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Subject: Re: BUG: iio: mpu3050: Wrong temperature scale
To:     Andy Shevchenko <andy.shevchenko@gmail.com>,
        Lars-Peter Clausen <lars@metafoo.de>
Cc:     Linus Walleij <linus.walleij@linaro.org>,
        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>,
        Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
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From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <af175b05-e37f-f9b3-cfd9-8bf48add3e99@gmail.com>
Date:   Wed, 21 Apr 2021 23:58:17 +0300
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21.04.2021 23:04, Andy Shevchenko пишет:
> On Wed, Apr 21, 2021 at 11:02 PM Andy Shevchenko
> <andy.shevchenko@gmail.com> wrote:
>>
>> On Wed, Apr 21, 2021 at 11:00 PM Lars-Peter Clausen <lars@metafoo.de> wrote:
>>> 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.
>>
>> Cool! So, that cast needs a very good comment then :-)
> 
> "will need" (since it's not yet in upstream)
> 
> 

The exact value I'm getting is 52144, which is -13392 and gives us 35C.
I see that the datasheet says it should be a temperature of the sensor
die, so maybe it's a correct value, although it means that it's warm as
much as ARM CPU core, which is questionable to me whether it's a correct
value.