Re: [lm-sensors] [PATCH v3] hwmon: ntc: fix iio raw to microvolts conversion

[Guenter Roeck <linux@roeck-us.net>]

On 06/01/2015 02:21 PM, Chris Lesiak wrote:
> On 06/01/2015 02:54 PM, Guenter Roeck wrote:
>> On 06/01/2015 09:27 AM, Chris Lesiak wrote:
>>> The function ntc_adc_iio_read was assuming both a 12 bit ADC and that
>>> pullup_uv is the same as the ADC reference voltage.  If either
>>> assumption is false, then the result is incorrect.
>>>
>>> Attempt to use iio_convert_raw_to_processed to convert the raw value to
>>> microvolts.  It will fail for iio channels that don't support support
>>
>> But we need millivolts ?
>
> No.  I think we really do want microvolts.
>
> Don't let the old comment "/* unit: mV */ " lead you astray.  It is incorrect.
>
Ok.

> The value eventually makes its way to the function get_ohm_of_thermistor.
> That function is expecting the value to be in microvolts.
>
> It is true that get_ohm_of_thermistor promptly converts both uv and pullup_uv
> to millivolts by dividing by 1000.  But I don't think that conversion to
> millivolts is necessary.  In fact it is hurting the accuracy a little.
>
> For example, take the ncpXXwb473 connected to 5000 mV and pulled down through
> a 47000 ohm resistor.  At 25 C, the resistance of the thermistor is 47000 ohms.
> The measured voltage will be 2500 mV.  If we measure instead 2501 mV, then
> the calculated resistance will be 46962 ohms - a difference of 38 ohms.
> So the precision of the resistance estimate could be increased by 38X by
> doing the calculations in microvolts.
>
> But how does that resolution affect the temperature estimate?
> At room temperature, the sensitivity of the thermistor is -5.3x10^-4 C/ohm.
> So, the resolution of the temperature measurement is 0.02 C.  That probably
> is good enough for most purposes.
>
> The result will be worse at lower temperatures, but if you wanted to measure
> low temperatures, you'd probably connect the thermistor to ground and pull it
> up through the resistor.
>
> I think things could be improved in one of two ways:
> 1.  Accept the precision that you get by doing the calculation millivolts.
>      Simplify things by using millivolts throughout, although pullup_uv
>      needs to remain becaus it is part of the interface.
> 2.  Create an additional patch doing the calculations with microvolts
>      instead of millivolts.
>
> What are your thoughts?
>

You explain above that doing the calculations in mV looses precision.
Given that, what would be the point of the first proposal, especially
since the uV are already available (and may still be needed in
ntc_adc_iio_read if conversion through iio is not available) ?

Thanks,
Guenter


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