Please do tell me more about that AUD support you worked with, sounds you have experience with SuperH4 targets (and possibly debug probes/equipment I could borrow?
I'll be level with you, I have no SH4 experience beyond reading a few datasheets. What I have worked on a fair bit is SH2 automotive stuff , mostly SH7055/SH7058.
On those, there's the 8-pin "AUD" interface, luckily not soft-disabled. It has two modes, the most interesting being Branch mode : every bra/jsr/b**/interrupt, the destination PC (or at least the changed bits) is output on AUD.
So I made a crappy board to keep the AUD port in the correct mode, then hooked up a logic analyzer to capture the branches. Even wrote a sigrok PD for it (
https://sigrok.org/wiki/Protocol_decoder:Aud ).
Interesting as this is, it won't help you I'm afraid...
For JTAG, you seem to be in good hands with that Glasgow ticket you filed. I was going to suggest permutations of holding/releasing nRST, asserting NMI, etc... I find it strange that you couldn't get at least an IDCODE.
UBC isn't relevant here unless you can reflash a modified firmware. I've used that to trace reads/writes to certain RAM vars that I could just not find with static analysis : UBC gives you a hardware breakpoint that triggers your interrupt code, in which you can log stuff etc.
Very interesting what the Glasgow folks said about HUDI. I thought it depended entirely on debug helpers compiled in the firmware (a kind of semihosting perhaps, where the debug hooks write to SDIR/etc and the JTAG probe interacts with that).
Have you looked at this ?
https://www.lauterbach.com/adaud.htmlHold on... this 7750, it doesn't have onboard flash, does it ? Couldn't you just watch the external address + bus and follow along ... (sorry if this was covered earlier, details are fuzzy)
[EDIT - more links]
This guy seems to have it figured out, with a parallel (!) interface :
https://renesasrulz.com/older-tools/hew_target_server/f/hew-target-server---forum/3486/eclipse-hts-hew-target-mcucouldn't see his link / attachment though.
Maybe relevant... the ST40 (some popular set-topbox thing) has an SH4 core with the probably-same UDI peripheral; maybe another lead. e.g.
https://project-magpie.github.io/2014/06/15/sti7111-st40-jtag-preparations/