Timers

For most MiraOS applications, timers will be essential. The timers are used for checking if a time period has passed, scheduling tasks, etc. The timers are also used by applications to let the OS work with other things or enter low power mode for a time period before resuming execution.

Event timers

An event timer, or etimer, is used for scheduling processes to be executed after a period of time. Etimers are used in processes to wait for a time period while the rest of the system can work or enter low power mode. An etimer will send an event to the process when it expires.

Example 1

In the following example an etimer is used to pause the execution of the current process for one second before it continues.

This is done in these steps:

1. Defining a etimer variable (note: this must be static to preserve it over context switches).
2. Setting the etimer with the etimer_set(...) call. An event will be generated when timer expires.
3. Telling the process to wait for an event.

PROCESS_THREAD(test_hello, ev, data)
{
  /* An event-timer variable. Note that this variable must be static
     in order to preserve the value across yielding. */
  static struct etimer et;

  PROCESS_BEGIN();

  while (1) {
    etimer_set(&et, CLOCK_SECOND); /* Set the timer to 1 second. */
    PROCESS_WAIT_EVENT_UNTIL(etimer_expired(&et));
    printf("Hello, world!\n");
  }

  PROCESS_END();
}

Example 2

The second example will use two etimers, one to print a character every 0.5 seconds, and one that prints a newline every 10 seconds.

PROCESS_THREAD(test_print, ev, data)
{
  /* Our two timer variables */
  static struct etimer et, et2;

  PROCESS_BEGIN();

  etimer_set(&et, CLOCK_SECOND / 2); /* Set the timer to 0.5 seconds. */
  etimer_set(&et2, CLOCK_SECOND * 10); /* Set the timer to 10 seconds. */

  while (1) {
    PROCESS_WAIT_EVENT(); /* wait for any event */
    if (etimer_expired(&et)) {
      etimer_set(&et, CLOCK_SECOND / 2); /* reset the timer */
      printf(".");
    }
    if (etimer_expired(&et2)) {
      etimer_set(&et2, CLOCK_SECOND * 10); /* reset the timer */
      printf("\n");
    }
  }

  PROCESS_END();
}