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
