Esp32 HAL - Dynamic ADC attenuation

[simon-jouet]

The ESP32 ADC operates in the range of 0V to ~1.1V (1.0V -- 1.2V). In order for this ADC to work with the full range input of 3.3V the attenuation must be configured to attenuate the input voltage below 1.1V.

In the current implementation of the HAL the ADC attenuation is always the maximum of 11dB allowing the full 3.3V range input (well almost but that's for another discussion). Using the full 111dB attenuation works but also decreases precision when the voltage is lower.

In this patch I use the ADC calibration characteristics to get the operating voltage range of each attenuation level. Using the last ADC reading the attenuation level can be changed to the nearest operating range. This allows much higher precision, which is especially relevant for higher temperatures (close to 0V).

In this code the attenuation is switched when the reading is 100mV below the nominal characteristics and 50mV above the previous attenuation. This range is used to compensate for inaccuracies when shifting range. (e.g when you change to a higher attenuation the value might be less precise and therefore change back to a higher attenuation, repeating multiple times and oscillating around two attenuations).

The 100mV and 50mV values have not been picked for any specific reason and some (better) values might be more suitable, but with this the attenuation do shift properly without oscillations.

[vivian-ng]

@simon-jouet
Due to the change here, is there a need to redefine HAL_ADC_RESOLUTION for the ESP32's HAL to 12 instead of the current 10?

Analog to Digital Converter