How to make a stopwatch with Arduino and millis()

An Arduino stopwatch is two glances at the clock and a subtraction. When it starts, save millis() in a variable; when it stops, take that away from millis() again. The difference is the time in milliseconds: elapsed = millis() − startedAt. A bool remembers whether it's running, so one button can start and stop it.

Why the start must be saved

millis() counts the milliseconds since the board powered up, and it never pauses. By the time you press stop, it has moved on, so the start has to be written down, like the time stamped on a car park ticket. That's startedAt, declared above setup() so it survives every pass of loop().

Live experiment

Stop it at exactly 2.0 seconds

One press starts the stopwatch and writes millis() on its ticket, startedAt; the next stops it and prints the time in tenths of a second. Try to stop it at exactly 2.0 s. Dividing by 100 throws away what's left over, so anything from 2000 to 2099 ms counts.

Try it yourself: free lesson

“Stopwatch” is lesson 4 of 6 in Clockwork Cove, Embedded Systems World 2. Free with an account. Lessons unlock in order, so you’ll do the 10 before it first.

The sketch

A button from pin 2 to GND, read with INPUT_PULLUP. Each press is caught once, on the change from HIGH to LOW, ignoring the pin for 50 ms after each change while the contacts bounce:

bool running = false;
unsigned long startedAt = 0;
bool last = HIGH;
unsigned long changedAt = 0;

void setup() {
  pinMode(2, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  bool now = digitalRead(2);
  if (now != last && millis() - changedAt > 50) {
    changedAt = millis();
    last = now;
    if (now == LOW) {                     // a new press
      if (!running) {
        startedAt = millis();             // start: note the time
      } else {
        unsigned long elapsed = millis() - startedAt;
        Serial.print("tenths: ");
        Serial.println(elapsed / 100);
      }
      running = !running;                 // flip the state
    }
  }
}

The first press starts it, the second stops it and prints, the third starts again. running = !running flips the state, true to false or false to true, in one line.

Printing it nicely

Milliseconds are hard to read, so the sketch divides by 100 to print tenths of a second. Both numbers are whole, so the division drops what's left over: 1250 ms prints 12, and 1980 ms prints 19, not 20.

Serial.print writes without ending the line and Serial.println ends it, so a label and its number share one line: tenths: 12. For seconds with decimals, divide by 1000.0 and choose the places: Serial.println(elapsed / 1000.0, 2) prints 1.25.

Make it a game

The same two glances time anything: a reaction-time game that lights an LED after a random wait and times how fast you press, a lap timer, or how long a door was left open.