Not really a secret, more that it's very application specific. To enter a low power mode all unnecessary tasks and peripherals need to be suspended first. That usually involves running down the list of tasks, notifying each one to go into suspend after powering down it's associated hardware, along with a wakeup source if it' related to the task. And a task not yet suspended may hang waiting on a suspended task, so there are race conditions to avoid in order to ensure tasks are in a known state before going into STOP mode.
This includes shutting down Systick or whatever timer is used for the FreeRTOS timebase. Which means no task must be blocked for some tick count, since the scheduler and systick aren't running.
When a wakeup event occurs all the suspended tasks and peripherals need to be resumed, so another rundown list is needed for all the suspended tasks. Like suspending the resume process is sensitive to race conditions so some type of synchronization is needed )I use event groups for this).
It's not a trivial process. I create a power management task to handle all this. It's the first task to start, last to suspend, first to resume. Every task uses an event queue. The p[ower management sends messages to trigger task suspension, synchronized with an event group. On wakeup the power task runs down the resume list, restarting each task. All tasks wait on an event group before continuing, to ensure everything is synchronized.
I have no idea how this can be done with the HAL and Cube, so no advice there.
Jack Peacock
