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I have a Nucleo F446RE, which writes to UART which is handled by the built-in ST-Link over VCP on USB.

I'd like to be able to see the serial signals on my oscilloscope as well. There are two exposed connectors, labelled TX and RX, which seem to connect to the USART2 (the same one connected to ST-Link). However, I see no signal on them on my scope.

Looking at the Nucleo data sheet:

6.8 USART communication
The USART2 interface available on PA2 and PA3 of the STM32 microcontroller can be connected to ST-LINK MCU, ST morpho connector, or to ARDUINO® connector. The choice can be changed by setting the related solder bridges. By default, the USART2 communication between the target STM32 and ST-LINK MCU is enabled, in order to support virtual COM port for Mbed™ (SB13 and SB14 ON, SB62 and SB63 OFF). If the communication between the target STM32 PA2 (D1) or PA3 (D0) and shield or extension board is required, SB62 and SB63 must be ON, while SB13 and SB14 must be OFF. In such a case, it is possible to connect another USART to ST-LINK MCU using flying wires between the ST morpho connector and CN3

I'm having trouble deciphering that, but it sounds like the USART is connected to either the ST-Link or the connector, not both, and that changing that requires soldering the board. Is that correct?

Is there anyway, without resoldering the Nucleo, to see a USART signal on an exposed pin or connector? I don't necessarily need USART2 - any one will do.

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3 Answers 3

Reset to default
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By default, SB13 and SB14 are soldered ON so they connect the target PA2 and PA3 to ST-Link virtual COM port.

Unless someone has. modified your board, it should just work, and you can scope on SB13 and SB14.

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Check out the schematics in the STM32 Nucleo-64 User Guide. Here's the relevant excerpt:

Excerpt from schematics in STM32 Nucleo-64 User Guide

This schematic is from Rev 12 of UM1724 (available here on User Manual Wiki). The latest version (Rev 14) is here on st.com, but does not include the schematics.

Solder bridges SB62 and SB63 are open by default, so the RX and TX pins are disconnected from "TX/D1" and "RX/D0" as labeled on the board. If you don't want to solder, then you can just probe the one side of the solder bridge.

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  • \$\begingroup\$ Looks like you can also get to them on CN3 \$\endgroup\$
    – justsomeguy
    Sep 20 at 17:11
  • \$\begingroup\$ Thanks. If I probe the side of the bridge, will that damage or interfere with the ST-Link? The manual seems to warn "PA2 and PA3 on STM32 are connected to D1 and D0 (pin 2 and pin 1) on ARDUINO® connector CN9 and ST morpho connector CN10 as USART signals. Thus SB13 and SB14 must be OFF." \$\endgroup\$
    – SRobertJames
    Sep 20 at 17:22
  • \$\begingroup\$ Also: Where do you see that you can get them on CN3? Which pins? The manual seems to state you can get a different USART (totally fine) if you add a "flying wire" (what's that? how do I add it?). \$\endgroup\$
    – SRobertJames
    Sep 20 at 17:22
  • \$\begingroup\$ @SRobertJames - probing the UART with a scope will not damage the board or interfere with the signal \$\endgroup\$
    – justsomeguy
    Sep 20 at 17:52
  • \$\begingroup\$ CN3 is the ST-LINK UART broken out to a connector appropriately labeled "TX" and "RX". If you break off the ST-LINK part of the board, you can connect these lines with a wire. Otherwise, you can probe these signals on this connector with your scope. \$\endgroup\$
    – tpimh
    Sep 21 at 8:03
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Figure 1 shows the main schematic connecting the CPU labels (USART_TX and USART_RX) to the ST-Link labels (STLK_TX and STLK_RX) for TX and RX.

Figure 2 shows the CPU labels and the solder jumpers (SB62 and SB63)

Figure 3 shows the STLink labels, the solder jumpers (SB13 and SB14) and CN3

I'm breaking up the documentation quote for easier reading:

By default, the USART2 communication between the target STM32 and ST-LINK MCU is enabled, in order to support virtual COM port for Mbed™ (SB13 and SB14 ON, SB62 and SB63 OFF)

If the communication between the target STM32 PA2 (D1) or PA3 (D0) and shield or extension board is required, SB62 and SB63 must be ON, while SB13 and SB14 must be OFF.

In such a case, it is possible to connect another USART to ST-LINK MCU using flying wires between the ST morpho connector and CN3

main schematic cpu schematic stlink schematic

Hope this helps.

Source of figures: https://www.st.com/content/ccc/resource/technical/layouts_and_diagrams/schematic_pack/group2/5a/85/d6/9a/34/e2/47/1d/MB1136-DEFAULT-C05_Schematic/files/MB1136-DEFAULT-C05_Schematic.pdf/jcr:content/translations/en.MB1136-DEFAULT-C05_Schematic.pdf

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