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Tutorial: Timers and events

George Oikonomou edited this page Apr 27, 2018 · 14 revisions

This tutorial will show how to make use of timers in Contiki-NG. It will also give a basic into to events. For more extensive documentation on these aspects, see doc:timers.

The etimer

The etimer (Event timer) will post an event when the timer is expiring. Since it posts an event you will need to have a process around it to handle the event.

PROCESS_THREAD(etimer_process, ev, data)
{
  static struct etimer et;
  PROCESS_BEGIN();
  /* set timer to expire after 5 seconds */
  etimer_set(&et, CLOCK_SECOND * 5);
  while(1) {
    PROCESS_WAIT_EVENT(); /* Same thing as PROCESS_YIELD */
    if(etimer_expired(&et)) {
      /* Do the work here and restart timer to get it periodic !!! */
      printf("etimer expired.\n"); 
      etimer_restart(&et);
    }
  }
  PROCESS_END();
}

In the above code the process is defined by PROCESS_THREAD which takes a name, an event variable and an event data variable as the arguments. When an etimer expires the event variable will be set to PROCESS_EVENT_TIMER and the event data variable will be set to point to the specific timer. Using that information the above example could also look like:

PROCESS_WAIT_EVENT_UNTIL(ev == PROCESS_EVENT_TIMER && data == &et);
/* Do the work here and restart timer to get it periodic! */
printf("etimer expired.\n"); 
etimer_restart(&et);

But when waiting for this event in this way -- all other events will be ignored so the first approach is more flexible as this will enable handling multiple types of events more easily.

The ctimer

The ctimer (callback timer) is very similar to the etimer but instead of posting an event it will call a callback function. This means that there is no need to set-up a process.

First -- somewhere in the code -- just set the ctimer's timeout and its callback (plus a pointer to data if you have any data that the timer needs a pointer to when being called -- NULL -- in the below example):

ctimer_set(&timer_ctimer, CLOCK_SECOND, ctimer_callback, NULL);

Then somewhere in the code you also need to define the callback function:

void
ctimer_callback(void *ptr)
{
  /* rearm the ctimer */
  ctimer_reset(&timer_ctimer);
  printf("CTimer callback called\n");
}

This example function will just print that the timer callback was called and reset the timer - which will set it to trigger again after the same timeout as before -- e.g. one second (CLOCK_SECOND).

Running the timer example

To try out all the timers available in Contiki-NG you can go to the examples/libs/timers directory and build the example. If you build it for the native platform you will be able to run it on your local laptop or desktop computer.

$ cd examples/libs/timers
$ make
$ ./all-timers.native
[INFO: Main      ] Starting Contiki-NG-4.0
[INFO: Main      ]  Net: tun6
[INFO: Main      ]  MAC: nullmac
[WARN: Tun6      ] Failed to open tun device (you may be lacking permission). Running without network.
[INFO: Main      ] Link-layer address 0102.0304.0506.0708
[INFO: Main      ] Tentative link-local IPv6 address fe80::302:304:506:708
RTimer callback called
CTimer callback called
RTimer callback called
RTimer callback called
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