Question I can see that there is a ADC module, but it is not clear how to configure the 2 ADC channels and connected to the external world.
QL Answer>>
EOS S3 has two channel of ADC, on the QuickFeather and BGA package, and 1 channel on the WLCSP package
It is a 12-bit ∑Δ Analog-to-Digital Converter (ADC), normally used to measure battery voltage level. If you look at the schematic for QuickFeather you will see that ADC0 is connected to the VBAT
ADC1 Is available on J3 pin 11
The specification of the ADC is as follows
The HAL source code is available in the QORC software in the qorc-sdk/HAL/src/eoss3_hal_adc.c file.