code: function(n, symbol, dp, callback) {
      if (n < 0 || n > 7) {
        throw "Invalid digit number";
      }
      if (!(symbol in MAX7219._Font)) {
        throw "Invalid symbol string";
      }
      var byte = MAX7219._Font[symbol] | (dp ? (1 << 7) : 0);
      this._shiftOut(MAX7219._Registers["Digit" + n], byte, callback);
    }
docs: Sets the symbol displayed in a digit.
  For this to work properly, the digit should be in decode mode.
  
  @param number n
  The digit number, from 0 up to and including 7.
  @param string symbol
  The symbol do display: "0".."9", "E", "H", "L", "P", "-" or " ".
  @param boolean dp
  Specifies if the decimal point should be on or off.
  
  @param function callback [optional]
  Invoked once the write to the SPI device finishes.