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Subject: Re: BUG: iio: mpu3050: Wrong temperature scale
To:     Linus Walleij <linus.walleij@linaro.org>,
        Lars-Peter Clausen <lars@metafoo.de>
Cc:     Andy Shevchenko <andy.shevchenko@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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 <b5a2ce37-b386-40b3-d9ad-6e77deda74e6@metafoo.de>
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From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <4c204f77-d560-43d3-c5d2-eced7a1124dc@gmail.com>
Date:   Thu, 22 Apr 2021 02:10:23 +0300
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22.04.2021 02:02, Linus Walleij пишет:
> On Wed, Apr 21, 2021 at 10:00 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
> 
> Oh nice find!
> 
>>> 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.
> 
> Yup there is the bug. (I am guilty.)
> 
> Dmitry will you test & send a patch?


I'll send the patch. Thank you all for yours help!