doc/fan-divisors also has a good explanation Philip Pokorny wrote: > > Jean Delvare wrote: > >>Jerome Hsiao @ Arima pointed out to me that the fan speeds from the > >>ADM1026 driver were not consistent with other drivers. > >> > >>Seems that most (all?) drivers assume two pulses per rev from the > >>fans. > >> I expected that users would need to use a "compute" directive to > >>modify the fan speed for the number of pulses-per-rev of their fans. > >>(All the fans I use are 4 ppr so I'm always having to "compute" a > >>correct value.) > >> > >>But it's not consistent with the other drives... > >> > >>So here is a patch to fix the ADM1026 so it reports fan speeds > >>consistent with the other drivers... > >> > >>This was done against CVS... > > > > > > Well, that's what fanX_div is meant for. As far as I know, the number of > > pulse/rotation depends mostly on the fan itself, not on the monitoring > > chipset. That's why we let the users change the fanX_div value through > > the config file. > > > > Maybe there's actually something to be fixed in the adm1026 driver, but > > maybe this is just a matter of setting the right fanX_div for unregular > > fans. > > No, fan_div doesn't work that way in practice. > > Most (all?) drivers include fan_div in the conversion from counts to > RPM. That means that when you change fan_div the RPM reading stays the > same. > > In practice, changing fan_div affects the minimum detectable fan speed > and the precision of the measured fan speed. > > With a 22.5kHz clock, and assuming 2 pulse/rev fans, you get the following: > > RPM > Min Typ Significant Bits in Count > Fan Div 254 153 3000 4400 10k 4ppr > -------- ------ ------ ------ ------ --------- > 1 2657 4411 7.81 7.26 5.08 > 2 1329 2206 6.81 6.26 4.08 > 4 664 1103 5.81 5.26 3.08 > 8 332 551 4.81 4.26 2.08 > > At a fan_div setting of 8, the 10k, 4ppr fan is only gating 4 counts on > average giving fan readings of: 14063 (3), 10547 (4), 8437 (5) > depending on how many counts you got that sample. Clearly, you want to > set the fan_div as low as possible and still detect your fans. > > :v) > > -- > Philip Pokorny, Director of Engineering > Tel: 415-358-2635 Fax: 415-358-2646 Toll Free: 888-PENGUIN > PENGUIN COMPUTING, INC. > www.penguincomputing.com
