ASC7521A - Andigilog, Inc. - Product.Network

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LOW-OLTAGE 1-WIRE DIGITAL TEMPERATURE SENSOR

aSC7521

PRODUCT SPECIFICATION

Product Description

The aSC7521A is a high-precision CMOS temperature sensor

and voltage monitor with Simple Serial Transport (SST)

compatible serial digital interface, intended for use in PC

hardware monitor applications.

Communication of device capabilities and temperature

readings takes place over the high-speed bi-directional SST

interface.

The SST temperature sensor provides a means for an analog

signal to travel over a digital bus enabling remote temperature

sensing in areas previously not monitored in the PC. The

temperature sensor supports an internal and external thermal

diode.

Sensor localization is aided by two address pins that

distinguish multiple sensors on the same bus.

The aSC7521A is available in MSOP-8 surface mount

package.

Features

• On-chip and remote temperature sensors

• Accuracy:

o +/- 3°C over operational range

o Internal +/- 2°C over 40°C to 70°C

o Remote +/- 1°C over 50°C to 70°C

• Operational Range: -40°C to 125°C

• Temperature resolution: 0.125°C

• 1-wire SST serial interface

• Negotiable SST signaling rate up to 2-Mbps

• Temperature and errors reported over SST

• Internally corrected for diode non-ideality and series

resistance

• 3-state address pins set one of 9 SST bus address

0x48 through 0x50

• 8-lead MSOP package

• MSL-1 per JEDEC J-STD-020C

• Pb-free Matte Sn leadfinish & RoHS Compliant

Packages

Pin Configuration

VDD 8 SST

GND

DD0

D+ GND

Applications

Desktop and Notebook Computers

Application Diagram

Ordering Information

Part Number Package Temp. Range and Operating

Voltage Marking Supplied In

aSC7521AM8 8-Lead MSOP -40°C to 125°C, 3.3V 521A

Ayww

2500 units Tape &

Reel

Ayww – Assembly site, year, workweek

D-

aSC7521A

ADD1

CPU

3.3V

aSC7521A

SST

Interface SST ADD0

ADD1

2, 6

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aSC7521A

Absolute Maximum Ratings1

Parameter Rating

Supply Voltage, VDD -0.3, +3.63V

Voltage on any Digital Input or Output3-0.3V to VDD +

0.3V

Input Current on any pin3±5mA

Package Input Current3±20mA

Relative Humidity (non-operating) 5% - 85% RH

@ 25°C to 70°C

Maximum Junction Temperature, TJmax 150°C

Storage Temperature Range -60°C to +150°

IR Reflow Peak Temperature 260°C

Lead Soldering Temperature (10 sec.) 300°C

Human Body Model 2000 V

Machine Model 250 V

ESD5

Charged-Device Model >1000 V

Notes:

1. Absolute maximum ratings are limits beyond which operation

may cause permanent damage to the device. These are

stress ratings only; functional operation at or above these

limits is not implied. For guaranteed specifications and test

conditions, see the Electrical Characteristics. The guaranteed

specifications apply only for the test conditions listed. Some

performance characteristics may degrade when the device is

not operated under the listed test conditions.

2. All voltages are measured with respect to GND, unless

otherwise specified.

3. When the input voltage (VIN) at any pin exceeds the power

supplies (VIN< (GND or GNDA) or VIN>V+, except for SST

and analog voltage inputs), the current at that pin should be

limited to 5mA. The 20mA maximum package input current

rating limits to number of pins that can safely exceed the

power supplies with an input current of 5mA to four.

4. The maximum power dissipation must be de-rated at elevated

temperatures and is dictated by TJmax, JA and the ambient

temperature, TA. The maximum allowable power dissipation at

any temperature is PD = (TJmax - TA) / JA. It must also take

into account self-heating that can adversely affect the

accuracy of internal sensors.

5. Human Body Model: 100pF capacitor discharged through a

1.5kΩ resistor into each pin. Machine Model: 200pF capacitor

discharged directly into each pin. Charged-Device Model is

per JESD22-C101C.

