Messages in this thread | | | Subject | Re: BUG: iio: mpu3050: Wrong temperature scale | From | Lars-Peter Clausen <> | Date | Wed, 21 Apr 2021 22:00:20 +0200 |
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On 4/21/21 9:41 PM, Andy Shevchenko wrote: > On Wed, Apr 21, 2021 at 10:05 PM Andy Shevchenko > <andy.shevchenko@gmail.com> wrote: >> On Wed, Apr 21, 2021 at 1:14 PM Linus Walleij <linus.walleij@linaro.org> wrote: >>> On Tue, Apr 20, 2021 at 11:26 PM Dmitry Osipenko <digetx@gmail.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? (13200 + x)/280 + 35 = (23000 + x)/280, which is what is in the driver. So the only bit missing is the cast to s16.
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