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FAQ: Common STM32 stumbling blocks

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Adds some commonly encountered issues.

Help wanted: if anyone has a working BDMA linker script setup, it would be good to add here.
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@ -231,3 +231,34 @@ Please refer to the STM32 documentation for the specific values suitable for you
Look for the `MEMORY` section and try to determine the FLASH and RAM sizes and section start.
If you find a case where the memory.x is wrong, please report it on [this Github issue](https://github.com/embassy-rs/stm32-data/issues/301) so other users are not caught by surprise.
== Known issues (details and/or mitigations)
These are issues that are commonly reported. Help wanted fixing them, or improving the UX when possible!
=== STM32H5 and STM32H7 power issues
STM32 chips with built-in power management (SMPS and LDO) settings often cause user problems when the configuration does not match how the board was designed.
Settings from the examples, or even from other working boards, may not work on YOUR board, because they are wired differently.
Additionally, some PWR settings require a full device reboot (and enough time to discharge any power capacitors!), making this hard to troubleshoot. Also, some
"wrong" power settings will ALMOST work, meaning it will sometimes work on some boots, or for a while, but crash unexpectedly.
There is not a fix for this yet, as it is board/hardware dependant. See link:https://github.com/embassy-rs/embassy/issues/2806[this tracking issue] for more details
=== STM32 BDMA only work out of some RAM regions
The STM32 BDMA controller included in some chips (TODO: list which ones) has a limitation in that it only works out of certain regions of RAM (TODO: list which ones), otherwise the transfer
will fail.
If you see errors that look like this:
[source,plain]
----
DMA: error on BDMA@1234ABCD channel 4
----
You likely need to set up your linker script to define a special region for this area, and copy data to that region before using with BDMA.
TODO: show how to do that