Unable to halt/connect STM32H7S3L8 using external debugger on Nucleo-H7S3L8 (JP1 closed)

[surender]

Hi everyone,

I'm facing issues connecting an external debugger to the NUCLEO-H7S3L8 board (STM32H7S3L8).

Current Setup:

• Board powered via CN5 (ST-LINK USB)
• JP1 closed (to disable on-board ST-LINK)
• External debugger connected to CN1 (MIPI20 connector)
• SWD mode at low speed (500 kHz to 1 MHz)
• Correct power sequence: Power CN5 first → wait for COM LED (LD8) to stabilize → then connect external probe

What I have already tried:

• BOOT0 = 1 (jumper on CN11[5-7]) to enter bootloader mode and halt using external debugger
• Internal ST-LINK works fine when JP1 is open (using STM32CubeProgrammer)
  

  Problem:

  The external debugger is unable to connect or halt the CPU, please guide me how to fix it.
  

  Thanks,
  regards,
  Surender.