A Press the Loop Misses

Part of the Timing Without Delay section of Coddy's Arduino journey. Lesson 12 of 18.

A sketch is sometimes busy with a slow job or a long delay(). A loop that reads the button only between jobs misses a press that starts and ends inside one.

An interrupt fixes that: when a pin changes, the board stops what it is doing, runs a function you choose, then carries on. The chip watches the pin itself, even during delay().

void onPress() {
  digitalWrite(ledPin, HIGH);
}

void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(buttonPin), onPress, FALLING);
}

attachInterrupt() takes three things: the interrupt for the pin (digitalPinToInterrupt(buttonPin) finds it), the function to run, and when. FALLING is the moment the pin goes from HIGH to LOW: with a pull-up, the moment the button goes down.

The function is called an interrupt service routine, or ISR. Keep it short: no delay() and no Serial.

Challenge

Easy

This alarm checks its button only once every 2 seconds, so a quick press is missed. Make a press light the LED at once:

write an ISR, onPress(), that turns the LED on
in setup(), attach it to the button pin with FALLING

Keep the slow job in loop(). The LED stays on after a press, like an alarm. Press Run, tap the button, then Check.

Try it yourself

const int ledPin = 13;
const int buttonPin = 2;

void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP);
  Serial.begin(9600);
  // attach the interrupt here
}

void loop() {
  // the slow job: keep it as it is
  Serial.println("working");
  delay(2000);

  // this only looks at the button once every 2 seconds,
  // so a short press is missed
  if (digitalRead(buttonPin) == LOW) {
    digitalWrite(ledPin, HIGH);
  }
}

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() Clock Times 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 MissesFALLING, RISING and CHANGE Sharing With volatile Count Presses by Interrupt

Practice on your own: Arduino playground