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I don’t know how many times I’ve written bad Arduino code to call a function every few milliseconds. Sometimes this bad code works well enough for my sketch to actually work. Often, it either doesn’t work at all or does something I really didn’t expect.

So on Arduino Day 2017, I’m glad I found out about bhagman/MillisTimer: A Wiring and Arduino library for working with millis(). It couldn’t be simpler to use: include the library, write the function you want to call every N milliseconds, set up the timer to run every N millis, and put timer.run() in a loop that’s called frequently. The library handles the timing and resetting all by itself.

As an example, here’s the eternal “Hello, World!” of the embedded world, Blink, rewritten to use MillisTimer:

// MillisTimerBlink - blink LED every second
//  using Brett Hagman's MillisTimer library
//  https://github.com/bhagman/MillisTimer
//  (or use Sketch → Include Library → Manage Libraries … to install)
// scruss - 2017-04-01

#include <MillisTimer.h>
MillisTimer timer1;               // new empty timer object
const int led_pin = LED_BUILTIN;  // use the built-in LED

void flash() {                    // function called by timer
  static boolean output = HIGH;
  digitalWrite(led_pin, output);  // set LED on or off
  output = !output;               // toggle variable state High/Low
}

void setup() {
  pinMode(led_pin, OUTPUT);       // use built-in LED for output
  timer1.setInterval(1000);       // set timer to trigger every 1000 millis
  timer1.expiredHandler(flash);   // call flash() function when timer runs out
  timer1.setRepeats(0);           // repeat forever if set to 0
  timer1.start();                 // start the timer when the sketch starts
}

void loop() {
  timer1.run();                   // trigger the timer only if it has run out
  // note that run() has to be called more frequently than the timer interval
  //  or timings will not be accurate
}

Note that MillisTimer only triggers when timer.run() is called. Sticking a delay(2000) in the main loop will cause it to fire far less frequently than the interval you set. So it’s not technically a true periodic timer, but is good enough for most of my purposes. If you want a true interrupt-driven timer, use the MsTimer2 library. It relies on the timer interrupts built into the Arduino hardware, and isn’t quite as easy to use as MillisTimer.