SWD Multidrop Support

[jebiga52]

It would be nice to have SWD Multidrop support so that RP2040s could be debugged with the J-Link in Rust. Issue probe-rs/probe-rs#1157 has some more details.

[jonas-schievink]

How does multidrop work? I don't think there's a hardware/firmware feature for that, so it might have to be implemented in probe-rs

[jebiga52]

I was able to connect to the RP2040 using SEGGER's JLinkExe, as shown in the issue I linked above. SEGGER's software exposes the RP2040 as two targets, RP2040_M0_0 and RP2040_M0_1. It seems like SEGGER already implemented multidrop support in the firmware last year, here's a thread I found related to that.

In terms of where support for this feature needs to be implemented, the probe-rs folks think it should be implemented in jaylink:

Unfortunately multidrop SWD isn't supported by probe-rs for jlink probes (I believe it would need implementing in the jaylink crate first, and then adding to the probe-rs jaylink driver).

Perhaps @adamgreig can comment because I am also unsure of what the implementation of multidrop would entail.

[jonas-schievink]

According to the ADI specification, multidrop just involves a special target selection register and more line resets, both of which should already be supported by this crate.