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M
MMich.2
Visitor II
October 5, 2020
Question

How to use USART (CN3 Rx and Tx pins) of ST-LINK (from Nucleo) for external target board?

  • October 5, 2020
  • 1 reply
  • 5186 views

Hi,

I'm using the ST-LINK from a Nucleo-F446RE as a programming/debugging adapter for a customized external target board. I have (un)jumpered the ST-LINK to "open" and I can program and debug my external board which is awesome! : - )

Now I'd like to use the two pins Rx and Tx of CN3 on the ST-LINK to communicate via USART with my own board. I'm not sure if this works "out of the box" or if I have to use other jumpers or even have to remove/insert some solder bridges. I'd also prefer not breaking the ST-LINK mechanically from the STM32 part of the Nucleo.

I'm not 100% sure how to interpret the chapter 6.8 in the manual here but I think that I can simply use the Rx and Tx from the ST-LINK without further changes, don't I?

I have also found additional info about transforming an ST-LINK to a fully external debug adaptor here. The strange thing is that there they talk about soldering in the JP4, then jumpering it and also desoldering a few of the solder bridges (or breaking the board mechanically in two pieces). Is all of this really required and correct?

Thanks a lot!

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    1 reply

    T
    Tesla DeLorean
    Graduate II
    October 5, 2020

    >>Is all of this really required and correct?

    ST does provide a schematic for the electrically minded to review.

    When snapped off some of the solder-bridges to disconnect the local target become irrelevant, whereas when it is not snapped off you need to remove connectivity so it does not connect/conflict. You don't want two target devices driving the RX pin at the same time.

    CN2 only deals with the SWDIO/SWCLK, so you still have SWO and NRST to contend with, and the VCP UART pins.

    SB13/SB14 will disconnect the target side UART

    SB12 NRST

    SB15 SWO

    JP4 used to connect the target NRST on older board designs, it's current function is not disclosed (connects to host PB10, so either a GPIO or USART3_TX)

    M
    MMich.2Author
    Visitor II
    October 6, 2020

    Thanks for the quick response! : - )

    "ST does provide a schematic for the electrically minded to review."

    Well, I think that I'm not electrically minded enough ; - )

    Shouldn't I have already have problems with NRST and SWO if the solder bridges SB12 and SB15 are important? Im' already using the ST-LINK successfully to program and debug everything according to the manual using the JTAG/SWD interface. I guess that I don't really understand the role of NRST and SWO here.

    So the problem with SB13 and SB14 is that they represent Rx and Tx directly to the STM32 processor on the Nucleo board? Since I would connect the Rx of the ST-LINK to the Tx of my custom board and vice versa you think that the electrical connection might be a problem? I guess that the connection's state on the Nucleo would be undefined and could interfere with my UART communication?

    I have not read anything about the JP4 in any official document so I guess that this is not required to tinker with.

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