code: function(firstByte, secondByte, callback) {
      if (!this._spi) {
        throw "SPI device not initialized";
      }
  
      for (var i = 0; i < this._buffer.length; i += 2) {
        this._buffer[i] = MAX7219._Registers.NoOp;
        this._buffer[i + 1] = 0x00;
      }
  
      var offset = this._activeController * 2;
      this._buffer[offset] = firstByte;
      this._buffer[offset + 1] = secondByte;
  
      this._spi.write(this._buffer, callback);
    }
docs: Shifts two bytes to the SPI device.
  
  @param number firstByte
  The first byte, as a number.
  @param number secondByte
  The second byte, as a number.
  @param function callback [optional]
  Invoked once the write to the SPI device finishes.