Chapter 2. API Reference
Macros
tmrCOMMAND_EXECUTE_CALLBACK_FROM_ISR
tmrCOMMAND_EXECUTE_CALLBACK
tmrCOMMAND_START_DONT_TRACE
tmrCOMMAND_START
tmrCOMMAND_RESET
tmrCOMMAND_STOP
tmrCOMMAND_CHANGE_PERIOD
tmrCOMMAND_DELETE
tmrFIRST_FROM_ISR_COMMAND
tmrCOMMAND_START_FROM_ISR
tmrCOMMAND_RESET_FROM_ISR
tmrCOMMAND_STOP_FROM_ISR
tmrCOMMAND_CHANGE_PERIOD_FROM_ISR
xTimerStart(xTimer, xTicksToWait)
BaseType_t xTimerStart( TimerHandle_t xTimer, TickType_t xTicksToWait );
Timer functionality is provided by a timer service/daemon task. Many of the public FreeRTOS timer API
functions send commands to the timer service task through a queue called the timer command queue. The
timer command queue is private to the kernel itself and is not directly accessible to application code. The
length of the timer command queue is set by the configTIMER_QUEUE_LENGTH configuration constant.
xTimerStart() starts a timer that was previously created using the xTimerCreate() API function. If the timer
had already been started and was already in the active state, then xTimerStart() has equivalent functionality to
the xTimerReset() API function.
Starting a timer ensures the timer is in the active state. If the timer is not stopped, deleted, or reset in the mean
time, the callback function associated with the timer will get called ‘n’ticks after xTimerStart() was called,
where ‘n’is the timers defined period.
It is valid to call xTimerStart() before the scheduler has been started, but when this is done the timer will not
actually start until the scheduler is started, and the timers expiry time will be relative to when the scheduler is
started, not relative to when xTimerStart() was called.
The configUSE_TIMERS configuration constant must be set to 1 for xTimerStart() to be available.
Example usage:
Return pdFAIL will be returned if the start command could not be sent to the timer command queue even
after xTicksToWait ticks had passed. pdPASS will be returned if the command was successfully sent
to the timer command queue. When the command is actually processed will depend on the priority of
the timer service/daemon task relative to other tasks in the system, although the timers expiry time is
relative to when xTimerStart() is actually called. The timer service/daemon task priority is set by the
configTIMER_TASK_PRIORITY configuration constant.
Parameters
• xTimer: The handle of the timer being started/restarted.
• xTicksToWait: Specifies the time, in ticks, that the calling task should be held in the Blocked
state to wait for the start command to be successfully sent to the timer command queue, should the
queue already be full when xTimerStart() was called. xTicksToWait is ignored if xTimerStart() is
called before the scheduler is started.
See the xTimerCreate() API function example usage scenario.
xTimerStop(xTimer, xTicksToWait)
BaseType_t xTimerStop( TimerHandle_t xTimer, TickType_t xTicksToWait );
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