Intel I7-975 User Manual

80

Datasheet 

Fan speed control solutions use a value stored in the static variable, T

CONTROL

. The DTS 

temperature data, which is delivered over PECI (in response to a GetTemp0() 

command), is compared to this T

CONTROL

 reference. The DTS temperature is reported 

as a relative value versus an absolute value. The temperature reported over PECI is 

always a negative value and represents a delta below the onset of thermal control 

circuit (TCC) activation, as indicated by PROCHOT#. Therefore, as the temperature 

approaches TCC activation, the value approaches zero degrees. 

The processor digital thermal sensor (DTS) provides an improved capability to monitor 

device hot spots, which inherently leads to more varying temperature readings over 

short time intervals. To reduce the sample rate requirements on PECI and improve 

thermal data stability versus. time the processor DTS implements an averaging 

algorithm that filters the incoming data. This filter is expressed mathematically as: 

PECI(t) = PECI(t–1)+1/(2^^X)*[Temp – PECI(t–1)]

Where: PECI(t) is the new averaged temperature; PECI(t-1) is the previous 

averaged temperature; Temp is the raw temperature data from the DTS; X is the 

Thermal Averaging Constant (TAC)

Only values read using the PECI interface are averaged. Temperature values read using 

the IA32_THERM_STATUS MSR are not averaged.

The Thermal Averaging Constant is a BIOS configurable value that determines the time 

in milliseconds over which the DTS temperature values are averaged. Short averaging 

times will make the averaged temperature values respond more quickly to DTS 

changes. Long averaging times will result in better overall thermal smoothing but also 

incur a larger time lag between fast DST temperature changes and the value read using 

PECI. Refer to the appropriate processor Thermal and Mechanical Design Guidelines 

(see 

) for further details on the Data Filter and the Thermal Averaging 

Constant.

Within the processor, the DTS converts an analog signal into a digital value 

representing the temperature relative to TCC activation. The conversions are in 

integers with each single number change corresponding to approximately 1 °C. DTS 

values reported using the internal processor MSR will be in whole integers.

As a result of the averaging function described above, DTS values reported over PECI 

will include a 6-bit fractional value. Under typical operating conditions, where the 

temperature is close to T

CONTROL

, the fractional values may not be of interest. But when 

the temperature approaches zero, the fractional values can be used to detect the 

activation of the TCC. An averaged temperature value between 0 and 1 can only occur 

if the TCC has been activated during the averaging window. As TCC activation time 

increases, the fractional value will approach zero. Fan control circuits can detect this 

situation and take appropriate action as determined by the system designers. Of 

course, fan control chips can also monitor the PROCHOT# pin to detect TCC activation 

using a dedicated input pin on the package. Further details on how the Thermal 

Averaging Constant influences the fractional temperature values are available in the 

Thermal Design Guide.