Electrical Characteristics6

(-40°C≤T≤+125°C, V = 3.3V unless otherwise noted. Specifications subject to change without notice)

ADD

Parameter Conditions Min Typ Max Units

Supply Voltage VDD 3.0 3.3 3.6 V

SST Signal Meets SST Specification Version 1.0

for 1.5V interface

-40°C≤TA≤+125°C ±3 °C

Local Sensor Accuracy7, 8

40°C≤TA≤70°C ±2 °C

Local Sensor Resolution 0.125 °C

0°C≤TA≤70°C,

-40°C≤TD ≤+125°C ±3 °C

Remote Diode Sensor Accuracy7, 8, 9

0°C≤TA≤70°C,

50°C≤TD ≤70°C ±1 °C

Remote Diode Sensor Resolution 0.125 °C

Temperature Monitor Cycle Time10 tC 0.2 Sec

Notes:

6. These specifications are guaranteed only for the test conditions listed.

7. Accuracy (expressed in °C) = Difference between the aSC7521A reported temperature and the device temperature.

8. The aSC7521A can be read at any time without interrupting the temperature conversion process.

9. Calibration of the remote diode sensor input is set to meet the accuracy limits with a CPU thermal diode that has a non-ideality factor of

1.009 with a series resistance of 4.52.

10. Total monitoring cycle time for all temperature and analog input voltage measurements is 0.2 second.

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aSC7521A

Pin Descriptions

Pin # Name Direction Description

1 VDD (3.3V) Supply Supply Voltage, 3.3V +/- 10%

2 GND Supply Ground

3 D+ Current Source Remote Diode Anode or Positive Lead

4 D- Current Sink Remote Diode Cathode or Negative Lead

5 ADD1 Input

Device Address Tri-State selector: Ground, Float or VDD select device

address

6 GND Supply Ground

7 ADD0 Input

Device Address Tri-State selector: Ground, Float or VDD select device

address

8 SST Input Digital Input / Output. SST Bi-directional Data Line.

Figure 1. Block Diagram

On-

Chip

Sensor

Control and SST Interface

ADC

Remote

Diode

Open / Short

ADD1ADD0 SST

D +

D -

Chip

emp

erature

Remote

Temp

erature

DIB Register

VDD VSS

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aSC7521A

SST Sensors

The SST temperature sensor provides a means for an analog

signal to travel over a single-wire digital bus enabling remote

temperature sensing in areas previously not monitored in the

PC. The temperature sensor supports an internal temperature

sensor and external thermal diodes.

This section outlines general requirements for Simple Serial

Transport (SST) sensors intended for use in PC desktop

applications that conform to SST Version 1.0 specification.

The aSC7521A reports external temperature sensed by a

remote diode-connected transistor and an internal temperature

measurement.

Addressing

The aSC7521A complies with the address range set aside for

fixed-address, discoverable devices as defined in the SST

Specification Version 1.0. Simple Temperature sensors use

fixed addresses in the range of 0x48 to 0x50. The aSC7521A

may be programmed to any of these addresses via the

address select pins AD0 and AD1.

Frame Check Sequence (FCS)

Each message requires a frame check sequence byte to

ensure reliable data exchange between host and client. The

message originator and client both make an FCS calculation.

One FCS byte must be returned from the message target to

the originator after all bytes including the header and the data

block are written. If data is read from the target, a second FCS

byte must follow the data block read.

The FCS byte is the result of an 8-bit cyclic redundancy check

(CRC) of the each data block preceding the FCS up to the

most recent, earlier FCS byte. The first FCS in the message

does not include the two address timing negotiation ‘0’ bits that

precede the address byte or the message timing negotiation bit

after the address byte. The first FCS does include the address

byte in its computation. The FCS is initialized at 0x00 and is

calculated in a way that conforms to a CRC-8 represented by

the CRC polynomial, C(x) = x8 + x2 + x + 1.

Bus Voltage

All SST sensor devices used for PC applications must be

capable of operating the SST interface portion of the sensor

device at 1.5 volts as defined in 1.5 Volt Static (DC)

Characteristics section of the SST Version 1.0 specification.

Bus Timing

All SST sensor devices must be able to negotiate timing and

operate at a maximum bus transfer rate of 2-Mbps. If the bus

address timing is negotiated at a lower rate due to the

performance limitations of other devices on the bus, the sensor

device will operate at that lower rate.

Device Po wer-on Timing

Following a power-on reset, such as a system transitioning

from S3-S5 to S0, the aSC7521A will be able to participate in

the address and message timing negotiation and respond to

required SST bus commands such as respond to a GetDIB()

command within 10ms of the device’s VDD rail reaching 90%.

The aSC7521A has an internal power on reset and will be fully

functional within 50ms of power on.

The aSC7521A does not employ any device power

management.

Voltage and Temperature Sensor Data

Little Endian Format

The bit level transfer is defined in the SST specification. The 2-

byte data values are returned in little Endian format, in other

words, the LSB is sent first followed by MSB.

