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STM32U5 Serial Bootloader AN3155 Discrepancy

dsurrett
Associate II

In looking at AN3155, USART protocol used in the STM32 bootloader, the following diagram describes the MCU's write protect command sequence from the device side:

dsurrett_1-1750720697920.png

There is a comment at the bottom which indicates that only certain F and L series chips perform the system reset.  I am working with a STM32U585QI, and I also see that this chip appears to perform a system reset when executing the write protect command.  Is that correct and this comment is just wrong, or am I seeing this unexpectedly?

1 REPLY 1
Imen.D
ST Employee

Hello @dsurrett ,

The note in the AN3155 is correct. A system reset when executing the write protect command doesn’t apply to STM32U5.

On the STM32F4/F7/L4 series listed on the AN3155, modifying the Option byte doesn’t generate a reset – it’s bootloader SW who add a NVIC_SystemReset after any OB change. But for the new STM32 we removed that as it’s not aligned with the HW behaviour.

Now, if a product generates a system reset after modifying the OB, this is due to the HW itself:

  • For example: STM32U5/L5/WBA.. generate a reset when OB_LAUNCH is done, so when OB is modified
  • This is not true on product like STM32H7, STM32H5..

You can refer to the RM0456 related to your product and check this detailed on "Embedded flash memory (FLASH)" part.

Hope I answered to your question and this clear for you now.

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Thanks
Imen