volatile int state = LOW; // must declare volatile, since it's
                          // modified within the  handler
volatile int HighToLowCount = 0;
volatile int LowToHighCount = 0;
volatile int val;
volatile int CurrentCount;
volatile int Period;
volatile int OldCount;
volatile int PulseWidth;
volatile int OverflowCount;
volatile int timeroverflow;

HardwareTimer timer1(1);

void setup()
{
    pinMode(BOARD_LED_PIN, OUTPUT);
    pinMode(0, INPUT);
    attachInterrupt(0, exti_handler, CHANGE);
    
    CurrentCount = 0;
    OldCount = 0;
    Period = 0;
    PulseWidth = 0;
    OverflowCount = 0;
    timeroverflow = 0;
    
        // Setup Counting Timers
    timer1.setChannel1Mode(TIMER_OUTPUT_COMPARE);
    timer1.pause();
    timer1.setCount(0);
    timer1.setPrescaleFactor(72);  //run at 72 MHz/72 = 1MHz count every 1us
    timer1.setOverflow(65535);
    timer1.setCompare1(1);   
    timer1.attachCompare1Interrupt(timer1_handler);
    timer1.refresh();
    timer1.resume();


}

void loop()
{
    SerialUSB.print("LowToHighCount: ");
    SerialUSB.println(LowToHighCount);
    SerialUSB.print("HighToLowCount: ");
    SerialUSB.println(HighToLowCount);
    SerialUSB.print("Period: ");
    SerialUSB.println(Period);
    SerialUSB.print("PulseWidth: ");
    SerialUSB.println(PulseWidth);
    SerialUSB.print("Overflow: ");
    SerialUSB.println(OverflowCount);
    delay(1000);
}

void exti_handler() 
{
    timer1.pause();
    CurrentCount = timer1.getCount();
    val = digitalRead(0);

    
    if (val == HIGH)
    {
        LowToHighCount++;
        Period = CurrentCount;
        timer1.setCount(0);
        timer1.refresh();
    } 
    else { // state must be LOW
        HighToLowCount++;
        PulseWidth = CurrentCount;
    }

    timer1.resume();
}

void timer1_handler()
{
  OverflowCount = 0;
  timeroverflow++;
}