Pine

#define VBATPIN 6

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);

  SerialUSB.begin(9600);
  while (!SerialUSB) {}
  delay(1000);

  SerialUSB.println("Start battery voltage measurement!");
}

void loop() {
  // SerialUSB.println("HIGH");
  digitalWrite(LED_BUILTIN, HIGH);
  delay(200);

  // SerialUSB.println("LOW");
  digitalWrite(LED_BUILTIN, LOW);
  delay(200);

  float measuredvbat = analogRead(VBATPIN);

  for (int i = 0; i < 10; i++) {
    measuredvbat += analogRead(VBATPIN);
    delay(10);
  }

  measuredvbat /= 10;
  measuredvbat *= 2;
  measuredvbat *= 4.5;  // Or the reference voltage E.g. 2.8V, 3.3V
  measuredvbat /= 1024;

  SerialUSB.print("VBat: ");
  SerialUSB.print(measuredvbat);
  SerialUSB.println("V");
}

BOARD?=arduino:samd:arduino_zero_native
PORT := $(shell ls /dev/cu.usbmodem*)

.PHONY: default lint all flash clean

default: lint all flash clean

lint:
	cpplint --extensions=ino --filter=-legal/copyright,-whitespace/line_length,-readability/casting,-readability/todo *.ino

all:
	# This custom PCB does not have a crytal on pins PA00 and PA01
	# Hence, use -DCRYSTALLESS to replace the extra_flags in boards.txt
	arduino-cli compile --fqbn $(BOARD) --build-properties build.extra_flags="-DCRYSTALLESS -D__SAMD21G18A__ {build.usb_flags}" ./

flash:
	arduino-cli upload -p $(PORT) --fqbn $(BOARD)

clean:
	rm -r build