For multi-function devices that allow access to multiple

sensors, the data is returned LSB followed by the MSB for the

first sensor, LSB followed by the MSB for the second sensor,

and so on. The specific order is explicitly specified in the

command description.

Atomic Readings

The aSC7521A ensures that every value returned is derived

from a single analog to digital conversion and is not skewed

(e.g. the MSB and the LSB come from two different

conversions).

Conversion Time

The maximum refresh time for all temperature values is

200ms. The aSC7521A provides the logic to ensure all

readings meet the conversion time requirements.

Temperature Data

Data Precision, Accuracy and Resolution

The temperature data meets the following minimum

requirements:

• Operational Range: -40°C to +125°C

• Internal Sensor Accuracy:

o +/- 3°C over operational range

o +/- 2°C over 40°C to 70°C

• Remote Sensor Accuracy (when TA is from 0°C to

70°C):

o +/- 3°C over operational range

o +/- 1°C over 50°C to 70°C

• Resolution: 0.125°C

Temperature Data Format

The data format is capable of reporting temperature values in

the range of +/-512°C. The temperature sensor data is

returned as a 2’s complement 16-bit binary value. It represents

the number of 1/64°C increments in the actual reading. This

allows temperatures to be represented with approximately a

0.016°C resolution.

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aSC7521A

Values that would represent temperatures below -273.15°C (0

K or absolute zero) are reserved and are not be returned

except as specifically noted.

For the aSC7521A the required resolution is 0.125°C. Bits

[2:0] will be defined but they are beyond the required

resolution. The sign bit will indicate a negative temperature

except when reporting an error condition (see Sensor Error

Condition).

Temperature 2’s complement representation

80°C 0001 0100 0000 0000

79.875°C 0001 0011 1111 1000

1°C 0000 0000 0100 0000

0°C 0000 0000 0000 0000

-1°C 1111 1111 1100 0000

-5°C 1111 1110 1100 0000

Table 1. Temperature Representation

Sign Integer Temperature 0°C to 512°C

Fractional

Temperature

LSB 0.125°C

Always Zero

15 14 13 12 11 10 9 8 7 6  5 4 3 2 1 0

Figure 2. Temperature Reading

A to D Converter Resolution and Mapping

The mapping of the A-D converter bit values is a two’s

complement representation with the binary point between bits

5 and 6 of the 16-bit data word. Bit 15 is the sign, bits 14

through 6 are integer temperature in degrees, bits 5 down to

3 are the fractional part with 0.125°C as the LSB. The lowest

3 bits are set to zero.

Temperature Inputs

The simple temperature sensor has an internal thermal

sensor plus an external sensor using a remote diode. Both

temperature readings are internally corrected for lead

resistance and non-ideality for the thermal diode of a

Pentium™ 4, 65nM process (1.009 non-ideality, 4.35 lead

resistance). The range of measurement currents falls within

the Intel recommended range of 10A and 170A to

minimize the impact of Beta variation in the CPU substrate

thermal diode. Note that Pentium 4, 90 nM process is 1.011

non-ideality and series resistance of 3.33.

If a diode connected discrete transistor is used instead of a

CPU diode, a correction must be applied to the reading to

compensate for the difference in non-ideality. A 2N3904 NPN

transistor has a non-ideality () factor of approximately 1.04.

To correct the value reported to the actual temperature use

the following formula:

TACTUAL = TREPORTED x Transistor / 1.009

It is recommended that the actual transistor type and

manufacturers chosen for the remote sensor be

characterized for non-ideality as part of system qualification.

Sensor Error Condition

The aSC7521A has the capability to detect and report open

or shorted external diode inputs per Sensor Error Condition.

When an error or failure condition is detected, the sensor

device must return a large negative value in response to

either the GetIntTemp() or GetExtTemp() command. In this

manner software is provided with a means to determine

whether or not the sensor is working normally and that the

data returned is good.

The aSC7521A will write one of the values from the table

below to appropriate memory locations for GetIntTemp()

and/or GetExtTemp().

The aSC7521A uses the OEM defined values of 0x8102

(open) and 0x8103 (short) rather than the generic errors

defined for codes 0x8000 to 0x8003.

Error Code Description

0x8000 to 0x80FF Reserved

0x8102 Remote Diode Open

0x8103 Remote Diode Short

0x8100-0x81FF Reserved

Table 2. Error Codes

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aSC7521A

SST Interface

Multi Client Mode

Sensors operate in multi-client mode for read bit timing. Reference the SST Specification Version 1.0 for details.

