we are using STM32H755IIT6 MCU we made 5 boards with the same MCU STM32H755IIT6 all the board can be programable with verification but only one board of start to run what we can check on the board to find the problem

[SLeit.2]

the problem is that we are using the same code for evaluation board and it is running well

we also have one out of 6 boards that working well all other dost work at all we just successes to program

[Tesla DeLorean]

Ok, but then you've got to figure out what's different in the design/implementation.

That the boards are programmable suggests you're not completely dead in the water.

You'll likely want to code something simple starting at the Reset_Handler, and establish some beach-head functionality from which you can inspect from within.

Perhaps some infinite loop of NOPs would allow for the debugger to connect initially ?

[GLASS]

You can also give a look to Option Bytes and check that STM default values (look at Reference Manual) are ​not altered on all boards. You can read Option Bytes with STM Cube Programmer or perhaps St Link utility ( not sure that this 'old' SW know H7)...

[LCE]

... and you have checked and measured every supply voltage and ground pin?

Checked the crystals / oscillators?

Reset?

If one board is running, maybe it's some hardware problem, bad caps, bad soldering?

IIT6 is LQFP-package? If yes, at least you can optically check and measure at each pin.

[LCE]

I would further try:

• connecting crystal / HSE
• using LDO instead of SMPS