   Wire.onReceive(i2cReceive);
   READY(BOARD);
 
+#ifdef DEBUG
+  Serial.begin(9600);
+#endif
+
   environment_sensor.begin();
   // not: READY(ENVIRONMENT_SENSOR);
   // (do not make it ready until the first measurement)

#define PIN_ANALOG_I2C_SDA      4 // i2c enabled by jumper
#define PIN_ANALOG_I2C_SCL      5 // i2c enabled by jumper

// TODO: read these from Arduino Nano (protocol = ?)
unsigned long encoder_left_front = 0, encoder_left_back = 0,
              encoder_right_front = 0, encoder_right_back = 0;

    Wire.write((byte *)(&status), sizeof(status));
    break;
  case REGISTER(BOARD) + BOARD_BATTERY:
    // centiV = analog * 1.7581
    // Range = 10.6V .. 12.6V
    value = analogRead(PIN_ANALOG_BATTERY);
    Wire.write((byte)(value * 1.7581 / 2));
    break;
  case REGISTER(LIGHT_SENSOR):
    value = analogRead(PIN_ANALOG_LIGHT_SENSOR);
