I have a quick question:

In the ST-Link V3 datasheet the pinout of the STDC14 connector is mentioned as follows:


I have a question regarding:

  1. Pin 3: Should I connect the 3.3V signal from the target power supply? As far as I know, this is not for powering, but rather for setting the correct reference voltage, therefore connecting it is important;

  2. Pin 11: I've seen designs which connect this pin directly to ground and ones with a 100Ω resistor to ground, what is the correct or "more" common thing to do?

  3. Pin 13 and 14: Is this the interface for the USART connection, so that USART_RX from the uC is connected to pin 13 and USART_TX vice versa? Theoretically, could I leave the pins unwired if I am only interested in SWD debugging or is there any other benefit to it I am not recognising currently?

  • \$\begingroup\$ What is your target board and the logic Voltage levels? At this point is sound almost like your asking about using 6 PIN SWD connection setup verse 10 Pin ARM setup. \$\endgroup\$
    – ben
    Commented Feb 20, 2022 at 8:32
  • \$\begingroup\$ The T_ prefix presumably means "Target" (meaning the device being programmed/debugged). For example, T_XYZ would refer to signal XYZ from the target board on the ST-LINK as this could help avoid ambiguity if there were another XYZ signal on that board. \$\endgroup\$
    – jacobq
    Commented Jan 14, 2023 at 12:24

1 Answer 1


This question is similar to one posted on ST's community discussion forum. A good way to verify that what you're doing is OK is to compare it with a known-good reference board, such as one of ST's NUCLEO offerings.

For example, the NUCLEO-G474's user manual includes Table 14 on page 28 (like OP but with slightly more helpful descriptions), and if you click on the "CAD Resources" tab of the NUCLEO/DK/eval board page you'll often find complete board design & manufacturing files as well as schematic. In this case you can peek at the schematic and see how CN4 is connected.

CN4 pinout on NUCLEO-G474 schematic

  1. Yes, you should connect pin 3 to the MCU's VCC/VDD/V+ supply.
  2. I would include 100ohm resistor as ST's reference boards do. However, I do not think it matters unless your (target) MCU needs to detect the presence of the ST-LINK debugger hardware. If you look at note 6 of the table referenced it says, "Tied to GND by STLINK-V3SET firmware; may be used by the target for detection of the tool." Perhaps it is intended to be pulled up to T_VCC with weak pull-up resistor for that case?
  3. You can leave them unwired if you don't care about the VCP (virtual COM port) functionality. However, I would double check whether Rx/Tx needs to be flipped or not. The STLink-MINIE user manual section 6.3 / Table 4 shows T_VCP_TX as an input and T_VCP_RX as an output.

VCP Rx/Tx connections


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.