Using interrupts with Arduino

Interrupts allow the microcontroller to execute a function when an event occurs on one of the interrupt pins. Instead of constantly reading the value of a sensor, the program will only be triggered when the value of the sensor changes. This solves many task layout problems.

Material

• Arduino UNO
• push button (or other sensor)
• USB A/ USB B cable

Scheme

On the Arduino UNO board, there are two pins that support the interrupts: pins 2 and 3. So we will connect the output of the button to pin 2 of the Arduino.

Reminder of the digital pins compatible with the interrupts

Interrupt management code

The syntax to initialize an interrupt is as follows:

attachInterrupt(digitalPinToInterrupt(pin), ISR, mode)

With pin, the pin used, ISR, the function to activate when the event is detected and mode, the detection mode used.

• LOW to enable detection when the spindle is in the LOW state
• CHANGE to activate the detection when the state of the spindle changes
• RISING to enable detection when the pin status changes from LOW to HIGH
• FALLING to enable detection when the pin status changes from HIGH to LOW

const byte ledPin = 13;
const byte interruptPin = 2;
volatile byte state = LOW;

void setup() {
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);
  pinMode(interruptPin, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(interruptPin), onEvent, CHANGE);
  Serial.println(F("Système intialisé"));
}

void loop() {
  digitalWrite(ledPin, state);
}

void onEvent() {
  state = !state;
  Serial.print(F("Switch LED 13 : "));
  if(state){
    Serial.println(F("ON"));
  }else{
    Serial.println(F("OFF"));
  }
}

Warning: In the onEvent() function, the millis() and delay() functions, which depend on interrupts, will not work.

Functions to know to go further in the management of interruptions

• attachInterrupts
• detachInterrupt(digitalPinToInterrupt(pin))disable the specified interrupt
• interrupts() re-enables interrupts
• noInterrupts() to disable interrupts

Result

Once the code is uploaded, if you press the button, the LED on the Arduino should turn on and off.

Sources

• Arduino reference on interrupts
• Management of a push button