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Legacy Fan Speed Control
Appendix E Legacy Fan Speed
Control
A motherboard design may opt to use a SIO or ASIC based fan speed control device
that uses the existing look up or state based fan speed control.
The fan speed control implementations consist of the following items
• A motherboard designed with a fan speed controller with the following
functionality:
⎯ PWM fan control output
⎯ Remote digital thermal sensor measurement capability over the PECI bus.
• A motherboard with a 4 pin fan header for the processor heatsink fan.
• Processor heatsink with 4–wire PWM controlled Fan.
A thermistor in the fan hub is recommended, but not a requirement. The
reference solution and the Boxed Processor will implement a thermistor into the
design.
The following sections will discuss the necessary steps to implement Legacy Fan
Speed Control.
E.1
Thermal Solution Design
The first step is to select or design a processor thermal solution that meets the
thermal profile for the processor. See Section 2.2.2 for the definition of the thermal
profile and consult the processor datasheet for the specific values.
The designer needs to ensure that when the heat sink fan is operating at full speed
the thermal solution will meet the T
profile will allow the designer to make tradeoffs in thermal performance versus the
inlet temperature to the processor fan heatsink.
E.1.1
Determine Thermistor Set Points
A thermistor implemented in the hub of a fan is a first level of fan speed control. It
provides an easy and cost effective means to begin acoustic noise reduction. It will,
by design, run the fan at an appropriate speed based on the ambient conditions.
Chapter 5 discussed in detail the reference thermal solution, including the target Ψ
fan speed based on temperature to ensure that T
at a given ambient temperature. The resulting variable speed fan (VSF) curve is the
upper limit on fan speed.
The benefit of this upper limit will become more apparent when the fan speed
controller is responding to the digital thermal sensor.
Thermal and Mechanical Design Guidelines
limits at TDP. The slope of the thermal
C-MAX
is not exceeded for TDP power
C-MAX
,
CA
97
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