ZX81 ULA-in-a-CPLD

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RetroTechie
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ZX81 ULA-in-a-CPLD

Post by RetroTechie » Sat Jun 02, 2012 11:00 pm

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. :oops:
  • 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!
Last edited by RetroTechie on Mon Jun 11, 2012 5:18 pm, edited 1 time in total.
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PokeMon
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Re: ZX81 ULA-in-a-CPLD

Post by PokeMon » Sun Jun 03, 2012 1:00 am

Good job. And well documented. ;)

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

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RetroTechie
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Re: ZX81 ULA-in-a-CPLD

Post by RetroTechie » Sun Jun 03, 2012 2:15 am

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... :mrgreen:

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...
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PokeMon
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Re: ZX81 ULA-in-a-CPLD

Post by PokeMon » Sun Jun 03, 2012 11:45 am

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. :mrgreen:

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sirmorris
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Re: ZX81 ULA-in-a-CPLD

Post by sirmorris » Sun Jun 03, 2012 10:01 pm

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
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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Re: ZX81 ULA-in-a-CPLD

Post by RetroTechie » Mon Jun 04, 2012 3:41 am

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. :mrgreen:
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zx81user
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Re: ZX81 ULA-in-a-CPLD

Post by zx81user » Sat Apr 13, 2013 8:17 am

Hi,

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

Best regards,
Michel

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PokeMon
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Re: ZX81 ULA-in-a-CPLD

Post by PokeMon » Sat Apr 13, 2013 9:11 pm

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

Rink
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Re: ZX81 ULA-in-a-CPLD

Post by Rink » Sun Apr 14, 2013 12:14 pm

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.

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Paul
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Re: ZX81 ULA-in-a-CPLD

Post by Paul » Sun Apr 14, 2013 2:56 pm

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
In theory, there is no difference between theory and practice. But, in practice, there is.

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