Arduino: Why You Should Use millis() Instead of delay()

You want to blink an LED and read a button. You use delay(1000) for the blink, and now the button only works if you press it at exactly the right moment.

Quick answer: delay() stops the entire program. Use millis() to check "has enough time passed?" instead, so your loop never blocks.

Why delay() is a problem

void loop() {
  digitalWrite(LED, HIGH);
  delay(1000);            // nothing else can run here for 1 second
  digitalWrite(LED, LOW);
  delay(1000);            // ...or here
}

During those two seconds the Arduino ignores buttons, sensors, serial data — everything. That's fine for a demo, and terrible for a real project (like a clock that has to update a display, check a button and blink a colon at the same time).

The millis() pattern

millis() returns the milliseconds since the board started. Instead of waiting, you remember when you last did something and compare.

const int LED = 13;
const unsigned long INTERVAL = 1000;
 
unsigned long previousMillis = 0;
bool ledState = false;
 
void setup() {
  pinMode(LED, OUTPUT);
}
 
void loop() {
  unsigned long now = millis();
 
  if (now - previousMillis >= INTERVAL) {
    previousMillis = now;
    ledState = !ledState;
    digitalWrite(LED, ledState);
  }
 
  // Anything else runs freely here, every loop
}

loop() now runs thousands of times per second, and the blink code only acts once per interval.

Do multiple things at once

Each task gets its own timestamp:

unsigned long lastBlink = 0;
unsigned long lastSensor = 0;
 
void loop() {
  unsigned long now = millis();
 
  if (now - lastBlink >= 500) {
    lastBlink = now;
    toggleLed();
  }
 
  if (now - lastSensor >= 2000) {
    lastSensor = now;
    readTemperature();
  }
 
  checkButton(); // runs every loop, no delay
}

Now the button is responsive, the LED blinks and the sensor is polled — all independently.

The rules

1. Use unsigned long for time variables. int overflows after about 32 seconds.
2. Always subtract (now - previous >= interval) rather than comparing (now >= previous + interval). The subtraction keeps working correctly when millis() wraps around after about 49 days.
3. Update the timestamp inside the if, otherwise the code runs every loop.

Is delay() ever OK?

Yes: in setup(), in quick tests, or in a project that truly does one thing. The moment you need two things at once, switch to millis().

Checklist

• ✅ No delay() in loop() for anything longer than a few milliseconds
• ✅ Each timed task has its own unsigned long timestamp
• ✅ Time is compared using subtraction
• ✅ Timestamps are refreshed inside the if block