[lm-sensors] Intel 2500K stock fan control

[Soeren D. Schulze]

It's me with my NCT6776 again.

It doesn't seem to be the driver that is causing problems.

When I force the PWM to full duty cycle at low CPU temperature, I get
about 2200 RPM.  I get the same thing when I disconnect the PWM pin
completely (so apparently the driver sets the PWM duty cycle correctly).
Voltage on the supply pin of the fan is 12 V.

When put load on the system and CPU temperature rises, though, the fan
RPM rise up to 2800.  The question is:  What is the reason for this RPM
rise?  It can't be the PWM signal, as the PWM pin is disconnected.  It
can't be the voltage, because the voltage stays at 12 V (I measured it).

The only explanation that comes to my mind is that the fan does not
actually go to full speed even at full PWM duty cycle until it senses
high temperature itself.  This implies that the fan has some integrated
temperature sensor.

Is this possible?  And if so, why would they do such a thing?


Sorry if this was a bit off-topic.

Sören