SST Device Commands

GetDIB() Command (0xF7)

Read the Device Identifier Block (DIB). The read length of the command is either 8 or 16 bytes. 8 bytes is the minimum number of

bytes populated by a fixed address discoverable client.

Write Data Length: 0x01

Read Data Length: 0x08/0x10

Command Code: 0xF7

Note: Un-shaded table entries are created by the host. Shaded entries are the response bytes from the aSC7521A to the host.

# Bits # Bits

Host Sending aSC7521A Sending

Hex Value Hex Value

8 8 8 8 8

Target Address Write Length Read Length GetDIB Cmd FCS

0x48 0x01 0x10 0xF7 0xDC

8 8 8 8 8

DIB Byte 1 … DIB Byte 15 DIB Byte 16 FCS

(data) (data) (data) (data) (data dependent)

Figure 3. GetDIB() Command (16-byte read length)

8 8 8 8 8

Target Address Write Length Read Length GetDIB Cmd FCS

0x48 0x01 0x08 0xF7 0x23

8 8 8 8 8

DIB Byte 1 … DIB Byte 7 DIB Byte 8 FCS

(data) (data) (data) (data) (data dependent)

Figure 4. GetDIB() Command (8-byte read length)

Ping() Command

The Ping() command provides a safe means for software to verify that a device is responding at a particular address.

Write Data Length: 0x00

Read Data Length: 0x00

Command Code: none

8 8 8 8

Target Address Write Length Read Length FCS

0x48 0x00 0x00 0xD7

Figure 5. Example of Ping()

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aSC7521A

ResetDevice() Command

The ResetDevice() command is used to reset all device functions to their power-on reset values. It is used by the system to recover

from serious hardware or bus errors.

Write Data Length: 0x01

Read Data Length: 0x00

Command Code: 0xF6

8 8 8 8 8

Target Address Write Length Read Length ResetDevice

Command FCS

0x48 0x01 0x00 0xF6 0x8C

Figure 6. ResetDevice() format ta rgeting a non-default address

8 8 8 8

Target Address Write Length Read Length ResetDevice

Command

0x00 0x01 0x00 0xF6

Figure 7. ResetDevice() format ta rgeting the default address

Sensor Command Summary

GetIntTemp()

Returns the temperature of the device’s internal thermal sensor.

Write Data Length: 0x01

Read Data Length: 0x02

Command Code: 0x00

Example bus transaction for a thermal sensor device located at address 0x48 returning a value of 60°C:

8 8 8 8

Target Address Write Length Read Length Command

0x48 0x01 0x02 0x00

8 8 8 8

FCS LSB MSB FCS

0x6A 0x00 0x0F 0x2D

Figure 8. Get Internal Temperature Command Example

GetExtTemp()

Returns the temperature of the external thermal diode.

Write Data Length: 0x01

Read Data Length: 0x02

Command Code: 0x01

GetAllTemps()

Returns a 4-byte block of data containing both the Internal and External temperatures in the following order Internal then External

temperatures.

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aSC7521A

Write Data Length: 0x01

Read Data Length: 0x04

Command Code: 0x00

Optional SST Device Commands

The optional SST commands Alert(), Suspend() are not supported in the aSC7521A.

Vendor Specific Extensions

The vendor specific command codes are in the range from 0xE0 and 0xE7. Reading and writing to specific internal registers is

provided for custom tuning of sensor response characteristics.

WriteReg()

Writes to the sensor’s internal registers.

Write Data Length: 2+N (command + address + Number of bytes to write)

Read Data Length: 0x00

Command Code: 0xE0

Example bus transaction to write to a sensor located at address 0x48. This example writes 2 consecutive locations (0x20 and 0x21)

to values 0x25 and 0x28.

8 8 8 8

Target Address Write Length Read Length Command

0x48 0x04 0x00 0xE0

8 8 8 8

RAM Addr Write Data Write Data FCS

0x20 0x25 0x28 0x1B

Figure 9. Example Register Write

ReadReg()

Reads from the sensor’s internal registers.

Write Data Length: 0x02 (command + address)

Read Data Length: N (Number of bytes to read)

Command Code: 0xE1

Example bus transaction to read a sensor located at address 0x48. This example reads 2 consecutive locations (0x20 and 0x21).

