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attachInterrupt()

Description

Specifies a named Interrupt Service Routine (ISR) to call when an interrupt occurs. Replaces any previous function that was attached to the interrupt. Most Arduino boards have two external interrupts: numbers 0 (on digital pin 2) and 1 (on digital pin 3). The table below shows the available interrupt pins on various boards.

Boardint.0int.1int.2int.3int.4int.5
Uno, Ethernet23    
Mega25602321201918
32u4 based (e.g Leonardo, Micro)32017 
Due, Zero(see below)

The Arduino Due board has powerful interrupt capabilities that allows you to attach an interrupt function on all available pins. You can directly specify the pin number in attachInterrupt().

The Arduino Zero board allows you to attach an interrupt function on all available pins except for pin 4. You can directly specify the pin number in attachInterrupt().

Note

Inside the attached function, delay() won't work and the value returned by millis() will not increment. Serial data received while in the function may be lost. You should declare as volatile any variables that you modify within the attached function. See the section on ISRs below for more information.

Using Interrupts

Interrupts are useful for making things happen automatically in microcontroller programs, and can help solve timing problems. Good tasks for using an interrupt may include reading a rotary encoder, or monitoring user input.

If you wanted to insure that a program always caught the pulses from a rotary encoder, so that it never misses a pulse, it would make it very tricky to write a program to do anything else, because the program would need to constantly poll the sensor lines for the encoder, in order to catch pulses when they occurred. Other sensors have a similar interface dynamic too, such as trying to read a sound sensor that is trying to catch a click, or an infrared slot sensor (photo-interrupter) trying to catch a coin drop. In all of these situations, using an interrupt can free the microcontroller to get some other work done while not missing the input.

About Interrupt Service Routines

ISRs are special kinds of functions that have some unique limitations most other functions do not have. An ISR cannot have any parameters, and they shouldn't return anything.

Generally, an ISR should be as short and fast as possible. If your sketch uses multiple ISRs, only one can run at a time, other interrupts will be executed after the current one finishes in an order that depends on the priority they have. millis() relies on interrupts to count, so it will never increment inside an ISR. Since delay() requires interrupts to work, it will not work if called inside an ISR. micros() works initially, but will start behaving erratically after 1-2 ms. delayMicroseconds() does not use any counter, so it will work as normal.

Typically global variables are used to pass data between an ISR and the main program. To make sure variables shared between an ISR and the main program are updated correctly, declare them as volatile.

For more information on interrupts, see Nick Gammon's notes.

Syntax

attachInterrupt(interrupt, ISR, mode)
attachInterrupt(digitalPinToInterrupt(pin), ISR, mode);
attachInterrupt(pin, ISR, mode) (Arduino Due, Zero only)

Note

If the syntax:

attachInterrupt(digitalPinToInterrupt(pin), ISR, mode);

is used, the pin on the board is automatically translated to the corresponding interrupt number like showed in the table above.

Parameters

interrupt: the number of the interrupt (int)

pin: the pin number (Arduino Due, Zero only)

ISR: the ISR to call when the interrupt occurs; this function must take no parameters and return nothing. This function is sometimes referred to as an interrupt service routine.

mode:

defines when the interrupt should be triggered. Four contstants are predefined as valid values:

  • LOW to trigger the interrupt whenever the pin is low,
  • CHANGE to trigger the interrupt whenever the pin changes value
  • RISING to trigger when the pin goes from low to high,
  • FALLING for when the pin goes from high to low.

The Due board allows also:

  • HIGH to trigger the interrupt whenever the pin is high.
(Arduino Due, Zero only)

Returns

none

Example

int pin = 13;
volatile int state = LOW;

void setup()
{
  pinMode(pin, OUTPUT);
  attachInterrupt(0, blink, CHANGE);
}

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

void blink()
{
  state = !state;
}

See also

Reference Home

Corrections, suggestions, and new documentation should be posted to the Forum.

The text of the Arduino reference is licensed under a Creative Commons Attribution-ShareAlike 3.0 License. Code samples in the reference are released into the public domain.

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