Re: BUG: iio: mpu3050: Wrong temperature scale

[Lars-Peter Clausen]

On 4/21/21 9:41 PM, Andy Shevchenko wrote:
> On Wed, Apr 21, 2021 at 10:05 PM Andy Shevchenko
> <andy.shevchenko@xxxxxxxxx> wrote:
> > On Wed, Apr 21, 2021 at 1:14 PM Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
> > > On Tue, Apr 20, 2021 at 11:26 PM Dmitry Osipenko <digetx@xxxxxxxxx> 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.