How to Use Task Notifications Within RTOS

We are proud to present a range of Real Time Operating System (RTOS) training materials for you to build and refresh your real time OS skills. This tutorial is an introduction to the basics of Task Notifications and how they can offer significant performance and RAM benefits over traditional methods. Task notifications can receive a task’s 32bit notification value in a variety of ways providing increased flexibility to your system.

What are RTOS Task Notifications?

An RTOS has task notifications feature set provides a lightweight alternative to traditional RTOS Queues, Semaphores and Event Groups. Task notifications are suitable where only one task consumes the information being provided and offer significant performance and RAM benefits over traditional methods.

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 up to 45% faster and uses less RAM than unblocking a task with a Binary Semaphore.

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Obriel Obriel Mariga

RTOS Engineer at WHIS
Last updated 10th May 2024

Used as Lightweight Binary Semaphores Within an RTOS

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.

From the RTOS point of view, binary semaphores are essentially a special case of a traditional RTOS queue. Although no data is actually sent through the queue, the queue control structure is still necessary. Using task notifications in place of a binary semaphore, removes the need for the queue control structure, therefore saving RAM and simplifying the internal RTOS processes.

From an application point of view, the creation of the Binary Semaphore is no longer necessary and the calls to ‘take’ and ‘give’ the semaphore are replaced with calls to ‘take’ and ‘give’ the Task Notification. Typical uses are shown in Figure 1 and Figure 2.

Figure 1: Using Task Notifications as a binary semaphore between tasks

Figure 1: Using RTOS Task Notifications as a Binary Semaphore Between Tasks

Figure 2: Using Task Notifications as a binary semaphore between an ISR and a task.

Figure 2: Using RTOS Task Notifications as a Binary Semaphore Between an ISR and a Task.

Performance Benefits and Usage Restrictions

Task Notifications have speed & RAM size advantages over other RTOS features with equivalent functionality. However, these benefits require some use case limitations:

RTOS Task Notifications can only be used when there is only one task that can be the recipient of the event. However this condition is met in the majority of real world applications.
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.

Used as a Lightweight Counting Semaphore

An RTOS Counting Semaphore is a semaphore that can have a count value of zero up to a maximum value set when the semaphore is created. A task can only ‘take’ the semaphore if it is available, and the semaphore is only available if its count is greater than zero.

As with Binary Semaphores, from the RTOS point of view, Counting Semaphores are essentially a special case of a traditional RTOS queue. Although no data is actually sent through the queue, the queue control structure is still necessary. Using Task Notifications in place of a Counting Semaphore, removes the need for the queue control structure, therefore saving RAM and simplifying the internal RTOS processes.

When a Task Notification is used in place of a Counting Semaphore, the receiving task’s notification value is used in place of the Counting Semaphore’s count value, and is therefore incremented whenever the Task Notification is ‘given’ and decremented whenever the Task Notification is ‘taken’. The example in Figure 3. uses the receiving task’s notification value as a counting semaphore.

Figure 3: Using Task Notifications as a counting semaphore between an ISR and a task

Figure 3: Using RTOS Task Notifications as a Counting Semaphore Between an ISR and a Task

Used as a Lightweight Event Group in an RTOS

An Event Group is a set of binary flags (or bits), to each of which the application writer can assign a meaning. The task notification value can be used as such an Event Group, allowing the RTOS task to enter the Blocked state to wait for one or more flags within the group to become active as shown in Figure 4.

Figure 4: Using RTOS Task Notifications as an Event Group

Used as a Lightweight Mailbox (Queue)

RTOS Task Notifications can be used to send data to a task, but in a much more restricted way than can be achieved with an RTOS queue because:

Only 32-bit values can be sent;
The value is saved as the receiving task’s notification value, and there can only be one notification value at any one time.

Hence the phrase ‘lightweight mailbox’ is used in preference to ‘lightweight queue’. The task’s notification value is the mailbox value. Figure 5 demonstrates the use of task notifications as a lightweight mailbox.

Figure 5: Using Task Notifications as a lightweight mailbox

Figure 5: Using RTOS Task Notifications as a Lightweight Mailbox

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How To Use Task Notifications For RTOS