Cron Jobs in Arduino

ArduinoHardwareSoftware & Coding

The CronAlarms library in Arduino helps you set cron jobs in Arduino. If you aren't aware of cron jobs, they are scheduled tasks to be carried out at a fixed time interval. An example can be sending a health packet to a server every day at midnight.

In order to install this library, search for CronAlarms in the Library Manager and install the library by Martin Laclaustra.

Once installed, go to - File → Examples → CronAlarms.

Open CronAlarms_example. If you go through the example, you will see that they are doing the following −

  • Set the time to Saturday 8:29:00am Jan 1 2011, using the tm struct (time library)

  • Sets 6 Cron alarms, daily, weekly, at interval of few seconds, and even a one-time triggered non-repeating alarm.

  • The one-time triggered non-repeating alarm, in fact, disables the 2-second cron.

  • Within the loop, the current time is printed every second.

The cron creation syntax is -

CronID_t create(const char * cronstring, OnTick_t onTickHandler, bool isOneShot);


  • cronstring provides the cron interval,

  • onTickHandler is the function that will be called when the alarm is triggered,

  • isOneShot determines if the alarm is repeating or is a one-time interval.

This function returns a cron ID, which can be used to reference the alarm elsewhere in the code, as is done in the 10-second alarm, to disable the 2-second alarm, using

//creating the 2-second repeating alarm and 10-second single-shot alarm

// timer for every 2 seconds
id = Cron.create("*/2 * * * * *", Repeats2, false);

// called once after 10 seconds
Cron.create("*/10 * * * * *", OnceOnly, true);

// 2-second alarm function
void Repeats2() {
   Serial.println("2 second timer");

//disabling 2-second alarm within the 10-second alarm
void OnceOnly() {
   Serial.println("This timer only triggers once, stop the 2 second timer");
   // use to disable a timer and recycle its memory.;
   // optional, but safest to "forget" the ID after memory recycled
   // you can also use Cron.disable() to turn the timer off, but keep
   // it in memory, to turn back on later with Alarm.enable().

The most important part of creating the CronAlarm is the cron string. Its syntax is −

"seconds minutes hours days_of_month month days_of_week"

A * for any of these means 'all values'. Thus,

  • "0 30 8 * * *" → Indicates trigger at 8:30:00 every day.

  • "30 30 8 * * 6" → Indicates 8:30 trigger at 8:30:30 every Saturday (day_of_week = 6)

  • A / in the expression indicates interval.

  • "*/15 * * * * *" → Indicates every 15 seconds.

If you run this example on your Arduino, the Serial Monitor output will be −

As you can see, the alarms get triggered as expected.

Published on 26-Jul-2021 11:15:14