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µC/OS-III provides a number of services to manage timers as summarized in the table below.

A timer needs to be created before it can be used. You create a timer by calling OSTmrCreate() and  specify a number of arguments to this function based on how the timer is to operate. Once the timer operation is specified, its operating mode cannot be changed unless the timer is deleted and recreated. The function prototype for OSTmrCreate() is shown below as a quick reference:

void  OSTmrCreate (OS_TMR              *p_tmr,            /* Pointer to timer     */
                   CPU_CHAR            *p_name,           /* Name of timer, ASCII */
                   OS_TICK              dly,              /* Initial delay        */
                   OS_TICK              period,           /* Repeat period        */
                   OS_OPT               opt,              /* Options              */
                   OS_TMR_CALLBACK_PTR  p_callback,       /* Fnct to call at 0    */
                   void                *p_callback_arg,   /* Arg. to callback     */
                   OS_ERR              *p_err)

Once created, a timer can be started (or restarted) and stopped as often as is necessary. Timers can be created to operate in one of three modes: One-shot, Periodic (no initial delay), and Periodic (with initial delay).

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You terminate the countdown process of a timer (before it reaches zero) by calling OSTmrStop(). In this case, you can specify that the callback function be called or not.

As shown in the figure below, a one-shot timer can be retriggered by calling OSTmrStart() before the timer reaches zero. This feature can be used to implement watchdogs and similar safeguards.

Periodic (no initial delay)

As indicated in the in the figure below, timers can be configured for periodic mode. When the countdown expires, the callback function is called, the timer is automatically reloaded, and the process is repeated. If specifying a delay of zero (i.e., dly == 0) when the timer is created and, when started, the timer immediately uses the “period” as the reload value. You can call OSTmrStart() at any point in the countdown to restart the process.

Periodic (with initial delay)

As shown in the figure below, timers can be configured for periodic mode with an initial delay that is different than its period. The first countdown count comes from the “dly” argument passed in the OSTmrCreate() call, and the reload value is the “period”. You can call OSTmrStart() to restart the process including the initial delay.