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I have a quick question:

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

Pinout

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?

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  • \$\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

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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

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