Re: [lm-sensors] Tachometer speed returned rather than absolute fan speed?

[Laszlo Papp]

On Sat, Mar 8, 2014 at 11:50 PM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> 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@xxxxxxx> 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.

Hmm, interesting, but then I wonder how it works fine when getting the
data from userspace with ioctl (i.e .without the driver) through
/dev/i2c-1. There must be some trick in that case that I am not yet
aware of. I will double check the schematics on Monday.

Thanks!
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