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Specifies a function to call when an external 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.

Board int.0 int.1 int.2 int.3 int.4 int.5
Uno, Ethernet 2 3
Mega2560 2 3 21 20 19 18
Leonardo 3 2 0 1
Due (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().


(:table border=0 cellpadding=5 cellspacing=0 width=80%:) (:cell width=400px:) attachInterrupt(interrupt, function, mode) (:cellnr width=400px:) attachInterrupt(pin, function, mode) (:cell:) (Arduino Due only) (:tableend:)


(:table border=0 cellpadding=5 cellspacing=0 width=80%:) (:cell width=150px:) interrupt: (:cell width=240px:) the number of the interrupt (int)

(:cellnr width=150px:) pin: (:cell width=240px:) the pin number (:cell:)(Arduino Due only)

(:cellnr width=150px:) function: (:cell width=240px:) the function 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.

(:cellnr width=150px:) mode: (:cell width=240px:) 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.

(:cellnr width=150px:) (:cell width=240px:) The Due board allows also:

(:cellnr width=150px:) (:cell width=240px:) * HIGH to trigger the interrupt whenever the pin is high. (:cell:)(Arduino Due only)





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.

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.


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

* detachInterrupt() Source:

arduino/external_interrupts/attachinterrupt.txt · Last modified: 2013/02/15 20:45 (external edit)