AS99127F PWM Fan Control Experiment on P3B-F

[ticket]

I conducted some experiment on Asus P3B-F motherboard with AS99127F (Ver. 1).

I confirm that 0x59 register control the CPU_Fan Header on this motherboard, and 0x5a register control PWR_Fan.

In order to reduce the dependency of specific fan, the measurement is conducted with a digital scope without fan connected. I found out that P3B-F actually output variable DC voltage on fan header center pin, looks like PWM is filtered on this motherboard.

Here are some of measurements:

0x80 20 mV
0x81 20 mV
0x82 232 mV
0x83 1.2V
0x84 2.31V
0x85 3.44V
0x86 4.62V
0x87 5.81V
0x88 7.01V
9x89 8.22V
0x8a 9.42V
0x8b 10.6V
0x8c 11.9V
0x8d 12.4V
0x8e 12.4V
0x8f 12.4V

If you have any questions, feel free to send a email.

[khali]

Replying to ticket:

I found out that P3B-F actually output variable DC voltage on fan header center pin, looks like PWM is filtered on this motherboard.

It might just be that the AS99127F outputs a DC signal and not a PWM signal on the fan control pins. We tend to use "PWM" as a generic term for "fan control" regardless of the actual technique being used.

Thanks for the information. I've added it to the w83781d driver documentation (in r5355). It seems to correlate with Hector Martin's findings. This may be enough to write a patch to the w83781d driver adding fan control support for the AS99127F rev. 1. However, I wouldn't take the risk myself. Without a datasheet, we can't be certain that our implementation will be correct for all boards out there. And these boards start being old anyway, I wonder how many people are still using them.