Add analogReference function to switch between default VDD/4 reference and internal reference for ADC.

[stuthedew]

Problem

The default analog reference for nRF52840-based boards (Xenon, Argon, Boron) uses VDD/4. This is fine for stable input voltage (usb), but super not great if you're running off a battery as the input voltage (and therefor reference voltage) decreases over time. The nRF52840 does have an internal 0.6v reference that would solve this problem, but the ability to use it is not currently implemented in the Particle OS. This is pretty annoying in a device that markets itself as a battery-based sensor node.

Solution

I've implented an Arduino-style AnalogReference function, that allows you to select the internal 0.6v reference instead of the default VDD/4 if desired.

Steps to Test

user/tests/hal/adc/test_adc.cpp

Example App

#include "Arduino.h"

const int16_t ANALOG_READ_PIN = A0;
int analog_value;

void setup() {
 pinMode(ANALOG_READ_PIN, INPUT);
 Serial.begin(9600);
}

void loop() {
 analog_value = analogRead(ANALOG_READ_PIN); //get value using default VDD/4 reference.
 Serial.print("Analog Value (VDD/4 reference): ");
 Serial.println(analog_value);

 delay(250);

 analogReference(INTERNAL);  // use INTERNAL 0.6v reference 
 analog_value = analogRead(ANALOG_READ_PIN); //get value using default VDD/4 reference.
 Serial.print("Analog Value (Internal 0.6v reference): ");
 Serial.println(analog_value);

 analogReference(AR_DEFAULT);  // switch back to default VDD/4 ADC reference 

 delay(1000);
}

References

https://community.particle.io/t/internal-voltage-reference-for-accurate-voltage-calibration/6344
https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.sdk5.v12.0.0%2Fgroup__nrf__drv__saadc.html
https://www.arduino.cc/reference/en/language/functions/analog-io/analogreference/

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Completeness

• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)