Attach Interrupts

From: https://www.arduino.cc/reference/en/language/functions/external-interrupts/attachinterrupt/


Intro Description Digital Pins With Interrupts Notes And Warnings Using Interrupts About ISRs Syntax Parameters	Returns Example Code Interrupt Numbers


attachInterrupt()
[External Interrupts]



Description



Digital Pins With Interrupts
The first parameter to attachInterrupt() is an interrupt number. Normally
 you should use digitalPinToInterrupt(pin) to translate the actual digital
 pin to the specific interrupt number. For example, if you connect to pin
 3, use digitalPinToInterrupt(3) as the first parameter to
 attachInterrupt().


Board
Digital Pins Usable For Interrupts
Notes
Uno Rev3, Nano, Mini, other 328-based 2, 3
UNO R4 Minima, UNO R4 WiFi 2, 3
Uno WiFi Rev2, Nano Every All digital pins
Mega, Mega2560, MegaADK 2, 3, 18, 19, 20, 21 (pins 20 & 21 are not
 available to
 use for interrupts while they are used for I2C communication; they also
 have external pull-ups that cannot be disabled)
Micro, Leonardo 0, 1, 2, 3, 7
Zero 0-3, 5-13, A0-A5 Pin 4 cannot be used as an interrupt.
MKR Family boards 0, 1, 4, 5, 6, 7, 8, 9, A1, A2
Nano 33 IoT 2, 3, 9, 10, 11, 13, A1, A5, A7
Nano 33 BLE, Nano 33 BLE Sense (rev 1 & 2) all pins
Nano RP2040 Connect 0-13, A0-A5
Nano ESP32 all pins
GIGA R1 WiFi all pins
Due all digital pins
101 all digital pins (Only pins 2, 5, 7, 8, 10, 11, 12, 13 work with
 CHANGE))




Notes and Warnings
ote
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 ensure 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( digitalPinToInterrupt(pin), ISR, mode) (recommended)
attachInterrupt( interrupt, ISR, mode) (not recommended)
attachInterrupt( pin, ISR, mode) (Not recommended. Additionally, this syntax
	only works on Arduino SAMD Boards, Uno WiFi Rev2, Due, and 101.) 



Parameters
interrupt: the number of the interrupt. Allowed data types: int.

pin: the Arduino pin number.

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 constants 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, Zero and MKR1000 boards allow also:
HIGH to trigger the interrupt whenever the pin is high.



Returns
Nothing



Example Code
	/*H*****************************************************
*
*******************************************************/
// ========================== DEFINES ==============================

// ========================== PROTOTYPES ===========================

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

/*F*****************************************************
*
*******************************************************/
void 
setup() 
{
	pinMode( ledPin, OUTPUT );
	pinMode( interruptPin, INPUT_PULLUP );
	attachInterrupt( digitalPinToInterrupt( interruptPin ), blink, CHANGE);
}
/*F*****************************************************
*
*******************************************************/
void 
loop() 
{
	digitalWrite(ledPin, state);
}
/*F*****************************************************
*
*******************************************************/
void 
blink() 
{
	state = !state;
}



Interrupt Numbers
Normally you should use digitalPinToInterrupt(pin), rather than place an
interrupt number directly into your sketch. The specific pins with
interrupts and their mapping to interrupt number varies for each type of
board. Direct use of interrupt numbers may seem simple, but it can cause
compatibility trouble when your sketch runs on a different board.

However, older sketches often have direct interrupt numbers. Often number 0
(for digital pin 2) or number 1 (for digital pin 3) were used. The table
below shows the available interrupt pins on various boards.

Note that in the table below, the interrupt numbers refer to the number to
be passed to attachInterrupt(). For historical reasons, this numbering
does not always correspond directly to the interrupt numbering on the
ATmega chip (e.g. int.0 corresponds to INT4 on the ATmega2560 chip).


Board
int.0
int.1
int.2
int.3
int.4
int.5
Uno, Ethernet 2 3 
Mega2560 2 3 21 20 19 18
32u4 based (e.g Leonardo, Micro) 3 2 0 1 7


For Uno WiFi Rev2, Due, Zero, MKR Family and 101 boards the interrupt
 number = pin number.

Board	Digital Pins Usable For Interrupts	Notes
Uno Rev3, Nano, Mini, other 328-based	2, 3
UNO R4 Minima, UNO R4 WiFi	2, 3
Uno WiFi Rev2, Nano Every	All digital pins
Mega, Mega2560, MegaADK	2, 3, 18, 19, 20, 21	(pins 20 & 21 are not available to use for interrupts while they are used for I2C communication; they also have external pull-ups that cannot be disabled)
Micro, Leonardo	0, 1, 2, 3, 7
Zero	0-3, 5-13, A0-A5	Pin 4 cannot be used as an interrupt.
MKR Family boards	0, 1, 4, 5, 6, 7, 8, 9, A1, A2
Nano 33 IoT	2, 3, 9, 10, 11, 13, A1, A5, A7
Nano 33 BLE, Nano 33 BLE Sense (rev 1 & 2)	all pins
Nano RP2040 Connect	0-13, A0-A5
Nano ESP32	all pins
GIGA R1 WiFi	all pins
Due	all digital pins
101	all digital pins	(Only pins 2, 5, 7, 8, 10, 11, 12, 13 work with CHANGE))

Board	int.0	int.1	int.2	int.3	int.4	int.5
Uno, Ethernet	2	3
Mega2560	2	3	21	20	19	18
32u4 based (e.g Leonardo, Micro)	3	2	0	1	7