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Subject: Re: BUG: iio: mpu3050: Wrong temperature scale
To:     Andy Shevchenko <andy.shevchenko@gmail.com>,
        Linus Walleij <linus.walleij@linaro.org>
Cc:     Dmitry Osipenko <digetx@gmail.com>,
        Jonathan Cameron <jic23@kernel.org>,
        "linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Svyatoslav Ryhel <clamor95@gmail.com>,
        Nathan Royer <nroyer@invensense.com>,
        Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
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From:   Lars-Peter Clausen <lars@metafoo.de>
Message-ID: <b5a2ce37-b386-40b3-d9ad-6e77deda74e6@metafoo.de>
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.