Custom external loader for STM32N655X0

[ArunJohn]

Hi,

I’m developing a custom external loader (.stldr) for an STM32N655X0 custom board to program external XSPI NOR flash using STM32CubeProgrammer via a J-Link interface (board does not have ST-LINK).

I have the loader framework working partially:

• Init() executes

• Sector erase succeeds

• Programming fails during download phase

• CubeProgrammer logs show repeated “Init flashloader...” calls and eventually errors like:

  • “Init function fail”

  • “Could not start CPU (ErrorCode: -1)”

  • “Cannot read R0 while CPU is running”

The external flash is S25FL064L and erase operation works correctly, so communication with flash seems functional.

I wanted guidance specifically for developing external loaders intended to be used through J-Link in STM32CubeProgrammer:

1. Are there any additional requirements or limitations when using .stldr loaders via J-Link compared to ST-LINK?

2. Does CubeProgrammer execute the loader lifecycle differently when J-Link is selected (Init / Erase / Write / Read sequencing)?

3. Are there RAM regions that CubeProgrammer reserves for staging write buffers which the loader must avoid?

4. Is there a recommended linker memory layout / maximum loader RAM footprint?

5. Does CubeProgrammer call Init() multiple times during programming, and should Init() be designed to be fully re-entrant?

6. Are there guidelines on cache/MPU/XSPI memory-mapped mode usage inside external loaders?

7. Any recommended debug method to trace loader execution?

If anyone has successfully developed and used a custom STM32CubeProgrammer external loader with J-Link on STM32N6, I’d appreciate any checklist or lessons learned.

Thanks.