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© Andigilog, Inc. 2006
www.andigilog.com
December 2006 - 70A05012
aSC7531A / aSC7531B
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 aSC7531 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 aSC7531A has an internal thermal sensor plus an
external sensor using a remote diode. The remote sensor is
calibrated for a 2N3904 NPN transistor that has a non-
ideality (η) factor of approximately 1.0046. Use of the remote
diode is discussed in the Applications Information section.
The aSC7531B also has an internal thermal sensor plus an
external sensor using a remote diode however the remote
sensor is calibrated for an Intel CPU (Pentium 4, 65nM) that
has a non-ideality (η) factor of approximately 1.009. Use of
the remote diode is discussed in the Applications Information
section.
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 aSC7531 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 aSC7531 will write one of the values from the table
below to appropriate memory locations for GetIntTemp()
and/or GetExtTemp().
The aSC7531 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
Voltage Data
Accuracy and Resolution
The aSC7531 measures the following voltages:
•
Measured voltages:
o
12V
o
5V
o
3.3V
o
2.5V
o
VCCP
•
Measurement Error: +/- 2% of full scale
•
Resolution: 7.8 mV
Voltage Data Format
The data format used to report voltage allows values in the
range of +/-32V. The voltage sensor data is returned as a 16-
bit 2’s complement binary value. It represents the number of
1/1024 volts in the reading if extended the full 16-bits. This
allows voltages to be represented with approximately a 1mV
resolution.
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