[Bug]: Cannot debug and connect to serial output at the same time

[KeilChris]

Description

Using older DAP-Link or LPC-Link2 implementations, it is not possible to have a debug connection and monitor the output of the UART on the SERIAL MONITOR at the same time. Once the serial connection is opened, the debug connect stalls and throws these two errors:

 Cannot execute this command while the target is running.
Use the "interrupt" command to stop the target and then
try again.

And

Unable to read memory.

Using an NXP board with the new MCU-Link FW, this problem does not occur

Steps to Reproduce

Take any old development board with LPC-Link2/DAP-Link from NXP and try to debug while obeserving the UART output.

Expected Behavior

It should work simultaneously.

Additional Information

No response

[ReinhardKeil]

I believe the solution is to upgrade the firmware to MCU-Link FW. IMHO, we should document this (just briefly analyze if there is a potential to fix it).

[KeilChris]

I'm not aware that older firmware implementations can be updated to MCU-Link. Let Jens check this; might be fixed via documentation.

[jreineckearm]

Had a quick look at DAPLink on a FRDM-K64F where I see slightly different but similar behavior:

Daplink Mode: Interface
Interface Version: 0254
Bootloader Version: 0244

With this firmware, I see a proper connection loss with SWD communication breakdown when I connect to UART with Serial Monitor.
The board ran a Blinky example which I had paused. As soon as I connected to UART, the board started running. I suspect a board reset.

Very likely related to resets in this function: https://github.com/ARMmbed/DAPLink/blob/8f4e9e40cf1a0902b88088a5a55a8c0559f709a5/source/daplink/usb2uart/usbd_user_cdc_acm.c#L115C9-L115C31
See also ARMmbed/DAPLink#738

@KeilChris , let's handle by documentation in the troubleshooting section. Firmware implementations need to support parallel operation of UART and debug to allow the full experience.

[KeilChris]

Is this a similar failure mechanism as in #855?