Nickzoic/circuitpython nrf touchin 1048

[nickzoic]

A much-simplified approach to touch sensing for #1048 !

This is a capacitive touch sensing routine using a single digital
pin. The pin should be connected to the sensing pad, and to ground
via a 1Mohm or thereabout drain resistor. When a reading is taken,
the pin's capacitance is charged by setting it to a digital output
'high' for a few microseconds, and then it is changed to a high
impedance input. We measure how long it takes to discharge through
the resistor (around 50us), using a busy-waiting loop, and average
over N_SAMPLES cycles to reduce the effects of noise.

[nickzoic]

Manually tested on Feather NRF52832 and Feather NRF52840 Express boards, with code like:

import board
import touchio

t0 = touchio.TouchIn(board.A0)
t1 = touchio.TouchIn(board.A1)

while True:
    print("%4d %d %4d %d" % (t0.raw_value, t0.value, t1.raw_value, t1.value))

[tannewt]

How about auto setting a threshold on object creation? That's what the SAMD does already. That way folks don't need to set it themselves before using it.

[nickzoic]

Sure, we can totally do that. I'll add it in.

[nickzoic]

Threshold is now automatically set: I've adjusted the example code accordingly.

[tannewt]

Awesome! Thank you!

[ladyada]

tested, works well - thanks!