cJTAG support

[thvdveld]

Hi! I was wondering how I could add cJTAG support to the project such that I can use with rust embedded toolchain.
Not sure if this is the place to ask this, but could you point me out in the correct direction such that I can help implement it?
I will gladly create a PR then!

[jonas-schievink]

Hi! Check out the wip branch, it should in theory allow you to switch to cJTAG mode and then use jtag_io with it. I haven't tested it though, because I don't have any MCU that speaks cJTAG.

[thvdveld]

Thanks for the quick response!

Ah yes, I didn't looked into wip.
I copied the PIC32 example to test if cJTAG works, however, I don't know anything about JTAG.
Is there any jtag_io sequence that could tell if cJTAG works?

I'm using a CC2538 and sections 5.2.1 and 5.2.2 talk about JTAG commands etc., but this is gibberish for me.

[jonas-schievink]

I know fairly little about cJTAG unfortunately. What happens if you run the jtag_scan example but with the cJTAG transport? Do you get an error or does it result in a valid scan chain already?

[thvdveld]

I get the following:

Checking for cJTAG connection...
error: could not detect closed JTAG chain (check connection, or consider increasing the max chain length)

[jonas-schievink]

Hmm, of course it's not that simple... The next step would be to look at the USB packets in wireshark between jaylink and the official tooling (or libjaylink, if that supports cJTAG), and possibly also hook up a logic analyzer to look at the signals. In any case, you'll have to get some "known-good" cJTAG setup with another tool, and then we can try to match what it does here.

[thvdveld]

Thanks for the pointer with wireshark! I will have a look at that today and will let you know how it goes!

[thvdveld]

It's been a long time, but I tried debugging it further.
I tried connecting with JLinkExe to the CC2538 (which uses cJTAG) using the cJTAG protocol and even the official tool of Segger does not seem to be able to communicate with it.
I also have an XDS110 debugger which is able to connect using cJTAG, so I think it is a Segger related problem and not this library.

As a side note, when using JLinkExe, with the JTAG protocol, it manages to create a connection.
It does that by using a sequence that tells the CC2538 to use JTAG instead of cJTAG.
I found this sequence in OpenOCD.
After the sequence, the JLink uses regular JTAG.