Most 3-wire fans use an open-collector type output for the speed pulse and
output 2 or 4 pulses per revolution.

You could just hook up your fan and measure it.

I would also use a capture input if one is available as it frees up a lot
of CPU cycles for doing other things. You'll get an elapsed time between
pulses which can be easily converted to RPMs when you know how many pulses
per revolution the fan provides.

Matt Pobursky
Maximum Performance

On Mon, 01 Feb 2010 15:47:59 -0500, MCH wrote:
> Thanks for the great info on the backlight.
>
> Does anyone have some info/ASM code on fan speed checking? I need to use
> the 'third wire' of a PC type fan to check the RPM.
>
> I *think* it's just a matter of checking the line for X mS for ground
> pulses, then calculate how many pulses per minute that would equate. For
> example, if I check it for 100 mS, and I see 5 pulses, that would be 5 x
> 600, or 3000 RPM. The 600 multiplier came from 1 minute = 600 x 100 mS.
>
> Does anyone know if this is how this third wire works? (ground pulses)
>
> and if so does this sound correct for how to calculate the RPM?
>
> Thanks again,
> Joe M.

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