
- 5 - 
 Andigilog, Inc. 2006 
www.andigilog.com
October 2006 - 70A05011 
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 
80°C 
79.875°C 
1°C 
0°C 
-1°C 
-5°C 
2’s complement representation 
0001  0100  0000  0000 
0001  0011  1111  1000 
0000  0000  0100  0000 
0000  0000  0000  0000 
1111  1111  1100  0000 
1111  1110  1100  0000 
Table 1. Temperature Representation
Sign 
Integer Temperature 0°C to 512°C 
Fractional 
Temperature 
LSB 0.125°C 
5 
4 
Always Zero 
15 
14 
13 
12 
11 
10 
9 
8 
7 
6 
●
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
μ
A and 170
μ
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 (
η
) factor of approximately 1.04. 
To correct the value reported to the actual temperature use 
the following formula: 
T
ACTUAL
 = T
REPORTED
 x 
η
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 
0x8000 to 0x80FF 
Reserved 
0x8102 
Remote Diode Open 
0x8103 
Remote Diode Short 
0x8100-0x81FF 
Reserved 
Description 
Table 2. Error Codes