NCT7802Y Datasheet, PDF (19/87 Pages) Nuvotem Talema – Hardware Monitoring IC NCT7802Y with PECI 3.0 interface

NCT7802Y
In this illustration, when PECI temperature is -20, the PWM duty cycle for fan control is at Duty2.
When CPU is getting hotter and the PECI temperature is -10, the PWM duty cycle is at Duty1.
At Tcontrol PECI temperature, the recommendation from Intel is to operate the CPU fan at full speed.
Therefore Duty1 is 100% if this recommendation is followed. The value of Tcontrol can be obtained by
reading the related Machine Specific Register (MSR) in the Intel CPU. The Tcontrol MSR address is
usually in the BIOS Writer‟s guide for the CPU family in question. Refer to the relevant CPU
documentation from Intel for more information. In this example, Tcontrol is -10.
When the PECI temperature is below -20, the duty cycle is fixed at Duty2 to maintain a minimum (and
constant) RPM for the CPU fan.
NCT7802Y‟s fan control circuit can only accept positive real-time temperature inputs and limits setting
(when SMART FAN^TM IV mode). The device provides offset registers to „shift‟ the negative PECI
readings to positive values otherwise the fan control circuit will not function properly. The offset
registers are the Tbase registers located at Bank1, Index09h and Index0Ah. All default values of these
Tbase registers are 8‟h00.These registers should be programmed with (positive) values so that the
resultant value (Tbase + PECI) is always positive. The unit of the Tbase register contents is “count” to
match that of PECI values. The resultant value (Tbase + PECI) should not be interpreted as the
“temperature” (whether in count or °C) of the PECI client (CPU).
Figure 6-4 Temperature and Fan Speed Relation after Tbase Offsets shows the temperature/fan
speed relationship after Tbase offsets are applied (based on Figure 6-3 PECI Temperature). This view
is from the perspective of the NCT7802Y fan control circuit.
Figure 6-4 Temperature and Fan Speed Relation after Tbase Offsets
Assuming Tbase is set to 100 and the PECI temperature is -15 , the real-time temperature value to the
fan control circuit will be 85 (-15 + 100). The value of 55 (hex) will appear in the relevant real-time
temperature register.
While using SMART FAN^TM IV control function of NCT7802Y, BIOS/software must include Tbase in
determining the thresholds (limits). In this example, assuming Tcontrol is -10 and Tbase is set to 100,
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Feb, 2012
Version: 1.2