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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Wed, 21 Apr 2021 22:41:19 +0300
Message-ID: <CAHp75Vckb-B=rLsZA3At2zU95No9SUvRo24xNodObZEv-UU81Q@mail.gmail.com>
Subject: Re: BUG: iio: mpu3050: Wrong temperature scale
To:     Linus Walleij <linus.walleij@linaro.org>
Cc:     Dmitry Osipenko <digetx@gmail.com>,
        Jonathan Cameron <jic23@kernel.org>,
        "linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Svyatoslav Ryhel <clamor95@gmail.com>,
        Nathan Royer <nroyer@invensense.com>,
        Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
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On Wed, Apr 21, 2021 at 10:05 PM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Wed, Apr 21, 2021 at 1:14 PM Linus Walleij <linus.walleij@linaro.org> =
wrote:
> >
> > On Tue, Apr 20, 2021 at 11:26 PM Dmitry Osipenko <digetx@gmail.com> wro=
te:
> >
> > > 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 valu=
e
> > > as a signed integer, while upstream assumes it's unsigned.
> > >
> > > [1]
> > > https://github.com/blaisejarrett/Arduino-Lib.MPU3050/blob/master/MPU3=
050lib.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=C2=
=B0C.
>
> Range: -30=C2=B0C .. +85=C2=B0C
> Sensitivity: 280 LSB/=C2=B0C
> Room temperature offset: 35=C2=B0C =3D -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=C2=B0C

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?

--=20
With Best Regards,
Andy Shevchenko