DRAFT: stm32wb55 - sync cpu2 with hsem when entering sleep

[ckrenslehner]

I did not find where to mark this as draft, but here is a basic implementation of using the HSEM for syncing the with the CPU2 when entering sleep.
The stm32wb55 requires this when accessing some peripherals

Among these are:

• RNG
• Crypto
• Stop Mode
• Flash

The concrete flow charts for entering and exiting STOP mode are in this document
Page 23f.

I have some general questions:

• How should I handle the HSEM creation
• Now its created via the init and then passed to the executor again similar to the RTC
• The HSEM is needed in other places too. But with other underlying HSEM IDs. Should I split the HSEM into separate ID-Structs or how could I model such usecase in rust/embassy the best way?

I would greatly appreciate feedback on how to model this "correctly" :-)

[ckrenslehner]

Okay I also realized that keeping the HSEM initialized all the time needs to be avoided. Otherwise, entering the stop mode is not possible because, of the RCC init from the HSEM Module.

So to enter sleep the low power executor needs to init the HSEM peripheral on the inside to sync with CPU 2.