RTOS Task Notifications
[Inter-task communication and synchronisation]


Each RTOS task has a 32-bit notification value. An RTOS task notification is an event sent directly to a task that can unblock the receiving task, and optionally update the receiving task’s notification value.

Task notifications can update the receiving task’s notification value in the following ways:

  • Set the receiving task’s notification value without overwriting a previous value
  • Overwrite the receiving task’s notification value
  • Set one or more bits in the receiving task’s notification value
  • Increment the receiving task’s notification value
That flexibility allows task notifications to be used where previously it would have been necessary to create a separate queue, binary semaphore, counting semaphore or event group. Unblocking an RTOS task with a direct notification is 45% faster * and uses less RAM than unblocking a task with a binary semaphore.

Notifications are sent using the xTaskNotify() and xTaskNotifyGive() API functions (and their interrupt safe equivalents), and remain pending until the receiving RTOS task calls either of the xTaskNotifyWait() or ulTaskNotifyTake() API functions. If the receiving RTOS task was already Blocked waiting for a notification when one arrives the receiving RTOS task will be removed from the Blocked state and the notification cleared.

RTOS task notification functionality is enabled by default, and can be excluded from a build (saving 8 bytes per task) by setting configUSE_TASK_NOTIFICATIONS to 0 in FreeRTOSConfig.h.

Performance Benefits and Usage Restrictions

Task notifications have both speed and RAM footprint advantages over other FreeRTOS features that can be used to perform equivalent functionality. As would be expected, these benefits require some use case limitations:
  1. RTOS task notifications can only be used when there is only one task that can be the recipient of the event. This condition is however met in the majority of real world applications.

  2. Only in the case where an RTOS task notification is used in place of a queue; While a receiving task can wait for a notification in the Blocked state (so not consuming any CPU time), a sending task cannot wait in the Blocked state for a send to complete if the send cannot complete immediately.

Uses and Examples

The related API is designed to allow RTOS task notifications to be used as a faster and lighter weight alternative to binary and counting semaphores, event groups and in some cases queues (referred to in the documentation as a mailbox as the queue length must equal 1). Links to documentation pages that demonstrate how this is done can be found at the bottom of this page.

From this page:

* Measured using the binary semaphore implementation from FreeRTOS V8.1.2, compiled with GCC at -O2 optimisation, and without configASSERT() defined. A 35% improvement can still be obtained using the improved binary semaphore implementation found in FreeRTOS V8.2.0 and higher.

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