[FR] add JTAG support for STM32F103

[ildar]

I successfully use dap42 on a general STM32F103 board (BluePill exactly).
I guess nothing prevents JTAG be on for it. Thanks.

[devanlai]

Hi, it is technically possible to add JTAG support, but I haven't spent much time integrating it because I usually don't need full JTAG and from the last time I looked into it, the performance was not great. This is in part because the USB handling is not particularly optimized, and in part because CMSIS-DAP 1.0 is not very fast as far as JTAG transports go.

[ildar]

I see your point. I suggest this FR stay opened, just hanging out. Thanks!

[ildar]

probably related: devanlai/dap42-hardware#2

[xobs]

I have initial JTAG support in a branch: https://github.com/xobs/dap42/tree/add-jtag-support

This has been tested on STM32F042 hardware, and I have a new PCB coming that fixes the hardware issues. I'll make a PR once that's actually tested and working.