8 8 8 8

Target Address Write Length Read Length Command

0x48 0x02 0x02 0xE1

8 8 8 8

RAM Addr FCS Read Data Read Data FCS

0x20 0x9D 0x25 0x28 0x37

Figure 10. Example Register Read

VenCmdEnable()

Vendor Command Enable enables the Vendor Specified Extensions.

Write Data Length: 0x01

Read Data Length: 0x00

Command Code: 0xE2

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aSC7521A

8 8 8 8 8

Target Address Write Length Read Length Command FCS

0x48 0x01 0x00 0xE2 0xE0

Figure 11. Vendor Command Enable

VenCmdDisable()

Vendor Command Disable disables the Vendor Specified Extensions.

Write Data Length: 0x01

Read Data Length: 0x00

Command Code: 0xE3

8 8 8 8 8

Target Address Write Length Read Length Command FCS

0x48 0x01 0x00 0xE3 0xE7

Figure 12. Vendor Command Disable

Reserved or Unsupported Commands

Attempts to access the sensor using a reserved or unsupported command will not result in the device or bus failure. The sensor will

return a modified FCS when any of the following commands are received. To modify the FCS the sensor will invert all of the bits in

the correct FCS (1’s complement). A modified FCS is also called an Abort FCS.

The sensor will return an Abort FCS (modified FCS) for a reserved and unsupported command code (commands codes between

0xE4 to 0xF5 and 0xF8 to 0xFF).

The sensor will return an Abort FCS (modified FCS) for reserved commands (command codes 0x02 to 0xDF.

The sensor will return an Abort FCS (modified FCS) for unused vendor specific test and manufacturing command codes (command

codes 0xE8 to 0xEF). If any of these types of commands exist, they will be disabled during normal operation.

Malformed Commands

A malformed command is one which is valid but has an incorrect write or read length for the given command.

If a get temperature command with a write length not equal to 1 is sent, then the aSC7521A will send an Abort FCS and wait for a

new command. An Abort FCS will be formed by creating a 1’s complement of the the good FCS.

If a get temperature command and the read length is not equal to 2 or 4 then te aSC7521A will send an Abort FCS and wait for a

stop on the SST bus. See the Command Summary section for the expected Write and Read lengths of the legal commands.

There will be no checking for malformed WriteReg() and ReadReg() commands (Vendor Specific Extensions).

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aSC7521A

Command Summary

Hex Cmd Command Name Received Bytes Wr Len Rd Len Bytes Sent by Client

- Ping() 3(target,wr,rd) 0 0 FCS

0x00 GetIntTemp() 4(target,wr,rd,cmd) 1 2 FCS/2/FCS

0x01 GetExtTemp() 4(target,wr,rd,cmd) 1 2 FCS/2/FCS

0x00 GetAllTemps() 4(target,wr,rd,cmd) 1 4 FCS/4/FCS

0x02-0xDF Unsupported Abort FCS

0xE0 WriteReg() 4(target,wr,rd,cmd) 3+ 0 FCS

0xE1 ReadReg() 4(target,wr,rd,cmd) 2 1+ FCS/1+/FCS

0xE2 VenCmdEnable() 4(target,wr,rd,cmd) 1 0 FCS

0xE3 VenCmdDisable() 4(target,wr,rd,cmd) 1 0 FCS

0xE4-0xF5 Unsupported Abort FCS

0xF6 ResetDevice() 4(target,wr,rd,cmd) 1 0 FCS

0xF6 ResetDevice() 4(target,wr,rd,cmd) 1 0 None if default address (0x00)

0xF7 GetDIB() 4(target,wr,rd,cmd) 1 8 FCS/8/FCS

0xF7 GetDIB() 4(target,wr,rd,cmd) 1 16 FCS/16/FCS

0xF8-0xFF Unsupported Abort FCS

Table 3. Command Summary

Device Identifier Block (DIB)

The Device Identifier Block describes the identity and functions of a client device on the SST bus. Sixteen bytes are allocated for

this function as shown in Figure 13. Device Identifier Block is returned by the aSC7521A with a GetDIB() command. The aSC7521A

returned values are shown with the description of each field below.

