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xSemaphoreGiveFromISR
[Semaphores]

semphr. h

xSemaphoreGiveFromISR( 
                         xSemaphoreHandle xSemaphore, 
                         portBASE_TYPE *pxHigherPriorityTaskWoken 
                      )

Macro to release a semaphore. The semaphore must have previously been created with a call to vSemaphoreCreateBinary() or xSemaphoreCreateCounting().

Mutex type semaphores (those created using a call to xSemaphoreCreateMutex()) must not be used with this macro.

This macro can be used from an ISR.

Parameters:

xSemaphore	A handle to the semaphore being released. This is the handle returned when the semaphore was created.
pxHigherPriorityTaskWoken	xSemaphoreGiveFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE if giving the semaphoree caused a task to unblock, and the unblocked task has a priority higher than the currently running task. If xSemaphoreGiveFromISR() sets this value to pdTRUE then a context switch should be requested before the interrupt is exited.

Returns:

pdTRUE if the semaphore was successfully given, otherwise errQUEUE_FULL.

Example usage:

#define LONG_TIME 0xffff
#define TICKS_TO_WAIT    10

xSemaphoreHandle xSemaphore = NULL;

/* Repetitive task. */
void vATask( void * pvParameters )
{
    /* We are using the semaphore for synchronisation so we create a binary
    semaphore rather than a mutex.  We must make sure that the interrupt
    does not attempt to use the semaphore before it is created! */
    vSemaphoreCreateBinary( xSemaphore );

    for( ;; )
    {
        /* We want this task to run every 10 ticks of a timer.  The semaphore 
        was created before this task was started.
        
        Block waiting for the semaphore to become available. */
        if( xSemaphoreTake( xSemaphore, LONG_TIME ) == pdTRUE )
        {
            /* It is time to execute. */
            
            ...
            
            /* We have finished our task.  Return to the top of the loop where
            we will block on the semaphore until it is time to execute 
            again.  Note when using the semaphore for synchronisation with an
            ISR in this manner there is no need to 'give' the semaphore back. */
        }
    }
}

/* Timer ISR */
void vTimerISR( void * pvParameters )
{
static unsigned portCHAR ucLocalTickCount = 0;
static portBASE_TYPE xHigherPriorityTaskWoken; 
 
    /* A timer tick has occurred. */
    
    ... Do other time functions.
    
    /* Is it time for vATask() to run? */
    xHigherPriorityTaskWoken = pdFALSE;
    ucLocalTickCount++;
    if( ucLocalTickCount >= TICKS_TO_WAIT )
    {
        /* Unblock the task by releasing the semaphore. */
        xSemaphoreGiveFromISR( xSemaphore, &xHigherPriorityTaskWoken );
        
        /* Reset the count so we release the semaphore again in 10 ticks time. */
        ucLocalTickCount = 0;
    }
    
    /* If xHigherPriorityTaskWoken was set to true you
    we should yield.  The actual macro used here is 
    port specific. */
    portYIELD_FROM_ISR( xHigherPriorityTaskWoken );
}
 

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