KernelSoftware timers

Updated Jun 2025

xTimerStopFromISR

[Timer API]

timers.h

1 BaseType_t xTimerStopFromISR
2             (
3                 TimerHandle_t xTimer,
4                 BaseType_t *pxHigherPriorityTaskWoken
5             );

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A version of xTimerStop() that can be called from an interrupt service routine.

Parameters:

• xTimer

  The handle of the timer being stopped.

• pxHigherPriorityTaskWoken

  The timer service/daemon task spends most of its time in the Blocked state, waiting for messages to arrive on the timer command queue. Calling

  xTimerStopFromISR()

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  writes a message to the timer command queue, so has the potential to transition the timer service/daemon task out of the Blocked state. If calling

  xTimerStopFromISR()

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  causes the timer service/daemon task to leave the Blocked state, and the timer service/daemon task has a priority equal to or greater than the currently executing task (the task that was interrupted), then

  pxHigherPriorityTaskWoken

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  will get set to

  pdTRUE

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  internally within the

  xTimerStopFromISR()

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  function. If

  xTimerStopFromISR()

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  sets this value to

  pdTRUE

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  , then a context switch should be performed before the interrupt exits.

Returns:

• pdFAIL

  pdFAIL

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  will be returned if the stop command could not be sent to the timer command queue.

• pdPASS

  pdPASS

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  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. The timer service/daemon task priority is set by the

  configTIMER_TASK_PRIORITY

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  configuration constant.

Example usage:

1/* This scenario assumes xTimer has already been created and started. When
2   an interrupt occurs, the timer should be simply stopped. */
3
4/* The interrupt service routine that stops the timer. */
5void vAnExampleInterruptServiceRoutine( void )
6{
7BaseType_t xHigherPriorityTaskWoken = pdFALSE;
8
9    /* The interrupt has occurred - simply stop the timer.
10       xHigherPriorityTaskWoken was set to pdFALSE where it was defined
11       (within this function). As this is an interrupt service routine, only
12       FreeRTOS API functions that end in "FromISR" can be used. */
13    if( xTimerStopFromISR( xTimer, &xHigherPriorityTaskWoken ) != pdPASS )
14    {
15        /* The stop command was not executed successfully. Take appropriate
16           action here. */
17    }
18
19    /* If xHigherPriorityTaskWoken equals pdTRUE, then a context switch
20       should be performed. The syntax required to perform a context switch
21       from inside an ISR varies from port to port, and from compiler to
22       compiler. Inspect the demos for the port you are using to find the
23       actual syntax required. */
24    if( xHigherPriorityTaskWoken != pdFALSE )
25    {
26        /* Call the interrupt safe yield function here (actual function
27           depends on the FreeRTOS port being used). */
28    }
29}

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