This Arduino code measures the time interval between a pin going high and another pin going low. It waits for pin 4 to go high, starts a timer, lights an LED, then waits for pin 5 to go low, stops the timer, and reports the elapsed time in microseconds. The code provides a simple way to time events and can reliably measure intervals down to about 60 microseconds.

1. Simple timer code for Arduino
Raw
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);

2. 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);
}