Thanks for the reply, I got it working just the other day. I believe the entire time it was trying to boot and use UART1 on dedicated IO, rather than UART0 that we have routed on our custom board. We soldered wires and a 232 transceiver to UART1 on our board to see the boot loop it was stuck in (didn’t like the file system and then would just keep rebooting- hence the pulsing NRST) - then I fixed the file system and sd card image - and it all worked as expected on UART0 via shared IO. I thought that U-Boot would not need the filesystem (at all) or Linux device tree or anything other than U-boot itself to run, but I think that it does indeed need a good RFS. I think I proved that it has to have some root file system, even if it has almost no files in it.
As far as the link you sent (thank you) I have followed a similar process using the socedsgettingstarted guide from the Altera wiki, and was able to at least inspect ARM CPU registers. My issue seems to always be that the instructions are for Cyclone V/ArriaV, and while 90% similar, it’s different enough that I can’t get quite all the way there. For instance, with Arria 10 there is no preloader directory off the soceds install and there is no documentation that distinguishes this from the other soc devices that still use it for the preloader. According to the Altera support person, Arria 10 boot preloader is included in the Boot ROM so it’s not needed - just U-Boot and the Boot ROM are all that’s required (that would seem true given the preloader directory is gone for any Arria 10 ghrd). However, I have yet to find any documentation anywhere that mentions this.