FALLING, RISING and CHANGE

Part of the Timing Without Delay section of Coddy's Arduino journey. Lesson 13 of 18.

The Uno has two pins that can trigger an interrupt: pin 2 and pin 3. A button that needs an interrupt must be wired to one of them. Inside the chip they are interrupt 0 and interrupt 1, so let digitalPinToInterrupt() turn the pin into its number. attachInterrupt(3, ...) asks for interrupt 3, which the Uno does not have, and nothing happens.

The last part of attachInterrupt() says which change to react to:

FALLING: the pin goes from HIGH to LOW (with a pull-up, a press)
RISING: the pin goes from LOW to HIGH (a release)
CHANGE: either one

With CHANGE, the ISR runs on the press and on the release. It reads the pin to find out which one happened:

void onChange() {
  if (digitalRead(buttonPin) == LOW) {
    // it went down
  } else {
    // it came up
  }
}

attachInterrupt(digitalPinToInterrupt(buttonPin), onChange, CHANGE);

Challenge

Easy

The button is on pin 3, and loop() is busy with a slow job. Make the LED on pin 13 follow the button instantly:

in onChange(), read the button: LOW lights the LED, HIGH turns it off
attach onChange() to the button pin with CHANGE

Press Run, hold and release the button, then Check.

Try it yourself

const int ledPin = 13;
const int buttonPin = 3;

// Runs every time the button pin changes.
void onChange() {
  // light the LED when the button is down, turn it off when it is up
}

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

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

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 CHANGESharing With volatile Count Presses by Interrupt

Practice on your own: Arduino playground