.. highlight:: cpp

.. _lang-attachinterrupt:

attachInterrupt()
=================

Used to specify a function to call when an external interrupt (like an
GPIO changing from LOW to HIGH, a button getting pressed, etc.)
occurs.

.. contents:: Contents
   :local:

Library Documentation
---------------------

.. doxygenfunction:: attachInterrupt

.. doxygenenum:: ExtIntTriggerMode

.. doxygentypedef:: voidFuncPtr

Discussion
----------

Because the function will run in interrupt context, inside of it,
:ref:`lang-delay` won't work, and the value returned by
:ref:`lang-millis` will not increment. Serial data received while
in the function may be lost. You should declare as ``volatile`` any
global variables that you modify within the attached function.

There are a few constraints you should be aware of if you're using
more than one interrupt at a time; the :ref:`external-interrupts` page
has the details.


Using Interrupts
----------------

Interrupts are useful for making things happen automatically in
microcontroller programs, and can help solve timing problems. A
good task for using an interrupt might be reading a rotary encoder,
or monitoring user input.


If you wanted to insure that a program always caught the pulses
from a rotary encoder, never missing 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 doorbell.


Example
-------

::

    int maple_led_pin = 13;
    volatile int state = LOW; // must declare volatile, since it's
                              // modified within the blink handler

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

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

    void blink() {
      state = !state;
    }


Arduino Compatibility
---------------------

Most Arduino boards have two external interrupts: numbers 0 (on
digital pin 2) and 1 (on digital pin 3). The Arduino Mega has an
additional four: numbers 2 (pin 21), 3 (pin 20), 4 (pin 19), and 5
(pin 18).  On the Maple, you don't have to remember which interrupt
number goes with which pin -- just tell ``attachInterrupt()`` the pin
you want.


See also
--------


-  :ref:`detachInterrupt <lang-detachinterrupt>`
-  :ref:`external-interrupts`



.. include:: cc-attribution.txt
