or subtracted
from, the temperature measurement by the ADT7481.
• Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7481, IHIGH, is 233 μA. The low level current, ILOW, is
14 μA. If the ADT7481 current levels do not match the
current levels specified by the CPU manufacturer, it may
become necessary to remove an offset. The CPU data sheet
will advise whether this offset needs to be removed and how
to calculate it. This offset may be programmed to the offset
register. It is important to note that if more than one offset
must be considered, the algebraic sum of these offsets must
be programmed to the offset register.
If a discrete transistor is being used with the ADT7481, the best
accuracy is obtained by choosing devices according to the
following criteria:
• Base-emitter voltage greater than 0.25 V at 6 μA, at the
highest operating temperature.
• Base-emitter voltage less than 0.95 V at 100 μA, at the lowest
operating temperature.
• Base resistance less than 100 Ω.
• Small variation in hFE (say 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.
THERMAL INERTIA AND SELF-HEATING
Accuracy depends on the temperature of the remote sensing
diode and/or the local temperature sensor being at the same
temperature as that being measured. A number of factors can
affect this. Ideally, the sensor should be in good thermal contact
with the part of the system being measured; otherwise, the
thermal inertia caused by the sensor’s mass causes a lag in the
response of the sensor to a temperature change.
In the case of the remote sensor, this should not be a problem,
since it will either be a substrate transistor in the processor or a
small package device, such as an SOT-23, placed in close
proximity to it.
The on-chip sensor, however, will often be remote from the
processor and only monitors the general ambient temperature
around the package. In practice, the ADT7481 package will be
in electrical, and hence, thermal contact with a PCB and may
also be in a forced airflow. How accurately the temperature of
the board and/or the forced airflow reflects the temperature to
be measured will also affect the accuracy of the measurement.
Self-heating, due to the power dissipated in the ADT7481 or the
remote sensor, causes the chip temperature of the device (or
remote sensor) to rise above ambient. However, the current
forced through the remote sensor is so small that self-heating is
negligible. The worst-case condition occurs when the ADT7481
is converting at 64 conversions per second while sinking the
maximum current of 1 mA at the ALERT and THERM output.
In this case, the total power dissipation in the device is about
4.5 mW. The thermal resistance, θJA, of the MSOP-10 package is
about 142°C/W.
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments, and the
ADT7481 measures very small voltages from the remote sensor,
so care must be taken to minimize noise induced at the sensor
inputs. Take the following precautions:
• Place the ADT7481 as close as possible to the remote sensing
diode. Provided that the worst noise sources such as clock
ADT7481
Rev. 0 | Page 23 of 24
generators, data/address buses, and CRTs are avoided, this
distance can range from 4 inches to 8 inches.
• Route the D+ and D− tracks close together, in parallel, with
grounded guard tracks on each side. To minimize inductance
and reduce noise pick-up, a 5 mil track width and spacing is
recommended. Provide a ground plane under the tracks if
possible.
GND
D+
D–
GND
05466-017
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
Figure 22. Typical Arrangement of Signal Tracks
• Try to minimize the number of copper/solder joints that can
cause thermocouple effects. Where copper/solder joints are
used, make sure that they are in both the D+ and D− path
and at the same temperature.
• Thermocouple effects should not be a major problem as
1°C corresponds to about 200 mV, and thermocouple
voltages are about 3 mV/°C of temperature difference.
• Unless there are two thermocouples with a large temperature
differential between them, thermocouple voltages should be
much less than 200 mV.
• Place a 0.1 μF bypass capacitor close to the VDD pin. In
extremely noisy environments, an input filter capacitor may
be placed across D+ and D− close to the ADT7481. This
capacitance can affect the temperature measurement, so care
must be taken to ensure that any capacitance seen at D+ and
D− is a maximum of 1,000 pF. This maximum value includes
the filter capacitance, plus any cable or stray capacitance
between the pins and the sensor diode.
• If the distance to the remote sensor is more than 8 inches, the
use of twisted pair cable is recommended. A total of 6 feet to
12 feet of cable is needed.
• For really long distances (up to 100 feet), use shielded twisted
pair, such as Belden No. 8451 microphone cable. Connect the
twisted pair to D+ and D− and the shield to GND close to the
ADT7481. Leave the remote end of the shield unconnected to
avoid ground loops.
Because the measurement technique uses switched current
sources, excessive cable or filter capacitance can affect the
measurement. When using long cables, the filter capacitance
can be reduced or removed.
APPLICATION CIRCUIT
Figure 23 shows a typical application circuit for the ADT7481,
using discrete sensor transistors. The pull-ups on SCLK,
SDATA, and ALERT are required only if they are not already
provided elsewhere in the system.
The SCLK and SDATA pins of the ADT7481 can be interfaced
directly to the SMBus of an I/O controller, such as the Intel® 820
chipset.
5V OR 12V
SMBUS
CONTROLLER
FAN CONTROL
CIRCUIT
2N3904/06
OR
CPU THERMAL
DIODE
D1+
D1–
D2+
D2–
V
DD
SCLK
SDATA
ALERT
THERM
GND
ADT7481
0.1μF
V
DD
TYP 10kΩ
FAN ENABLE
3V TO 3.6V
TYP 10kΩ
05466-018
Figure 23. Typical Application Circuit
ADT7481
Rev. 0 | Page 24 of 24
OUTLINE DIMENSIONS
0.23
0.08
0.80
0.60
0.40
8°
0°
0.15
0.00 0.27
0.17
0.95
0.85
0.75
SEATING
PLANE
1.10 MAX
10 6
5
1
0.50 BSC
3.00 BSC
3.00 BSC
4.90 BSC
PIN 1
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-187-BA
Figure 24. 10-Lead Mini Small Outline Package [MSOP]
(RM-10)
Dimensions shown in millimeters
ORDERING GUIDE
Model Operating Temperature Range Package Description Package Option Branding SMBus Address
ADT7481ARMZ1−40°C to +125°C 10-Lead MSOP RM-10 T08 4C
ADT7481ARMZ-REEL1−40°C to +125°C 10-Lead MSOP RM-10 T08 4C
ADT7481ARMZ-REEL71−40°C to +125°C 10-Lead MSOP RM-10 T08 4C
ADT7481ARMZ-11−40°C to +125°C 10-Lead MSOP RM-10 T0M 4B
ADT7481ARMZ-1REEL1−40°C to +125°C 10-Lead MSOP RM-10 T0M 4B
ADT7481ARMZ-1REEL71−40°C to +125°C 10-Lead MSOP RM-10 T0M 4B
1 Z = Pb-free part.
©2005 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D05466–0–7/05(0)
