The millis() Clock

Part of the Timing Without Delay section of Coddy's Arduino journey. Lesson 1 of 18.

The board has a clock inside it. millis() returns how many milliseconds have passed since the board started: 0 at power-on, 1000 after one second, 60000 after a minute.

You never start or reset this clock. It counts on its own, even while your sketch is inside delay(), and you can read it at any moment:

Serial.println(millis());   // prints the time, in ms

Until now, delay() was the only way to time something, and it stops the whole sketch while it waits. Reading the clock stops nothing: the sketch keeps running and checks how much time has passed.

Challenge

Easy

Print the board's clock once a second:

in loop(), print millis() with Serial.println()
keep the delay(1000) after it

Press Run and watch the numbers in the Serial monitor, then Check.

Try it yourself

void setup() {
  Serial.begin(9600);
}

void loop() {
  // print the time since the board started here

  delay(1000);
}

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Timing Without Delay

1millis()

The millis() ClockTimes in unsigned long Blink Without Delay Two LEDs, Two Speeds Blink While a Button Toggles A Light on a Timer

4Precise Timing

Measure With micros()When millis() Rolls Over Button Stopwatch

2State Machines

Remember the Mode Choosing With switch Named States With enum Traffic Light State Machine Cycle Modes With a Button

3Interrupts

A Press the Loop Misses FALLING, RISING and CHANGE Sharing With volatile Count Presses by Interrupt

Practice on your own: Arduino playground