STLink V2 SWD operation without connecting VDD_TARGET PIN

[MJaworski]

Dear ST Community.

I am designing devices with STM32WL microcontrollers with highly limited board space. I routed out traces for all SWD signals (VDD, GND, NRST, SWDIO, SWCLK)

I have my Prototype Board, original ST-Link V2 with V2J45S7 Firmware and STM32CubeProgrammer version v2.20.0. During testing I realized that the connection to Prototype is stable only when I connect all 5 signals. For some reason I thought that "VDD_Target" pin on ST-Link side can be left floating.

I have found a previous post:
https://community.st.com/t5/stm32-mcus-boards-and-hardware/how-to-connect-the-stlink-v2-to-my-board/m-p/94389
but unfortunately in my case I have to have "VDD_Target" signal connected on both ends. When disconnected STMCubeProgrammer responds with error shown below:

Do I need to have all SWD signals being connected in order to program my Prototype or is there any hidden option in ST-Link/STMCubeProgrammer that I can turn on, so in final design I can omit routing VDD pin for SWD connector?

Kind regards
Marek Jaworski.

[mfgkw]

In theory VDD is required to inform the ST-Link about your CPU voltage.

If you are really out of space for connections it would be possible to fake your CPU voltage for ST-Link without a connection. So if you drive your STM with 3.3V it will work when you present 3.3V on ST-Link between VDD and GND without really connecting it from your STM to ST-Link.

Then you need to connect GND, SWCLK and SWDIO in minimum (provided you set VDD to your CPU voltage at ST-Link side), and optionally NRST for Debugging and optionally SWO for tracing if needed.

[MJaworski]

Dear mfgkw,
thank you for your reply. I did not specify in post above, my Prototype, or should I write my STM32WL is powered from 3,3V stable source. Creating artificial voltage for STLink was my first idea but I thought that there might be some legitimate way developed by tool's manufacturer. If there is no such thing, I will commit a voltage fraud.

Kid regards
Marek Jaworski.

[TDK]

It should work. The ST-LINK/V2 operates at 3.3 V regardless of VDD_TARGET level according to the user manual. Are you sure you have this exact debugger? Some debuggers use a level shifter. Perhaps link to the debugger you have.