How to write a FSBL Application for a custom STM32N6 board

[cp5900]

Context : I want to write a FSBL load and run application on a custom minimal PCB based on STM32N6-DK design. The board uses SMPS power supply and not the default power supply of the STM32N6-DK. I follow this original tutorial  as a main example.

Problem faced: CubeProgrammer SigningTool version compatibility and header size

1) Write a custom external bootloader to write and read external flash memory

• Modify the power supply parameter (System Core --> RCC --> SupplySource) in CubeMX to `PWR_SMPS_SUPPLY`
• Generate ExtMemLoader template from CubeMX 
• Build ExtMemLoader to generate .stldr file
  • make sure the post build script can be executed
  • manually copy the .stldr file generated in `/STM32CubeProgrammer.app/Contents/Resources/bin/ExternalLoader/` if necessary

2) Modify SigningTool header generation command in FSBL post-build

• Build FSBL & Application following the original tutorial
• Check heading size, it might be different from the 0x400 that is defined in CubeMX (caused by uninstalling then reinstalling CubeProgrammer thus SigningTool)
• look for this message *[Header v2.3] Do not align the payload to the 0x400 offset, No padding bytes at the beginning of the payload*
• Vector Table cannot be located at 0x240 for hardware alignment constraint, therefore it needs to be at 0x400 (following the header)
• Add `--align` at the end of the SigningTool command to add padding bytes and get a 0x400 header (`STM32_SigningTool_CLI -bin FSBL.bin ... --align`)
• Then rebuild and make sure header size is 0x400

3) Program flash and debug

• Select the custom .stldr as External loaders in CubeProgrammer
• Write trusted.bin files for both FSBL and Application following the original tutorial
• To use the Debugger in CubeIDE you also need to add the custom .stldr file previously generated in Debug Configurations --> C/C++ Application --> Debugger --> External loaders). Follow the instructions from this tutorial 

I hope this short guide will help some of you. Feel free to reach out if you have any question.

[Julian E.]

Hi @cp5900,

Thank you very much for sharing, I believe this will be useful for future readers!

Have a good day,

Julian

[Maxime_MARCHETTO1 · Accepted solution]

Hello @cp5900 @,

Thanks for sharing this case and the solution you found. ​

To make this post more visible and easier to find by others who may face the same issue, I mark my reply as the solution. 

---

@cp5900 wrote:

Context : I want to write a FSBL load and run application on a custom minimal PCB based on STM32N6-DK design. The board uses SMPS power supply and not the default power supply of the STM32N6-DK. I follow this original tutorial  as a main example.

Problem faced: CubeProgrammer SigningTool version compatibility and header size

1) Write a custom external bootloader to write and read external flash memory

• Modify the power supply parameter (System Core --> RCC --> SupplySource) in CubeMX to `PWR_SMPS_SUPPLY`
• Generate ExtMemLoader template from CubeMX 
• Build ExtMemLoader to generate .stldr file
  • make sure the post build script can be executed
  • manually copy the .stldr file generated in `/STM32CubeProgrammer.app/Contents/Resources/bin/ExternalLoader/` if necessary

2) Modify SigningTool header generation command in FSBL post-build

• Build FSBL & Application following the original tutorial
• Check heading size, it might be different from the 0x400 that is defined in CubeMX (caused by uninstalling then reinstalling CubeProgrammer thus SigningTool)
• look for this message *[Header v2.3] Do not align the payload to the 0x400 offset, No padding bytes at the beginning of the payload*
• Vector Table cannot be located at 0x240 for hardware alignment constraint, therefore it needs to be at 0x400 (following the header)
• Add `--align` at the end of the SigningTool command to add padding bytes and get a 0x400 header (`STM32_SigningTool_CLI -bin FSBL.bin ... --align`)
• Then rebuild and make sure header size is 0x400

3) Program flash and debug

• Select the custom .stldr as External loaders in CubeProgrammer
• Write trusted.bin files for both FSBL and Application following the original tutorial
• To use the Debugger in CubeIDE you also need to add the custom .stldr file previously generated in Debug Configurations --> C/C++ Application --> Debugger --> External loaders). Follow the instructions from this tutorial 

I hope this short guide will help some of you. Feel free to reach out if you have any question.

---

Regards,
Maxime