Choose an Inter-Task Communication Method in FreeRTOS

Owned by Hardy Yu
Last updated: 2024-03-20
2 min read

Intro

As you might have you know already, the EFS team in WARG is heavily using FreeRTOS to accomplish the multi-tasking needs in our projects, including the ZeroPilot. When one is working on a task with one of the Managers in ZeroPilot for example, they might need to transfer the processed data from their manager to another manager, or they might need to receive streams of raw data from an ISR. This type of communication that happens between task-to-task and ISR-to-task is called inter-task communication or inter-thread communication outside of the FreeRTOS context. This page is going to introduce some inter-task com methods available to you such that it can guide you to find the appropriate method to use in your scenario.

 

Options

Option 1 - Stream Buffer

  • Allows a stream of bytes to be passed from

    • ISR → Task

    • Task → Task

  • The length of the bytes can be totally arbitrary

  • Contents are passed by copy

  • Only single-reader single-writer scenarios***

  • Allows transmission complete callback

  • API

 

Option 2 - Queue

  • Allows messages to be passed from

    • ISR → Task

    • Task → Task

  • They are thread-safe FIFO buffers, meaning multiple reader/write fine

  • Contents are passed by copy

  • Allows a block time to be specified. When a task attempts to rad from an empty queue, the task can be blocked until the buffers are filled or time-out

  • Length is fixed

  • API

 

Option 3 - Task Notification

  • It is an event that sent directly to a task. No indirect buffers(queue, semaphore) during transmission

  • It can be used as a lightweight binary/counting semaphore, or 32-bit value direct sender, which is faster and cost less RAM than normal inter-task com

  • This functionality is only available after a certain release

  • API

 

Other Options

  • Binary Semaphore, Counting Semaphore, and Mutex. But they are more focused on task synchronization rather than message exchange

  • Global variables, static variables, and static functions are shared between tasks, but they are vulnerable to a race condition

  • You can also allocate a piece of memory on the heap, which is also shared between tasks, and you can free it when you don’t need it anymore. Functions are pvPortMalloc() and vPortFree . Also need multi-thread protection

image-20240320-014837.png
A Demonstration of FreeRTOS Memory Allocation

References: