Messages in this thread | | | Date | Sat, 08 Mar 2014 15:50:49 -0800 | From | Guenter Roeck <> | Subject | Re: [lm-sensors] Tachometer speed returned rather than absolute fan speed? |
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On 03/07/2014 10:17 AM, Guenter Roeck wrote: > On Fri, Mar 07, 2014 at 03:47:08PM +0000, Laszlo Papp wrote: >> On Fri, Mar 7, 2014 at 3:37 PM, Jean Delvare <jdelvare@suse.de> wrote: >>>>> I'm quite confused. While I admit that the term "tachometer speed" is >>>>> awkward, the max6650 driver is reporting fan speeds in RPM as every >>>>> other hwmon driver. So I really have no idea what you think is wrong. >>>>> What did you think "tachometer speed" was, if not the fan speed? Does >>>>> the max6650 driver not return correct fan speeds for you? >> >> That is some strange behavior. If I do "echo 1 > pwm1_enable; echo 0 > >> pwm1; cat fan1_input", I still see 30 for the connected fan, whereas I >> can see it stopped. Is this an expected behavior? I would expect zero >> as a user. >> > I seem to recall that I had seen that as well, with no fan connected. > Maybe the tachometer registers always read at least '1'. I would think > it is wrong, but we'll have to understand the chip a bit better > to be able to provide a fix. Unless you already have a fix ready, > of course. I'll try to re-test tonight if I find the time. >
The reason is (most likely) that your fan input does not have a pull-up resistor. Per datasheet, the fan inputs need a 10kOhm pull-up resistor. I confirmed this with my test board - with the pull-up resistor, inputs read 0, Without pull-up, the reported value is 1, which translates to 30 RPM.
You might also need the 10 uF capacitor on the FB pin.
Guenter
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