Posted on March 11, 2018 at 18:47Having been down this path, it's not for the faint of heart. As the previous replies suggested, you do need a compelling reason to add the extra hardware -- the stlink board that is a (removable) part of the nucleo is the best cheap solution. Really, any probe that talks ARM SWD will work fine (e.g. j-link) and a 10-pin mini connector is a lot less expensive than a spare processor + associated components.
In our application, we have a compelling reason. We're designing data collection tags for biologists studying the movement and activity of song birds. Our partners need a base to configure the tags, charge the batteries, and download data. One possible approach would have been to build this around the stlink from a nucleo, but we still needed a processor to handle battery charging. Once there's an extra processor, lots of other things become possible (e.g. power measurement). So, we replicated the stlink api and added another USB (logical) interface to control the auxiliary functionality. All much more work than I anticipated and I wouldn't have bothered had I known what I was in for.
If you are set on this and don't want to use the black magic probe or don't need much functionality, look at CMSIS DAP libraries. They're quite portable and (relatively) easy to understand. They are a bit bandwidth limited because the Host/Debugger interface is much lower level than stlink.
That said, the SWD interface (through stlink or other device) is a wonderful way to interact with a space and pin limited device. The Debug Handler interface makes it possible to implement an RPC mechanism through SWD. We even have a ChibiOS shell running through SWD.
Geoffrey
