Arduino Interrupts

With an Arduino (or any other CPU) there are 2 basic ways to use interrupts

Research the topic and write your own code
Use a library

With Arduinos, the problem is that each type of processor has a slightly different way of implementing interrupts. As a result, if you write it yourself the code will be faster but not as portable. However, if you use the EnableInterrupt library, the code will be slower, but completely portable.

Regardless of the method chosen, variables modified in an interrupt routine must be defined as volatile.

Background | EnableInterrupt library

Background

To get a solid background on Arduino interrupts, I suggest this article by Nick Gammon which is specific to the Arduino Uno (Atmega328).

The main differences between various Arduino processors are

Which pins support Rising Edge and Falling Edge interrupts
Whether an interrupt is attached to a single pin or a group of pins

With the Uno, all pins support a Pin Change interrupt, but, because they are ganged, the hardware can not tell the program which pin changed - only that one of the pins in the block changed. In addition, only 2 pins (D2 & D3) support individual interrupts and both Rising Edge and Falling Edge detection. (These 2 pins are referred to as external interrupts.)

EnableInterrupt library

The EnableInterrupt library hides the differences between various Arduino types and simulates all the interrupt types on every pin - even when the hardware does not actually support that type of interrupt. Of course, there is a cost for this - the library is fairly complex and the interrupt routines run much slower (but probably still much faster than you need).

Assuming that you only want to enable interrupts on a few pins, there is a mechanism to reduce the amount of code generated. Using this, you might save a few K of program space and 42 bytes (2%) of your available static RAM (where data is stored).

This is the library I used to develop my HC-SR04 Ultrasonic Sensor library.

Author: Robert Clemenzi