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RTOS Task Notifications
Used As Light Weight Binary Semaphores

Related pages:

Unblocking an RTOS task with a direct notification is 45% faster and uses less RAM than unblocking a task with a binary semaphore. This page demonstrates how this is done.

A binary semaphore is a semaphore that has a maximum count of 1, hence the 'binary' name. A task can only 'take' the semaphore if it is available, and the semaphore is only available if its count is 1.

When a task notification is used in place of a binary semaphore the receiving task's notification value is used in place of the binary semaphore's count value, and the ulTaskNotifyTake() (or ulTaskNotifyTakeIndexed()) API function is used in place of the semaphore's xSemaphoreTake() API function. The ulTaskNotifyTake() function's xClearOnExit parameter is set to pdTRUE so the count value is returned to zero each time the notification is taken - emulating a binary semaphore.

Likewise the xTaskNotifyGive() (or xTaskNotifyGiveIndexed()) or vTaskNotifyGiveFromISR() (or vTaskNotifyGiveIndexedFromISR()) functions are used in place of the semaphore's xSemaphoreGive() and xSemaphoreGiveFromISR() functions.

See the example below.

/* This is an example of a transmit function in a generic
peripheral driver.  An RTOS task calls the transmit function,
then waits in the Blocked state (so not using an CPU time)
until it is notified that the transmission is complete.  The
transmission is performed by a DMA, and the DMA end interrupt
is used to notify the task. */

/* Stores the handle of the task that will be notified when the
transmission is complete. */
static TaskHandle_t xTaskToNotify = NULL;

/* The index within the target task's array of task notifications
to use. */
const UBaseType_t xArrayIndex = 1;

/* The peripheral driver's transmit function. */
void StartTransmission( uint8_t *pcData, size_t xDataLength )
    /* At this point xTaskToNotify should be NULL as no transmission
    is in progress.  A mutex can be used to guard access to the
    peripheral if necessary. */
    configASSERT( xTaskToNotify == NULL );

    /* Store the handle of the calling task. */
    xTaskToNotify = xTaskGetCurrentTaskHandle();

    /* Start the transmission - an interrupt is generated when the
    transmission is complete. */
    vStartTransmit( pcData, xDatalength );

/* The transmit end interrupt. */
void vTransmitEndISR( void )
BaseType_t xHigherPriorityTaskWoken = pdFALSE;

    /* At this point xTaskToNotify should not be NULL as
    a transmission was in progress. */
    configASSERT( xTaskToNotify != NULL );

    /* Notify the task that the transmission is complete. */
    vTaskNotifyGiveIndexedFromISR( xTaskToNotify,
                                   &xHigherPriorityTaskWoken );

    /* There are no transmissions in progress, so no tasks
    to notify. */
    xTaskToNotify = NULL;

    /* If xHigherPriorityTaskWoken is now set to pdTRUE then a
    context switch should be performed to ensure the interrupt
    returns directly to the highest priority task.  The macro used
    for this purpose is dependent on the port in use and may be
    called portEND_SWITCHING_ISR(). */
    portYIELD_FROM_ISR( xHigherPriorityTaskWoken );

/* The task that initiates the transmission, then enters the
Blocked state (so not consuming any CPU time) to wait for it
to complete. */
void vAFunctionCalledFromATask( uint8_t ucDataToTransmit,
                                size_t xDataLength )
uint32_t ulNotificationValue;
const TickType_t xMaxBlockTime = pdMS_TO_TICKS( 200 );

    /* Start the transmission by calling the function shown above. */
    StartTransmission( ucDataToTransmit, xDataLength );

    /* Wait to be notified that the transmission is complete.  Note
    the first parameter is pdTRUE, which has the effect of clearing
    the task's notification value back to 0, making the notification
    value act like a binary (rather than a counting) semaphore.  */
    ulNotificationValue = ulTaskNotifyTakeIndexed( xArrayIndex,
                                                   xMaxBlockTime );

    if( ulNotificationValue == 1 )
        /* The transmission ended as expected. */
        /* The call to ulTaskNotifyTake() timed out. */

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