ZX81 ULA-in-a-CPLD

[RetroTechie]

Well... better late than never:

clicky

I'll write up a simple webpage later. Short specs:

• ZX81 ULA function implemented in a Xilinx XC9572 CPLD.
• Hardware-wise, a 1:1 drop-in replacement for a ZX81 ULA.
• 100% compatible. For me personally this has become my reference-ZX81. You'll find a real ZX81 behaves exactly the same way no matter what software you throw at it.
• Done using schematic entry - not as flexible as Verilog or VHDL, but many people might find a schematic easier to read.
• Very well documented, and published primarily for that reason - as a reference implementation of sorts. Perhaps useful for emulator writers as well (well, those that aren't scared of XOR gates or flip-flops, that is ).
• Very well tested (list of what software was used, is included). Has run 25thanni demo for 3 weeks non-stop... until AC mains power failure @ my place.
• A free, open design as far as I'm concerned.
• Yeah, it's got pix...

Feedback is appreciated, but before asking questions, please read the included documentation!

[PokeMon]

Good job. And well documented.

Will there be a commercial product later or just as it is ?

[RetroTechie]

Probably not...

Mostly due to lack of time. Not that it would be *that* hard, but it still takes persistence (and some money investment, too) to bring something from "it's alive!" to "order here" stage. And so many other projects-to-be are begging for attention...

There are some hints in the docs though... so if you're good with a PCB design package & wouldn't mind investing in further development / a small production run, please do!

For myself, I'll probably be looking at ways to move things onto a FPGA board (building with eg. T80 or TV80 soft core). Much easier to experiment with overclocking / card readers / VGA output / whatever on such a board. With that in mind, further developing an ULA replacement feels (a bit) like waste of time. I regard it more as a building block for use in future projects (and giving me a rock-solid, reliable Zeddy in the mean time). But that's just MHO...

[PokeMon]

Well that was just an informational question.
I still have two ULA boards from Andy here which are small and fit easily in a original ZX81 / ULA socket.
And I got them very cheap as an early bird.

But still not had the time to repair one old ISS3 board with non working ULA due to other projects which are begging for attention, too.

[sirmorris]

Fantastic stuff - thank you for sharing your work! It's great having multiple explanations of very complicated processes as details can be discovered in one way and reinforced in another!

Regards

C

[RetroTechie]

Btw: a while ago I found that image quality was improved by adding a small capacitor (I used 680 pF) to the composite video output. Likely by 'softening' the steep edges of the CPLD's digital outputs.

Just updated the "Read me" in above archive to include that info... no other changes, no need to re-download if you're only interested in the design's logic. But it's a small download so go ahead.

[zx81user]

Hi,

I am interested into looking at this CPLD ULA design, can I download it from somewhere?

Best regards,
Michel

[PokeMon]

This project was online before and is maybe temporarily (?) blocked.
You could send PM to RetroTechie.

[Rink]

Nice. Don't think I've seen this before.

I wonder what the utilisation of the CPLD is like? I'd be tempted to see if I can put the design in an Altera CPLD since I've been playing with those lately.

[Paul]

Andy Rea's ULA ist Altera based, fits into the Zeddy and has the interface to be reprogrammed.
And is still has plenty of pins and space free for enhancements

Maybe you ask him?
Kind regards
Paul