Wake up multiple tasks with single IO event

Kernel
1

Hello everyone,

I recently found out about FreeRTOS and been doing some experiments with it. I’m looking to make a “button” driver utilizing user buttons on the board I have.

Here’s the behavior I’m looking to implement:

  • allow multiple tasks to block and wait for a button “event” to occur.
  • once the button is pressed all tasks that were awaiting for the button press would be unblocked
  • If a button event happens and afterwards a task queries the button driver for an event, the task would block and await a future button event. Button events only apply for tasks already waiting for the event.

I’ve been looking into how to do it and I think using an event group is the way to go, but I just wanted to ask since I’m new to FreeRTOS.

Questions:

  1. Is an event group the best way to go in order to implement the behavior I’m looking for?
    I’m looking to make this driver as simple and as lightweight as possible (it’s a silly little driver), so I’m looking to spend as little time as possible in kernel-space & avoid creating driver tasks/daemons.

  2. My understanding is I don’t need to specify how many tasks are waiting for the same event group, is that correct?
    Ideally that’s what I want. I’m making a driver so I’d like my code to work with N tasks, although it’s not the end of the world if not.

2

Yes, An EventGroup is the way to handle multiple things waiting for a single ‘event’ to happen. When the event is posted to the group, EVERY task that was waiting in a way that the event is satisfied gets woken.

3

Thank you for clarifying Richard! I was 90% sure for that. My confusion mainly came from finding out about task notifications since the docs say it’s a lot more efficient to use, but it seems that it one can only notify a single task.

4

The Direct-to-Task notification system IS more efficient, largely because it only needs to handle waking up a single task, so it can be optimized for that. This removes a lot of resource contention.

If most of your events will only ever wake up a single task, and only occasionally do you need to wake up multiple tasks (but with a small upper limit for how many might be needed) you could replace the Event Group with a loop to signal each of the tasks with the direct-to-task system. I suspect that unless you are on the edge of your performance envelope, it won’t be worth the complexity.