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.

[MJaworski]

Thank you very much for your experiment mfgkw, it proves that STLink can be deceived by external power supply.

I do have a question though: does your setup work when you have Supply B disconnected from STLink's VDD? If so, I then believe that there is some error in my Prototype, I will look on signal lines with oscilloscope to detect possible faulty behaviour.

[mfgkw]

> I do have a question though: does your setup work when you have Supply B disconnected from STLink's VDD?

no, then I have no connection.

 

 

[MJaworski]

Alright, correct me if I'm wrong but then it means that STLink-V2 requires "VDD_Target" to be connected in order to work properly. That's a bummer.

[mfgkw]

VDD has to be provided to the ST-Link, but not necessarily from your board.

[Andrew Neil]

---

@mfgkw wrote:

VDD has to be provided to the ST-Link

---

... or must it?

@Suresh_M in this thread claims to have it working in Keil without a Target VDD connection ...

[MJaworski]

Okay people, let's wrap the thread up.

Thank you very much mfgkw and other users for help with this one, now jumping to conclusion:

1. STLink-V2 requires "VDD_TARGET" Pin connected in order to establish a connection with the Prototype BUT, it doesn't have to be connected to the Prototype itself, any other source of valid voltage on this pin allows to establish a connection between ST-Link and Prototype

2. When using ST-Link from Nucleo Boards, there is no need for connecting "VDD_TARGET" to Prototype. From what I saw an Nucleo schematics (Document MB1136, p. 4):

there is a pullup resistor on the VDD_TARGET pin, so without connecting it to the Prototype, pullup provides a valid voltage to PA0(AIN_1) Pin

3. There is unverified claim that this pin doesn't have to be connected when using Keil software.

If there is nothing more to add, we can close this thread and let it get indexed by google for future generations.

[Andrew Neil]

---

@MJaworski wrote:

If there is nothing more to add, we can close this thread and let it get indexed by google for future generations.

---

The way to do that is to Mark the solution (you can always change the marked post later, if further info comes to light)