code: function(limit, callback) {
      if (limit < 1 || limit > 8) {
        throw "Invalid scan limit number";
      }
      this._shiftOut(MAX7219._Registers.ScanLimit, limit - 1, callback);
    }
docs: Sets how many digits are displayed, from 1 digit to 8 digits.
  
  @param number limit
  The number of digits displayed, counting from first to last.
  E.g., to display only the first digit, limit would be 1.
  E.g., to display only digits 0, 1 and 2, limit would be 3.
  
  @param function callback [optional]
  Invoked once the write to the SPI device finishes.