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 RTOS 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 RTOS scheduler has been started, but
when this is done the timer will not actually start until the RTOS scheduler is
started, and the timers expiry time will be relative to when the RTOS 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.
The handle of the timer being started/restarted.
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. xBlockTime is ignored if xTimerStart() is called
before the RTOS scheduler is started.
pdFAIL will be returned if the start command could not be sent to
the timer command queue even after xBlockTime 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
Copyright (C) 2004-2010 Richard Barry. Copyright (C) 2010-2013 Real Time Engineers Ltd.
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