Timers

A timer provides the ability to execute an application-defined function upon expiration of a specified time interval. A timer can be started, stopped, reset, or restarted at any time from any code context, including from an interrupt service routine (ISR).

A timer has the resolution of the kernel tick rate. When specifying a time interval, the accuracy is +1/-0 of a system tick as the tick ISR is not synchronized with the starting time of a timer.

The graph below illustrates the remaining time before expiration for a timer as a function of run-time. The graph shows how the different API calls affect the timer's remaining time.

Basic Usage

A timer can be used to repeatedly call an application-defined function. The code snippet below creates a timer that expires every 100 kernel ticks (milliseconds by default). The timer is created with the options to have the timer started and to have the timer automatically reset upon each expiration which causes repeated calls to the timer callback function.

#include "Kernel/kernel_timer.h"
#include <assert.h>

static TIMER timer1;                                    /* Allocate a timer */

void APP_Example(void)
{
    STATUS status;


    status = TIMER_Create(&timer1,                      /* Create a timer */
                          "Timer1",                     /* A name for the timer (debugging purposes) */
                          APP_TimerCallback,            /* A function to be called when the timer expires */
                          NULL,                         /* An optional argument (not used here) */
                          100,                          /* Interval, in kernel ticks */
                          OPT_START | OPT_AUTORESET);   /* Start and configure as a repeating timer */

    assert(status == SUCCESS);
}

void APP_TimerCallback(TIMER* timer, void* arg)
{
    /* Called repeatedly every 100 kernel ticks (milliseconds by default) */
}