adc_vbat.ino

#include <Arduino.h>
#include <Adafruit_TinyUSB.h> // for Serial

#if defined ARDUINO_NRF52840_CIRCUITPLAY
#define  PIN_VBAT          A8   // this is just a mock read, we'll use the light sensor, so we can run the test
#endif

uint32_t vbat_pin = PIN_VBAT;             // A7 for feather nRF52832, A6 for nRF52840

#define VBAT_MV_PER_LSB   (0.73242188F)   // 3.0V ADC range and 12-bit ADC resolution = 3000mV/4096

#ifdef NRF52840_XXAA
#define VBAT_DIVIDER      (0.5F)          // 150K + 150K voltage divider on VBAT
#define VBAT_DIVIDER_COMP (2.0F)          // Compensation factor for the VBAT divider
#else
#define VBAT_DIVIDER      (0.71275837F)   // 2M + 0.806M voltage divider on VBAT = (2M / (0.806M + 2M))
#define VBAT_DIVIDER_COMP (1.403F)        // Compensation factor for the VBAT divider
#endif

#define REAL_VBAT_MV_PER_LSB (VBAT_DIVIDER_COMP * VBAT_MV_PER_LSB)


float readVBAT(void) {
  float raw;

  // Set the analog reference to 3.0V (default = 3.6V)
  analogReference(AR_INTERNAL_3_0);

  // Set the resolution to 12-bit (0..4095)
  analogReadResolution(12); // Can be 8, 10, 12 or 14

  // Let the ADC settle
  delay(1);

  // Get the raw 12-bit, 0..3000mV ADC value
  raw = analogRead(vbat_pin);

  // Set the ADC back to the default settings
  analogReference(AR_DEFAULT);
  analogReadResolution(10);

  // Convert the raw value to compensated mv, taking the resistor-
  // divider into account (providing the actual LIPO voltage)
  // ADC range is 0..3000mV and resolution is 12-bit (0..4095)
  return raw * REAL_VBAT_MV_PER_LSB;
}

uint8_t mvToPercent(float mvolts) {
  if(mvolts<3300)
    return 0;

  if(mvolts <3600) {
    mvolts -= 3300;
    return mvolts/30;
  }

  mvolts -= 3600;
  return 10 + (mvolts * 0.15F );  // thats mvolts /6.66666666
}

void setup() {
  Serial.begin(115200);
  while ( !Serial ) delay(10);   // for nrf52840 with native usb

  // Get a single ADC sample and throw it away
  readVBAT();
}

void loop() {
  // Get a raw ADC reading
  float vbat_mv = readVBAT();

  // Convert from raw mv to percentage (based on LIPO chemistry)
  uint8_t vbat_per = mvToPercent(vbat_mv);

  // Display the results

  Serial.print("LIPO = ");
  Serial.print(vbat_mv);
  Serial.print(" mV (");
  Serial.print(vbat_per);
  Serial.println("%)");

  delay(1000);
}