Which UART are you using?
As I was working from Garth's primer I used the MAX3100 SPI UART he recommends. It's flexible, has a simple interface and comes in a breadboard-friendly package for prototyping. I had a lot of fun writing the routines to big-bang a duplex'ed SPI interface using the 65C22 ...

Thanks for the tremendous support, guys. I'll go into the gory details below but tl;dr is that your suggestions addressed the issue.

You have BE , NMI(B) , RDY and SO(B) all tied to the same pullup resistor. RDY is a bi-directional pin and should be on its own pullup. If the 65C02 executes a WAI ...

Welcome, and congratulations on the success so far. It sounds like you have a good understanding of how to troubleshoot. I'm glad my 6502 primer was a help.
Thanks! It's nice to be finally be conversing here after years of lurking.
As you can see, there's no way the SRAM's chip select can go true ...

Thanks, Doc, I'll try to tone down the dad humor :oops:
Can you elaborate on what "looked more closely" means? Sounds like you're examining the stack upon entry to the ISR. The last items pushed to stack will be the address of a BRK opcode plus two, and the P register. So, you're examining the ...

Gladly! Nice to make your acquaintance, BDD. There's now a JPEG of the schematic in the repo here: https://github.com/contrarian-computing ... -8.sch.jpg

Greetings fellow 6502 enthusiasts! Following in what seems like every other 8-bit curmudgeon's footsteps, I've spent the last few months working on a compact, affordable 6502 SBC for my own enjoyment and (hopefully, one day) to serve as the core around which I'll build a wholesome afterschool ...