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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Wed, 21 Apr 2021 22:05:47 +0300
Message-ID: <CAHp75VcEsrM+uYSLo2iEta7C8LQtg26iwQVFX1GUk1Gp5TPT7g@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 1:14 PM Linus Walleij <linus.walleij@linaro.org> wr=
ote:
>
> On Tue, Apr 20, 2021 at 11:26 PM Dmitry Osipenko <digetx@gmail.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/MPU305=
0lib.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, i=
t
> > 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

> > Certainly a manual calibration is an option, but we will try to wait fo=
r
> > the answer from Nathans and Jean-Baptiste before going that route.
>
> The method I have seen used is:
> - Collect many bags of silica gel, those little packages of "dryer" that
> come in shoe boxes.
> - Put the device with all these in two layers of plastic bags and pull ou=
t
> cables, glue or strap many layers around the bags to make it really tight
> where the cables come out.
> - Submerge this into a mixture of ice and water which is known to be
> a calibration point for 0 degrees C, wait for some hour or so to
> stabilize, add some ice if it all melts.
>
> Now measures should be 0 deg C so any deviance will be the constant
> offset that need be added for the sensor.


--=20
With Best Regards,
Andy Shevchenko