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If my STM32 is externally powered with 2.5V at its VDD and I'm connecting it to the STLINK-V2 that is powered by USB, should the VAPP pin on the STLINK-V2 be connected to the STM32's VDD? I thought the VAPP pin was a 3.3V output from the STLINK-V2, but it's not clear to me from the table below if it's supposed to be supplying or receiving VDD from the STM32.

Would SWD work if I only connected the SWDIO, SWCLK, GND, and NRST pins from the STM32 to the STLINK-V2?

Why is the STLINK-V2 VDD pin even on the cable connection list if it's not connected to the STM32:

enter image description here

From the STLINK-V2 user manual: https://www.st.com/resource/en/user_manual/um1075-stlinkv2-incircuit-debuggerprogrammer-for-stm8-and-stm32-stmicroelectronics.pdf

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The table you posted from the manual says that the VAPP pin is an input on a genuine ST-Link/V2 for Target VCC and it must be connected to STM32 MCU VDD supply so the can use the same voltage reference for communications.

It is actually further explained in note (1) which you mostly cropped away from the picture.

So that pin is not an supply output to power an STM32 board and it is required to be connected or the genuine ST-Link/V2 has no IO voltage for communicating with the MCU.

If you have non-genuine clone product, all bets are off how they work, and they can have a 3.3V output on that pin, which makes them incompatible and dangerous as they can cause damage if they have a supply output in place of communication reference voltage input.

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