linux-kernel - Re: BUG: iio: mpu3050: Wrong temperature scale

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Date:   Wed, 21 Apr 2021 23:04:12 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Lars-Peter Clausen <lars@...afoo.de>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Dmitry Osipenko <digetx@...il.com>,
        Jonathan Cameron <jic23@...nel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Svyatoslav Ryhel <clamor95@...il.com>,
        Jean-Baptiste Maneyrol <jmaneyrol@...ensense.com>
Subject: Re: BUG: iio: mpu3050: Wrong temperature scale

On Wed, Apr 21, 2021 at 11:02 PM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
>
> On Wed, Apr 21, 2021 at 11:00 PM Lars-Peter Clausen <lars@...afoo.de> wrote:
> > On 4/21/21 9:41 PM, Andy Shevchenko wrote:
> > > On Wed, Apr 21, 2021 at 10:05 PM Andy Shevchenko
> > > <andy.shevchenko@...il.com> wrote:
> > >> On Wed, Apr 21, 2021 at 1:14 PM Linus Walleij <linus.walleij@...aro.org> wrote:
> > >>> On Tue, Apr 20, 2021 at 11:26 PM Dmitry Osipenko <digetx@...il.com> wrote:
> > >>>
> > >>>> I found a non-kernel example
> > >>>> which uses a similar equation [1], but in a different form. The main
> > >>>> difference is that the Arduino code interprets a raw temperature value
> > >>>> as a signed integer, while upstream assumes it's unsigned.
> > >>>>
> > >>>> [1]
> > >>>> https://github.com/blaisejarrett/Arduino-Lib.MPU3050/blob/master/MPU3050lib.cpp#L111
> > >>> Oh that's nice. Room temperature as mentioned is 20 deg C
> > >>> I think?
> > >>>
> > >>> The divide by 280 part seems coherent in all examples.
> > >>>
> > >>>> Still, even if assume that the raw temperature is a signed s16 value, it
> > >>>> gives us ~35C in a result, which should be off by ~10C.
> > >> Actually here [1] it says in chapter 3.1 that room temperature is 35°C.
> > >>
> > >> Range: -30°C .. +85°C
> > >> Sensitivity: 280 LSB/°C
> > >> Room temperature offset: 35°C = -13200 LSB
> > >>
> > >> [1]: https://www.cdiweb.com/datasheets/invensense/mpu-3000a.pdf
> > >
> > > So, if I'm reading this and the register description right the value
> > > is in the range
> > > -32768..32767.
> > > -13200 defines 35°C
> > >
> > > 50000 as mentioned by Dmitry is actually -15536. So, it means that the
> > > more negative a value is the higher temperature is shown.
> > >
> > > Since it's linearized scale, now we can see that
> > >
> > > (13200 -15536)/280 + 35 gives us 26.66.
> > >
> > > Does it make sense?
> > (13200 + x)/280 + 35 = (23000 + x)/280, which is what is in the driver.
> > So the only bit missing is the cast to s16.
>
> Cool! So, that cast needs a very good comment then :-)

"will need" (since it's not yet in upstream)


-- 
With Best Regards,
Andy Shevchenko