19.04.2021 13:07, Linus Walleij пишет: > On Mon, Apr 19, 2021 at 8:06 AM Dmitry Osipenko <digetx@xxxxxxxxx> wrote: > >> The driver uses >> (x+23000)/280 formula for the conversion of raw temperature value, which >> gives 82C for x=0, thus apparently formula is wrong because x=50000 >> should give us ~25C. >> >> I tried to search for the datasheet with the formula, but couldn't find it. > > There is no public datasheet. I have never seen a non-public datasheet > either. > > As the initial submission of the driver says: > > "This driver is based on information from the rough input driver > in drivers/input/misc/mpu3050.c and the scratch misc driver > posted by Nathan Royer in 2011. Some years have passed but this > is finally a fully-fledged driver for this gyroscope. It was > developed and tested on the Qualcomm APQ8060 Dragonboard." > > Nathans submission: > https://lore.kernel.org/lkml/1309486707-1658-1-git-send-email-nroyer@xxxxxxxxxxxxxx/ > (you find the threads at the bottom) > > This submission came from inside Invensense so it is the closest > authoritative source we have. > >> Linus, will you be able to check whether the formula used by the driver >> is correct? Thanks in advance. > > Sadly the code is the documentation when it comes to Invensense stuff, > I am CC:ing Nathans Invensense address in the vain hope he is still > working there and could help, also CC to Jean-Baptiste who was > there last year and maybe can help out. > > I don't anymore remember exactly how I found this equation, > but it wasn't from any datasheet. I vaguely remember browsing > through some Android userspace sensor code. > > What I tend to do is dig around in old mobile > phone Android trees, and there you sometimes find this information > in different GPL code drops. I bet I got it from browsing some of > those. > > Here is an example (Tegra): > https://android.googlesource.com/kernel/tegra/+/dba2740d025c8e7e7e3c61d84a4f964d2c1c0ac9/drivers/misc/inv_mpu > > Worst case what one *can* do is to calibrate the scale, like put > the device in a controlled environment of some two reasonably > far apart temperatures and measure, assuming it is at least > linear. Some professionals use controlled environment > chambers for this. But I hope there is a better way. Linus, thank you very much for the answer! 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 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. Certainly a manual calibration is an option, but we will try to wait for the answer from Nathans and Jean-Baptiste before going that route.