8 8 16 16 8

Vendor ID Device ID

Device

Capabilities

Version/

Revision LSB MSB LSB MSB

Device

Interface

8 8 8 16 24 8

Function

Interface

Device

Interface

Extension

Reserved Reserved Vendor

Specific ID

Client

Device

Address

Figure 13. Device Identifier Block

Device Capabilities Field (1-byte)

MSB 6 5 4 3 2 1 LSB

Address

Type Reserved Wake

Capable

Alert

Support

Suspend

Support

Slow

Device

110 0 0 0 0 0

Figure 14. Device Capabilities Field

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aSC7521A

Version / Revision Field (1-byte)

MSB 6 5 4 3 2 1 LSB

Pre-

release SST Version Minor Revision

1 001 0000 (default) for V1.0 Pre-production

0 001 0000 for V1.0 Production

Figure 15. Version / Revision Field

Vendor ID Field (2-by tes)

Andigilog Vendor ID is 16 bits = 0x19C9 (This field is stored in the format LS Byte, MS Byte = 0xC919). Vendor IDs can be found

at: http://www.pcisig.com/membership/vid_search

Device ID Field (2-bytes)

This field uniquely identifies the device from a specific vendor. Place the least significant byte as the first byte and the most

significant byte as the second byte.

Part Number Value (MS,LS) Stored Value (LS,MS)

aSC7521A 0x7521 0x2175

Device Interface Field (1-byte)

The vendor sets to ‘1’, bit positions in this field in the event the device supports higher layer protocols that are industry specific using

Table 4.

Value = 0x02

Bit Protocol Meaning

7 - Reserved for future use , must be set = ‘0’

6 - Reserved for future use , must be set = ‘0’

5 IPMI Device supports additional access and capabilities per the IPMI specification.

4 ASF Device supports additional access and capabilities per the ASF specification.

3 Serial-ATA Device supports additional access and capabilities per the serial-ATA specification.

2 PCI-Express Device supports additional access and capabilities per the PCI Express

specification.

1 SST

Device supports additional access and capabilities per the SST Functional

Descriptor Specification (to be published at a future date).

0 OEM

Device supports vendor-specific additional access and capabilities per the Vendor

ID and Device ID.

Table 4. Device Interface Field

Function Interface Field (1-byte)

This field provides a mechanism for a device to pass higher-layer SST device-specific information.

Value = 0x00

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aSC7521A

Device Interface Extension Field (1-byte)

This field is used to provide additional information about the device to the upper layers of software.

Value = 0x00

Reserved Field (3-bytes)

Value = 0x00 0x00 0x00

Vendor Specific ID Field (1-byte)

This field is set by the vendor in a way that uniquely identifies this device apart from all others with an otherwise common DIB

content.

Value = 0x00 – For Fixed address devices this field may be set to zero.

Client Device Address (1-by te)

SST Client Device Address is set according to the connection of pins ADD0 and ADD1. Float is defined as an unconnected pin.

ADD0 ADD1 Address

Ground Ground 0x48

Float Ground 0x49

VDD Ground 0x4A

Ground Float 0x4B

Float Float 0x4C

VDD Float 0x4D

Ground VDD 0x4E

Float VDD 0x4F

VDD VDD 0x50

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aSC7521A

Applications Information

Remote Diodes

The aSC7521A is designed to work with a variety of remote

sensors in the form of a diode-connected transistor or the

substrate thermal diode of a CPU or graphics controller.

Actual diodes are not suited for these measurements.

There is some variation in the performance of these diodes,

described in terms of its departure from the ideal diode

equation. This factor is called diode non-ideality,

.nf

The equation relating diode temperature to a change in

thermal diode voltage with two driving currents is:

VBE = (nf)KT

qln(N)

where:

nf = Pentium 4, 65nM non-ideality factor, (nominal 1.009).

= Boltzman’s constant, (1.38 x 10-23).

= diode junction temperature in Kelvins.

q= electron charge (1.6 x 10-19 Coulombs).

N= ratio of the two driving currents (10).

The aSC7521A is designed and trimmed for an expected

value of 1.009, based on the typical value for the Pentium 4,

65nM. There is also a tolerance on the value provided. The

values for CPUs may have different nominal values and

tolerances. Consult the CPU or GPU manufacturer’s data

sheet for the factor.

nf Table 5 gives a representative

sample of what one may expect in the range of non-ideality.

The trend with CPUs is for a lower value with a larger spread.

When thermal diode has a non-ideality factor other than

1.0046 the difference in temperature reading at a particular

temperature may be interpreted with the following equation:

⎟

⎠

⎞

⎜

⎝

⎛

=actual

reportedactual n

T T 009.1

where:

reported

T= reported temperature in temperature register.

actual

T= actual remote diode temperature.

actual

n= selected diode’s non-ideality factor, .

Temperatures are in Kelvins or °C + 273.15.

This equation assumes that the series resistance of the

remote diode 4.52.

