[Libre-soc-bugs] [Bug 763] Work on I/O Core Pad JTAG Tests

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=763

--- Comment #2 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
https://git.libre-soc.org/?p=c4m-jtag.git;a=blob;f=c4m/nmigen/jtag/tap.py;h=e0cf747150c03c45a7be16682b31422103ab75bc;hb=eb28aea69c7a785085364c2fb3c2cbdbe86f4890#l625

io_bd - and in fact io_sr itself directly - is set to directly be
sent out via TDOs, one bit at a time (line 673)

io_sr is only shifted "in" from tdi in "shift" mode (line 620)

io_sr only takes data *from* pads and core in *capture* mode (line 592)

so you cannot just set a few signals and "hope for the best", you have
to actually make sure that the correct "Modes" (shift, update, capture)
are set.

the JTAG example in soc that i referred you to does this correctly.
it took me something mad like 2 days to understand it.

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