STM32N6 BootROM flash boot XSPI port

[ArunJohn]

Hi,

I have a question regarding the STM32N6 BootROM flash boot process.

My understanding is that the BootROM loads the FSBL from an external XSPI flash device. However, it is not clear to me whether the BootROM:

1. Uses a fixed XSPI/XSPIM configuration (for example, a specific XSPIM port), or

2. Automatically probes and detects the flash location across available XSPI/XSPIM ports.

For a custom STM32N6 board, the external NOR flash is connected through XSPIM Port1. I would like to understand whether the BootROM is capable of automatically discovering and booting from a flash connected to Port1, or whether Flash Boot is restricted to a specific XSPI/XSPIM port configuration.

Could you please clarify:

• Does the STM32N6 BootROM scan multiple XSPI/XSPIM ports during Flash Boot?

• Is the boot source port fixed in ROM or configurable through OTP settings?

• If the flash is connected only to XSPIM Port1, can the BootROM still load the FSBL from it?

• Is there any documentation describing the exact XSPI/XSPIM configuration used by the BootROM during Flash Boot?

Thanks.

[RomainR. · Accepted solution]

Hello @ArunJohn 

Refer to UM3234 section 3.2.7 Boot ROM configuration determination 
https://www.st.com/resource/en/user_manual/um3234-how-to-proceed-with-boot-rom-on-stm32n6-mcus-stmicroelectronics.pdf

here some answers to your questions below:

• Does the STM32N6 BootROM scan multiple XSPI/XSPIM ports during Flash Boot?
  No, according UM3234, the BootRom uses XSPI1 on XSPIM-P2 for flash boot operation. The XSPI1 is configured in single line with the external NOR Flash (see 3.4.5 XSPI / XSPIM configuration for serial NOR device)
  In user application, the code can access to external NOR flash with XSPI2 and XSPIM-P2. Most of the template FSBL LRUN or XiP in STM32Cube N6 are configuring like this.
  
  

• Is the boot source port fixed in ROM or configurable through OTP settings?
  No, it's not possible to configure the XSPIM port by OTP.
  
  

• If the flash is connected only to XSPIM Port1, can the BootROM still load the FSBL from it?
  No, the NOR flash must be connected to XSPIM-P2. In DS14791 Table 18 page 106 define XSPIM-P2 in Octospi mode as Boot source by the BootROM.
  https://www.st.com/resource/en/datasheet/stm32n657a0.pdf

• Is there any documentation describing the exact XSPI/XSPIM configuration used by the BootROM during Flash Boot?
  All these information are detailed in the UM3234 rev4 (see the link shared above)

Best regards,

Romain,