Simple timer code for Arduino

timer_test.pde

// Timer test
// Kevin Karplus
// 12 July 2011

// One of the simplest data acquisition tasks is 
// to time the interval between two events.

// This program waits for pin 4 to go high, 
//   starts a timer, lights the on-board LED,
//   then waits for pin 5 to go low,
//   when it turns of the LED and resets the timer.
// It reports the time between pin 4 going high and pin 5 going low
//   in microseconds.

// It is easy to change the code to use 
// either transitions to high or transitions to low.
// Using a pull-up resistor can convert any make or break contact to
// a low-going or high-going edge.
// Opposite polarities were used in this example, so that the
// same square wave could be fed to both pins and the width of the
// positive pulse timed.


// The minimum time it can report (if pin 5 is already low when
// pin 4 goes high is about 12 microseconds +- 4 microseconds).
// The reported time seems to be always a multiple of 4 microseconds,
// which is most likely the resolution of the micros() call.

// Timing seems to be fairly reliable down to about 60 microseconds.

void setup()
{
  pinMode(4, INPUT);
  pinMode(5, INPUT);
  
  pinMode(13, OUTPUT);   // on-board LED
  Serial.begin(115200);
}

void loop()
{
     unsigned long start_time;
     unsigned long stop_time;
     
     digitalWrite(13,0);   // Turn LED off
     while(digitalRead(4)==0) {}
     start_time=micros();
     digitalWrite(13,1);   // Turn LED on
     while (digitalRead(5)) {}
     stop_time=micros();
     digitalWrite(13,0);
     Serial.print("time hi->lo=");
     Serial.print(stop_time-start_time);
     Serial.println(" microsec");
     delay(500);
}