The interrupt service routine and volatile variables

Operations like writing to the serial monitor that take more than a few processing cycles should be avoided.

In place of longer-running operations inside the ISR, you should write code that simply updates a volatile variable. When your sketch returns to the loop() function, it can read the data from the same volatile variable and process it as needed.

In other words, a volatile variable allows you to get data out of an ISR, to the rest of your sketch. This way, you can do the longer processing you need, without increasing the footprint of the ISR.

To declare a volatile variable, use the “volatile” keyword, like this:

volatile int temp = 0;

Then, you can use it like any other variable. Inside the ISR:

void isr()
{
      temp = analogRead(0); //Keep this block very short!
}

Finally inside the loop():

loop()
{
     Serial.println(temp); // plus lots more code
}

The volatile keyword informs the compiler that the flagged variable can change at any time, from any part of the sketch or program. In the Arduino that only has one processing unit and no true parallel programming is possible, this happens in the interrupt service routine only.

If you want to learn more about port expanders, interrupts and the volatile keyword, consider our comprehensive course Arduino Step by Step Getting Serious.