Intel QX9775 EU80574XL088N Data Sheet

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Fan speed control solutions based on PECI utilize a T

CONTROL

 value stored in the 

processor IA32_TEMPERATURE_TARGET MSR. The T

CONTROL

 MSR uses the same offset 

temperature format as PECI though it contains no sign bit. Thermal management 

devices should infer the T

CONTROL

 value as negative. Thermal management algorithms 

should use the relative temperature value delivered over PECI in conjunction with the 

T

CONTROL

 MSR value to control or optimize fan speeds. 

 shows a conceptual 

fan control diagram using PECI temperatures. 

The relative temperature value reported over PECI represents the data below the onset 

of thermal control circuit (TCC) activation as needed by PROCHOT# assertions. As the 

temperature approaches TCC activation, the PECI value approaches zero. TCC activates 

at a PECI count of zero.

The 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. The DTS sample interval range can be modified, and a data filtering algorithm 

can be activated to help moderate this. The DTS sample interval range is 82 us 

(default) to 20 ms (max). This value can be set in BIOS.

To reduce the sample rate requirements on PECI and improve thermal data stability vs. 

time the processor DTS also implements an averaging algorithm that filters the 

incoming data. This is an alpha-beta filter with coefficients of 0.5, and is expressed 

mathematically as: Current_filtered_temp = (Previous_filtered_temp / 2) + 

(new_sensor_temp / 2). This filtering algorithm is fixed and cannot be changed. It is on 

by default and can be turned off in BIOS.

Host controllers should use the min/max sample times to determine the appropriate 

sample rate based on the controller's fan control algorithm and targeted response rate. 

The key items to take into account when settling on a fan control algorithm are the DTS 

sample rate, whether the temperature filter is enabled, how often the PECI host will 

poll the processor for temperature data, and the rate at which fan speed is changed. 

Depending on the designer’s specific requirements the DTS sample rate and alpha-beta 

filter may have no effect on the fan control algorithm.