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

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Date:	Sat, 08 Mar 2014 15:50:49 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Laszlo Papp <lpapp@....org>
CC:	LKML <linux-kernel@...r.kernel.org>, lm-sensors@...sensors.org
Subject: Re: [lm-sensors] Tachometer speed returned rather than absolute fan
 speed?

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@...e.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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