[Libre-soc-bugs] [Bug 517] Define JTAG pins and set up with litex for ulx3s85f

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=517

--- Comment #86 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Cole Poirier from comment #78)

> > > Do you know of a jtag usb adapter that supports softcore targets?
> > 
> > i don't
> 
> Isn't that a problem as we are using a softcore and using gpio pins to do
> jtag instead of the stm module on the fpga?

you have confused multiple things.

* openocd talks to stlinkv2.
* stlinkv2 is instructed to enter JTAG MODE

at that point it doesn't matter what the hell gets connected, it's just JTAG.

* four wires speaking JTAG connect to FPGA
* those pins are connected to HDL
* that HDL understands JTAG

the "softcore" isn't even directly associated with JTAG, they are completely
separate subsystems.  the "softcore" does not know anything at all about JTAG.

the JTAG *protocol* is implemented in HDL by Staf's C4M-JTAG nmigen module.

that is its job.

what you are referring to, "is there a jtag usb that supports softcore targets"
completely confuses about four unrelated things, including assuming that the
HIGH LEVEL scripts in openocd are relevant and mandatory.

we are doing LOW LEVEL and the rest of openocd is completely and utterly
irrelevant at this early phase.

the only reason we are using openocd is because it contains a tiny relevant
fraction of code necessary to recognise the stlinkv2 and get it into JTAG mode.

THAT IS IT.


once this utterly simple task which took me 20 minutes to research and
implement is completed and confirmed, THEN AND ONLY THEN DO WE MOVE TO DEFINING
A PROCESSOR TARGET.


> > > Can you comment as to what your progress on JTAG is?
> > 
> > on the irc logs.  if you were on the irc channel you would have seen the
> > discussion of the past week.
> 
> irc should not be used as a substitute for the bug tracker. irc is only for
> short conversations, not critical development discussions. That is the
> explicit purpose of the bug tracker.

the solution there is to look up the irclog link and enter it into the
bugtracker.

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