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

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Date:   Wed, 21 Apr 2021 22:41:19 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     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>,
        Nathan Royer <nroyer@...ensense.com>,
        Jean-Baptiste Maneyrol <jmaneyrol@...ensense.com>
Subject: Re: BUG: iio: mpu3050: Wrong temperature scale

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?

-- 
With Best Regards,
Andy Shevchenko