Although the temperature error caused by non-ideality

difference is directly proportional to the difference from 1.009,

but a small difference in non-ideality results in a relatively

large difference in temperature reading. For example, if there

were a ±1% tolerance in the non-Ideality of a diode it would

result in a ±2.7 degree difference (at 0°C) in the result (0.01 x

273.15).

This difference varies with temperature such that a fixed

offset value may only be used over a very narrow range.

Typical correction method required when measuring a wide

range of temperature values is to scale the temperature

reading in the host firmware.

Part nf Min nf Nom nf Max Series

Res

Pentium™ III

(CPUID 68h) 1.0057 1.008 1.0125

Pentium 4,

130nM 1.001 1.002 1.003 3.64

Pentium 4, 90nM 1.011 3.33

Pentium 4, 65nM 1.000 1.009 1.050 4.52

Intel Pentium M 1.0015 1.0022 1.0029 3.06

2N3904 1.003 1.0046 1.005 0.6

Table 5 Representative CPU Thermal Diode and

Transistor Non-Ideality Factors

CPU or ASIC Substrate Remote Dio des

A substrate diode is a parasitic PNP transistor that has its

collector tied to ground through the substrate and the base

(D-) and emitter (D+) brought out to pins. Connection to

these pins is shown in Figure 16 CPU Remote Diode

Connection. The non-ideality figures in Table 5

Representative CPU Thermal Diode and Transistor Non-

Ideality Factors include the effects of any package resistance

and represent the value seen from the CPU socket. The

temperature indicated will need to be compensated for the

departure from a non-ideality of 1.0046 and series resistance

of 0.6 .

Figure 16 CPU Remote Diode Connection

Discrete Remote Diodes

When sensing temperatures other than the CPU or GPU

substrate, an NPN or PNP transistor may be used. Most

commonly used are the 2N3904 and 2N3906. These have

characteristics similar to the CPU substrate diode with non-

ideality around 1.0046. They are connected with base to

collector shorted as shown in Figure 17 Discrete Remote

Diode Connection.

D+

D-

CPU aSC7521

Substrate

- 14 -

aSC7521A

While it is important to minimize the distance to the remote

diode to reduce high-frequency noise pickup, they may be

located many feet away with proper shielding. Shielded,

twisted-pair cable is recommended, with the shield

connected only at the aSC7521A end as close as possible to

the ground pin of the device.

Figure 17 Discrete Remote Diode Connection

As with the CPU substrate diode, the temperature reported

will be subject to the same errors due to non-ideality variation

and series resistance. However, the transistor’s die

temperature is usually not the temperature of interest and

care must be taken to minimize the thermal resistance and

physical distance between that temperature and the remote

diode. The offset and response time will need to be

characterized by the user.

Series Resistance

Any external series resistance in the connections from the

aSC7521A to the CPU pins should be accounted for in

interpreting the results of a measurement.

The impact of series resistance on the measured

temperature is a result of measurement currents developing

offset voltages that add to the diode voltage. This is relatively

constant with temperature and may be corrected with a fixed

value in the offset register. To determine the temperature

impact of resistance is as follows:

Ω°×=

⎟

⎠

⎞

⎜

⎝

⎛

×=Δ

Δ××=Δ

/675.0

/200

135

CV A

R T

IT R T

SSR

DVSR

where:

TR= difference in the temperature reading from actual.

R= total series resistance of interconnect (both leads).

ID= difference in the two diode current levels (135µA).

T= scale of temperature vs. VBE (200µV/°C).

For example, a total series resistance of 10 would give an

offset of +6.75°C.

Board Layout Considerations

The distance between the remote sensor and the aSC7521A

should be minimized. All wiring should be defended from high

frequency noise sources and a balanced differential layout

maintained on D+ and D-.

Any noise, both common-mode and differential, induced in

the remote diode interconnect may result in an offset in the

temperature reported. Circuit board layout should follow the

recommendation of Figure 18. Basically, use 10-mil lines and

spaces with grounds on each side of the differential pair.

Choose the ground plane closest to the CPU when using the

CPU’s remote diode.

Figure 18 Recommended Remote Diode Circuit

Board Interconnect

Noise filtering is accomplished by using a bypass capacitor

placed as close as possible to the aSC7521A D+ and D-

pins. A 1.0nF ceramic capacitor is recommended, but up to

3.3nF may be used. Additional filtering takes place within the

aSC7521A.

It is recommended that the following guidelines be used to

minimize noise and achieve highest accuracy:

1. Place a 0.1µF bypass capacitor to digital ground as

close as possible to the power pin of the aSC7521A.

