[builtin] FR: ADC, frequency

[lure23]

ADC:

• a sample for reading a potentiometer, with commenting about the useful range of measurements (some 0.x .. 3.0V); references

frequency:

• a sample tying a PWM output to an input pin, and detecting the frequency in that input pin

[lure23]

Note: ADC within ESP32's suffers from non-linearity, but there is a range where meaningful measurements can be done.