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iio: sgp40: retain documentation in driver

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Retain documentation on how the voc index is actually calculated in
driver code as it'll be removed in Documentation.

This is a follow up on patch "[PATCH] iio: ABI: remove duplicate
in_resistance_calibbias" from David.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Link: https://patch.msgid.link/ZsWdFOIkDtEB9WGO@mail.your-server.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
This commit is contained in:
Andreas Klinger 2024-08-21 09:53:56 +02:00 committed by Jonathan Cameron
parent 6bcffddc14
commit d35099d3c6
1 changed files with 8 additions and 3 deletions
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11
drivers/iio/chemical/sgp40.c
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@ -14,11 +14,16 @@
* 1) read raw logarithmic resistance value from sensor
* --> useful to pass it to the algorithm of the sensor vendor for
* measuring deteriorations and improvements of air quality.
* It can be read from the attribute in_resistance_raw.
*
* 2) calculate an estimated absolute voc index (0 - 500 index points) for
* measuring the air quality.
* 2) calculate an estimated absolute voc index (in_concentration_input)
* with 0 - 500 index points) for measuring the air quality.
* For this purpose the value of the resistance for which the voc index
* will be 250 can be set up using calibbias.
* will be 250 can be set up using in_resistance_calibbias (default 30000).
*
* The voc index is calculated as:
* x = (in_resistance_raw - in_resistance_calibbias) * 0.65
* in_concentration_input = 500 / (1 + e^x)
*
* Compensation values of relative humidity and temperature can be set up
* by writing to the out values of temp and humidityrelative.