I have this development board with the following pinout:

enter image description here

And I am using the following FTDI to USB interface to communicate with a PC:

enter image description here

Here is a photo of the board and the FTDI wired to the board pins:

enter image description here

So above on the left two wires are connecting the pins as:

FTDI +5V ---> Nucleo +5V(CN6 pin5)
FTDI GND ---> Nucleo GND(CN6 pin7)

and on the right two wires are connecting UART pins as:

FTDI RX ---> Nucleo PB10: USART3_TX
FTDI TX ---> Nucleo PB11: USART3_RX

So first of all, I assume +5V of the Nucleo board means that it is a 5V power supply for some electronics interface such as sensors. And I also assume that +5V in the FTDI board means that it needs to be powered by 5V supply so it is not a power supply pin but a pin for receiving power.

I do the wiring as I mentioned above and plug the FTDI board to the PC via its USB. The thing is even though I do not power the Nucleo through its own USB port, I still can communicate with the Nucleo's uC through PC.

So how does the uC get power in this case? I am super confused.

  • 1
    \$\begingroup\$ Look at the Nucleo schematic - there’s a 5V to 3V3 regulator. Also look at the schematic for your usb->serial - the 5V is probably coming from USB. You can use a multimeter to measure. \$\endgroup\$
    – Kartman
    Apr 10, 2021 at 10:25
  • 1
    \$\begingroup\$ But isnt +5V pin on the Nucleo a power output? How come it acts as a power input then? \$\endgroup\$
    – cm64
    Apr 10, 2021 at 10:29
  • \$\begingroup\$ look at the schematic! It seems to work both ways! st.com/resource/en/user_manual/… section 6.3 \$\endgroup\$
    – Kartman
    Apr 10, 2021 at 10:36
  • \$\begingroup\$ you want to get a 3.3v ftdi board, do not connect power connect grounds if you wish but if these are plugged into the same computer/hub they may already be grounded. then tx to rx and rx to tx....no power pins The very rare and I mean have only seen one, would be if you are powering the vddio on the ftdi part and in that case for the stm32 parts you want to use 3.3v. easy way to tell is with the ftdi board by itself connect tx to rx and use a dumb terminal to see if it is looped back if so then the board supplies its own vddio \$\endgroup\$
    – old_timer
    Apr 10, 2021 at 16:43

1 Answer 1


You don't provide any details about your FTDI board.

But if an USB to serial converter has a 5V pin, there is no reason to assume it is a power input.

The 5V pin would be expected to give out the 5V USB supply provided by the PC.

When that ia connected to the 5V node on the Nucleo it will power it up. Same way like connecting it directly to USB from the Nucleo USB. So don't connect two supplies to it.

You can verify this by reading the USB serial adapter manual. Just make sure the TX/RX pin use 3.3V voltage levels instead of 5V. Some, but not all MCU pins are 5V tolerant.

  • \$\begingroup\$ But isnt +5V node on the Nucleo a power output? How come it acts as a power input then? \$\endgroup\$
    – cm64
    Apr 10, 2021 at 10:28
  • \$\begingroup\$ It is a wire onboard that connects all 5V pins together. Nobody defines it must be used for power output. And certainly the FTDI board does not need a separate 5V input. Unless you have manuals to show it does. \$\endgroup\$
    – Justme
    Apr 10, 2021 at 10:32
  • \$\begingroup\$ It's standard for a USB IC, such as a USB-serial converter, to have a USB Vbus input. It powers the front-end bus electronics so the device can meet the USB protocol electrical requirements and behave correctly on the bus. \$\endgroup\$
    – TonyM
    Apr 10, 2021 at 10:38

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