On Wed, Apr 21, 2021 at 11:02 PM Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote: > > On Wed, Apr 21, 2021 at 11:00 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 > > > > > > 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. > > Cool! So, that cast needs a very good comment then :-) "will need" (since it's not yet in upstream) -- With Best Regards, Andy Shevchenko