2. Match the trace routing of the D+ and D- leads and

use a 1.0nF filter capacitor close to the aSC7521A.

Use ground runs along side the pair to minimize

differential coupling as in Figure 18.

3. Place the aSC7521A as close to the CPU or GPU

remote diode leads as possible to minimize noise and

series resistance.

4. Avoid running diode connections close to or in parallel

with high-speed busses, staying at least 2cm away.

5. Avoid running diode connections close to on-board

switching power supply inductors.

6. PC board leakage should be minimized by maintaining

minimum trace spacing and covering traces over their

full length with solder mask.

Thermal Considerations

The temperature of the aSC7521A will be close to that of the

PC board on which it is mounted. Conduction through the

leads is the primary path for heat flow. The reported local

10 mils

D +

10 mils D -

GND

D -

2N3904

D +

aSC7521A

2N3906

D +

aSC7521A

D -

- 15 -

aSC7521A

sensor is very close to the circuit board temperature and

typically between the board and ambient.

In order to measure PC board temperature in an area of

interest, such as the area around the CPU where voltage

regulator components generate significant heat, a remote

diode-connected transistor should be used. A surface-mount

SOT-23 or SOT-223 is recommended. The small size is

advantageous in minimizing response time because of its low

thermal mass, but at the same time it has low surface area

and a high thermal resistance to ambient air. A compromise

must be achieved between minimizing thermal mass and

increasing the surface area to lower the junction-to-ambient

thermal resistance.

In order to sense temperature of air-flows near board-

mounted heat sources, such as memory modules, the sensor

should be mounted above the PC board. A TO-92 packaged

transistor is recommended.

The power consumption of the aSC7521A is relatively low

and should have little self-heating effect on the local sensor

reading. At the highest measurement rate the dissipation is

less than 2mW, resulting in only a few tenths of a degree

rise.

- 16 -

aSC7521A

M8 Package – 8-Lead MSOP Package Dimensions

Pb-Free Package

2.85mm (min)

3.05mm (max)

4.75mm (min)

5.05mm (max)

0.95mm BSC

2.90mm (min)

3.10mm (max)

0.25mm (min)

0.40mm (max)

2.90mm (min)

3.10mm (max)

4.75mm (min)

5.05mm (max)

0.65mm BSC

1.10mm (max)

0.525mm BSC

α 0° (min)

6°

(max)

2.85mm (min)

3.05mm (max)

0.10mm

0.78mm (min)

0.94mm (max)

0.05mm (min)

0.15mm (max)

2.90mm (min)

3.10mm (max)

0.25mm (min)

0.40mm (max)

0.25mm (min)

0.35mm (max)

0.13mm (min)

0.23mm (max)

0.13mm (min)

0.18mm (max)

0° (min)

6° (max)

9° (min)

15°

(max)

0.40mm (min)

0.70mm (max)

Detail B

Section A

Detail B

Andigilog, Inc.

8380 S. Kyrene Rd., Suite 101

Tempe, Arizona 85284

Tel: (480) 940-6200

Fax: (480) 940-4255

- 17 -

aSC7521A

Notes:

Andigilog, Inc.

8380 S. Kyrene Rd., Suite 101

Tempe, Arizona 85284

Tel: (480) 940-6200

Fax: (480) 940-4255

- 18 -

aSC7521A

Data Sheet Classifications

Preliminary Specification

This classification is shown on the heading of each page of a specification for products that are either under

development (design and qualification), or in the formative planning stages. Andigilog reserves the right to change or

discontinue these products without notice.

New Release Specification

This classification is shown on the heading of the first page only of a specification for products that are either under

the later stages of development (characterization and qualification), or in the early weeks of release to production.

Andigilog reserves the right to change the specification and information for these products without notice.

Fully Released Specification

Fully released datasheets do not contain any classification in the first page header. These documents contain

specification on products that are in full production. Andigilog will not change any guaranteed limits without written

notice to the customers. Obsolete datasheets that were written prior to January 1, 2001 without any header

classification information should be considered as obsolete and non-active specifications, or in the best case as

Preliminary Specifications.

Pentium™ is a trademark of Intel Corporation

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ANDIGILOG'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR

SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF ANDIGILOG,

INC. As used herein:

1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b)

support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the

labeling, can be reasonably expected to result in a significant injury to the user.

2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected

to cause the failure of the life support device or system, or to affect its safety or effectiveness.