Board Level PWM and Fan Speed Control Requirements
R
Thermal/Mechanical Design Guide 87
Appendix E Board Level PWM and Fan
Speed Control
Requirements
To use all of the features in the Intel reference heatsink design or the Boxed Intel Pentium 4
Processor in 775–land LGA package, system integrators should verify the following functionality
is present in the board design. Refer to the Fan Specification for 4 wire PWM Controlled Fans
and Chapter
5 for complete details on the Intel enabled thermal solution.
The basics of Fan Speed Control are discussed in Chapter
6; as a review, the FSC definitions are
listed in
Table 7.
Table 7. FSC Definitions
Item Definition
T
DIODE
Temperature reported from the processor on-die thermal diode.
T
CONTROL
T
CONTROL
is the specification limit for use with the on-die thermal diode
T
LOW
The temperature above which the fan will begin to accelerate in response to the on-die
thermal diode temperature.
Hysteresis The number of degrees below T-control the fans will remain on before slowing down.
T
HIGH
The temperature at which the fan is operating at full speed (100% PWM Duty Cycle). By
specification this is T
CONTROL
.
All Fans ON The processor temperature at which all fans in the system are increased to 100% Duty
Cycle.
Min PWM Minimum pulse width modulation (% duty cycle) that the fans will run at when T
DIODE
is less
than T
LOW
.
Spin-up Amount of time fan is run at 100% duty cycle to overcome fan inertia.
PWM Freq The operating frequency of the PWM signal.
T
AVERAGING
The time (in seconds) that elapses while the fan is gradually sped up in response to a
processor temperature spike.
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