ESP8266 Community Forum

It did seem odd to me that adc gave invalid (not just noisy) readings when wifi was active until I started reading disassembly of libphy.a.
Looks like adc's main purpose on this chip is to provide feedback for internal RF calibration. Apparently trying to read two different channels (some internal voltage and TOUT pin) at the same time (from phy task and user task) messes up the adc/mux registers.

Good point!

I see 2 purposes why the PHY may have usage for the ADC: to read the power supply voltage and maybe the temperature (through a possible internal temperature sensor, although it has not been mentioned anywhere), in order to adjust the RF power, frequency, or both.

This would indeed require switching the ADC from the TOUT input channel to some other internal channels for power supply voltage and/or temperature.