STM32H735VGT6 Custom Board: Unable to Upload Code via ST-LINK

[Dn1te]

I made a custom 4 layer board using STM32H735VGT6 (schematic and PCB layout attached).
I am trying to upload code into the MCU flash via ST-LINK V2, but it fails to connect.

At first, I realized that I had not connected VDDLDO to 3.3V, which I thought was the problem. However, even after connecting it to 3.3V, I still cannot upload code.

Error from CubeIDE:

STMicroelectronics ST-LINK GDB server. Version 7.11.0
Copyright (c) 2025, STMicroelectronics. All rights reserved.

Starting server with the following options:
Persistent Mode : Disabled
Logging Level : 1
Listen Port Number : 61234
Status Refresh Delay : 15s
Verbose Mode : Disabled
SWD Debug : Enabled
InitWhile : Enabled

Target no device found

Error in initializing ST-LINK device.
Reason: No device found on target.

I also tried the “hold reset while clicking connect, then release” method, but it still fails.

This is a newly soldered MCU. I have not uploaded any code to it yet. I also tried switching BOOT0 from low to high to enter system bootloader and erase flash via USART1 (PA9/PA10), but failed as well.

Power connections:

• I have 2 2.2µF capacitors connected to 2 of the 3 VCAP pins, and for the last one I connected it to one of the VCAP pin (as per datasheet).

• The measured voltage on VCAP pins is ~0.99–1.0V instead of 1.2V. I’m not sure if this is  indicates an issue.

At this point, I’m unsure whether the problem is hardware-related (power/VCAP), ST-LINK wiring, or boot issue. Any advice on what I might be missing and how to recover would be appreciated.

[Peter BENSCH]

Welcome @Dn1te, to the community!

I suspect that ST-LINK/V2 is the cause of your problem. Please post photos of your ST-LINK/V2, including the back if it has a plastic casing.

Regards
/Peter

[Dn1te]

Thank you for your help. The pinout of the ST-LINK I use is exactly the same as the one shown in the first picture below. I also tried using another ST-LINK (with the pinout in the second image), but that failed as well.

At this point, I suspect the issue might be related to the power supply on my board.



EDIT: I changed to another ST-LINK and was able to perform a flash erase successfully. Now I can connect to ST-LINK constantly. Thanks you Peter for your help

[Ozone]

I would check the hardware.
The voltage at the supply pins should be in range, and the current consumption as well. I'm not sure if the ST-Link can supply the target, I had trouble with that occasionally. Better try an external supply.

I cannot really comment on the ST-Link V2 issue. I know there are some non-functional fakes around.
You could try the onboard ST-Link from another ST board, or another debug pod like a JLink. Or try Segger's JLink firmware for ST-Links, which is reversible.