
- 4 - 
 Andigilog, Inc. 2006 
www.andigilog.com
October 2006 - 70A05011 
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) = x
 + x
 + 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 Power-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 V
DD
 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.