public static BV4502 getInstance(I2CInterface device, char channel) {
    BV4502.device = device;
    switch (channel) {
      case 'A' :
        if (channel_A==null) channel_A=new BV4502('A');
        return channel_A;
      case 'B' :
        if (channel_B==null) channel_B=new BV4502('B');
        return channel_B;
    }
    throw new IllegalArgumentException();
  }