XIAO BLE Sense battery level and charging status

davekw7x · March 15, 2022, 6:51pm

I just received my XIAO BLE Sense board and was disappointed to see this thread indicating problems reading the battery voltage.

I note that the return value trying analogRead from P0_31 is 0xFFFFFFFF, which is the traditional return value when trying to do an analogRead from a pin not connected to an ADC input.

So, poking around in the pins_arduino.h file in the variants directory for this board, I found the following:

#define PIN_VBAT (32u)

I decided to try PIN_VBAT in my analogRead, and, guess what?
Taa-daa!

Here’s my test sketch:

/*
 * Test to read a LiPo battery connected to VBAT on the
 * underside of XIAO BLE boards.
 * 
 * Note that the ADC input from the VBAT voltage divider is
 * NOT pin P0_31 as shown on the schematic!
 * 
 * March, 2022
 * davekw7x
 */
// For mbed compilers, the following allows us to use the
// standard printf() function.
REDIRECT_STDOUT_TO(Serial)

void setup()
{
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(115200);
  while (!Serial)
    ;
  printf("\nBattery Test compiled by davekw7x on %s at %s\n",   __DATE__, __TIME__);
  printf("Note: PIN_VBAT = %u\n\n", PIN_VBAT);
  
  pinMode(P0_14, OUTPUT);
  digitalWrite(P0_14, LOW);
}  // End of setup()

const double vRef = 3.3; // Assumes 3.3V regulator output is ADC reference voltage
const unsigned int numReadings = 1024; // 10-bit ADC readings 0-1023, so the factor is 1024

void loop()
{
  unsigned int adcCount = analogRead(PIN_VBAT);
  double adcVoltage = (adcCount * vRef) / numReadings;
  double vBat = adcVoltage*1510.0/510.0; // Voltage divider from Vbat to ADC
  
  printf("adcCount = %3u = 0x%03X, adcVoltage = %.3fV, vBat = %.3f\n",
             adcCount, adcCount, adcVoltage, vBat);
             
  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
  delay(1000);
} // End of loop()

And here’s the output, consistent with my very freshly charged LiPo battery:

Regards,

Dave