Measure voltage supplied to chip

[dhalbert]

Add microprocessor.cpu.voltage to measure the regulated voltage supplied to the chip. Monitoring of this value can detect when the regulated voltage is slumping due to a low battery.

This is typically 3.3V. Note that this does not measure the actual on-chip voltage used to urn the CPU, which is often lower due to an on-chip regulator (e.g., 1.2V).

Note that we also already have brownout-detection enabled, but that simply prevents causes a reset or makes the chip refuse to start. This can catch eventual brownout in advance, before the battery gets much too low.

I use the on-chip bandgap reference and compare it with a scaled version of the supplied voltage. The SAMD51 is very touchy about how to use this reference. See https://community.atmel.com/forum/samd51-using-intref-adc-voltage-reference.

• I did some code cleanup in the atmel-same Processor.c as well, to remove some confusion I introduced about ADC channels vs MUXPOS choices.
• The nRF chips have a simple way to calibrate the ADC, which is recommended on startup. I added this.
• I attempted to use the INTREF for the temperature measurement as well, based on new info about using it for the voltage, but this still does not work.

Tested using a variable bench power supply. The SAMD chips can clearly measure sag. On the nRF52840, I lost the USB connection very early when testing sag, but it still seems to work.

[ladyada]

yay! ill check next week i think

[tannewt]

Neat! One question but looks good otherwise

[dhalbert]

@tannewt all updated to handle recent PR's

[tannewt]

Great! Looks good. Thanks!