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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Wed, 21 Apr 2021 11:54:24 +0200
Message-ID: <CACRpkdbtnCoDdwJA9oi88NKStf5uhi72DgP_a=3Dpp_aT=kYNg@mail.gmail.com>
Subject: Re: BUG: iio: mpu3050: Wrong temperature scale
To:     Dmitry Osipenko <digetx@gmail.com>
Cc:     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 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/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.
>
> Certainly a manual calibration is an option, but we will try to wait for
> 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 out
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.

I guess the OTP (one time programmable memory) also contains
device-unique calibration, maybe also for the temperature sensor,
but IIUC that is read in automatically by the hardware since no
drivers seem to look into that.

Yours,
Linus Walleij