title: Can not get current output pin state on Arduino Due
body: <p>I have a simple task of toggling an output pin state on Arduino Due using Arduino IDE 1.5.2.
  I have tried the code like this:</p>
  
  <pre><code>int pinnum = 13;
  void setup() {
    pinMode(pinnum, OUTPUT);  // use on-board LED
  } 
  
  void loop() {
    digitalWrite(pinnum, !digitalRead(pinnum));
    delay(1000);  // wait around for 1 sec (1000 ms)
  }
  </code></pre>
  
  <p>This does nothing. Pin 13 stays HIGH all the time. What am I doing wrong?</p>