void setup() {
  Serial.begin(115200);
  while (!Serial) { /* wait for USB serial */ }

  // Configure on-board LED channels
  pinMode(PIN_LED_R, OUTPUT);
  pinMode(PIN_LED_G, OUTPUT);
  pinMode(PIN_LED_B, OUTPUT);

  analogWriteResolution(8);    // 8-bit resolution

  Serial.println("XIAO Ready. Send R<0-255>, G<0-255> or B<0-255>");
}

void loop() {
  if (!Serial.available()) return;

  char channel = Serial.read();          // 'R', 'G' or 'B'
  int value    = Serial.parseInt();
  Serial.read();                         // consume newline
  value = constrain(value, 0, 255);

  int pin;
  switch (channel) {
    case 'R': pin = PIN_LED_R; break;
    case 'G': pin = PIN_LED_G; break;
    case 'B': pin = PIN_LED_B; break;
    default:  return;
  }

  analogWrite(pin, value);
  Serial.printf("%c set to %d\n", channel, value);
}