Re: BUG: iio: mpu3050: Wrong temperature scale

[Linus Walleij]

On Wed, Apr 21, 2021 at 10:00 PM Lars-Peter Clausen <lars@xxxxxxxxxx> wrote:
> 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

Oh nice find!

> > 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.

Yup there is the bug. (I am guilty.)

Dmitry will you test & send a patch?

Yours,
Linus Walleij