Legacy Fan Speed Control
118 Thermal and Mechanical Design Guidelines
Table 7-4. ATX FSC Settings
Parameter Classification Processor Thermal
Sensor
PWM Output Notes
T
HIGH
Required T
CONTROL
3, 5
T
LOW
Required T
CONTROL
– 10 °C 3,5
Minimum PWM Duty
Cycle
Required 20%
PWM Frequency Required 21–28 kHz 1
Spin-up time Suggested 250 – ~500 ms 2, 6
T
AVERAGING
Suggested 35 sec 6
When T
SENSOR
< T
LOW
Suggested Minimum PWM%
All Fans ON Suggested T
CONTROL
+ 3 °C
Hysteresis Suggested 2 °C
Offset Required Thermal Diode
Correction Factor
4, 5
NOTES:
1. A PWM frequency of 25 kHz is the design target for the reference and for the Intel
®
Boxed Processor and the reference design.
2. Use the lowest time available in this range for the device selected.
3. To ensure compliance with the thermal specification, thermal profile and usage of the
T
SENSOR
for fan speed control these setting should not be user configurable.
4. If present in the FSC device the Thermal Diode Correction Factor should be input to this
register.
5. If the FSC device does not have a means to input a fixed temperature offset then:
T
HIGH
= T
CONTROL
+ Thermal Diode Correction Factor and
T
LOW
= T
CONTROL
+ Thermal Diode Correction Factor.
6. If this function is present on the device it must be enabled
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