code: function Ultrasonic(board, trigger, echo) {
      Module.call(this);
  
      this._type = 'HC-SR04';
      this._board = board;
      this._trigger = trigger;
      this._echo = echo;
      this._distance = null;
      this._lastRecv = null;
      this._pingTimer = null;
      this._pingCallback = function () {};
  
      this._board.on(BoardEvent.BEFOREDISCONNECT, this.stopPing.bind(this));
      this._messageHandler = onMessage.bind(this);
      this._board.on(BoardEvent.ERROR, this.stopPing.bind(this));
    }
docs: The Ultrasonic class.
  
  @namespace webduino.module
  @class Ultrasonic
  @constructor
  @param {webduino.Board} board The board the ultrasonic sensor is attached to.
  @param {webduino.Pin} trigger The trigger pin the sensor is connected to.
  @param {webduino.Pin} echo The echo pin the sensor is connected to.
  @extends webduino.Module