Time: delay, millis and micros

Every sketch so far has used delay(). It is the easiest way to make something happen slowly enough to see — and it is a blunt instrument.

delay(1000) does not mean "wait a second while other things carry on". It means stop. For that whole second the Arduino does nothing else: it will not read a button, will not check a sensor, will not answer the Serial Monitor. A press during that second is simply missed.

millis() is the alternative. It does not wait at all. It hands you the number of milliseconds since the board was switched on, and leaves you to decide whether enough time has passed. Nothing is blocked, so the sketch can watch several things at once.

micros() is the same idea a thousand times finer, for things too quick to measure in milliseconds.

The shift from "wait here" to "has it been long enough yet" is the single biggest step in this course. Everything that reacts while it is doing something else depends on it.

No wiring. Everything here is read in the Serial Monitor at 9600 baud.

→ Blink Without Delay — the canonical example, taken apart line by line.

The four calls

Call	Gives you	Range before it wraps
delay(ms)	nothing — it stops	—
delayMicroseconds(us)	nothing — it stops, briefly	—
millis()	milliseconds since power-on	about 49 days
micros()	microseconds since power-on	about 70 minutes

delay() stops everything

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

void loop() {
  Serial.println("tick");
  delay(1000);
}

One line a second, and for 999 of every 1000 milliseconds the board is asleep on its feet. Nothing else can happen.

delayMicroseconds() is the same, for very short waits — under about 16000 microseconds. Below three microseconds it is not accurate, and it is used mostly for timing signals to sensors.

millis() is a clock, not a pause

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

void loop() {
  Serial.println(millis());
  delay(500);
}

The numbers climb: 0, 500, 1000, 1500. That is time since the board started, not time since anything you did.

Reset the board and it starts again from zero. millis() has no idea what day it is — it only knows how long it has been awake.

Two clocks at once.

Using only millis() and no delay() anywhere, print:

• fast every 200 ms
• slow every 1000 ms

Both must keep their own rhythm. Every fifth fast should be followed by a slow.

You will need two variables remembering when each last ran, and the if pattern from the lesson above. This is the first sketch in the course that does two things at genuinely different rates.

Think about it: try writing the same thing with delay() only. You cannot — not without the two rhythms interfering. That is the whole argument, and it is why millis() is worth the extra lines.