Putting a ZX81 onto a CPLD

[RetroTechie]

I suspect mdave4953 means programmable logic devices (PAL/GAL/CPLD/...) in general. CPLD's are probably the most practical / cost-effective these days. Wish new users would check the archives before asking:

Hardware -> ZX81 ULA replacement revisited
Hardware -> List of ULA availability

among similar threads over the years.

Anyway welcome to this forum, mdave4953! Did you have anything specific in mind? Pop out ULA & put in replacement to bring an old Zeddy back to life? A starting point to do your own ULA? Considering to build a ZX81 clone from scratch?

Somebody re-did my design in an Atmel CPLD recently, see here: Development -> ZX81 ULA-in-a-CPLD
And wow.. a little digging shows that using GALs has indeed been done (back in 2011): Development -> ULA revisited.
Amazing... Andy Rea at it again! But I don't think you'd want to go that route.. might as well do discrete 74xx IC's then.

[Andy Rea]

Yep that was me, I have to date created 4 ula implementations

1. Full ttl using 74xx series chips approx 22 chips iirc.
2. GAL and ttl, 6x gals 1xtll
3. CPLD + GAL didn't work well at all
4. CPLD works well has additional functionality approx 75 made
5. W.I.P.... Wait what.... DIP 40 sized direct replacement 2 small CPLDS working together uses own clock so garantees oscillator will work unlike the previous one that occasionally needed tweaking.

I think I still have the code for the gals. It was a mixture of 20v8 and 16v8 I think the ttl chip was the paralell load serial out 8 bit shift register, ( don't remember the part no.)

Regards Andy

[sirmorris]

WAI... WHA..

I knew you'd been a bit too quiet

[mdave4953]

Yep that was me, I have to date created 4 ula implementations

1. Full ttl using 74xx series chips approx 22 chips iirc.
2. GAL and ttl, 6x gals 1xtll
3. CPLD + GAL didn't work well at all
4. CPLD works well has additional functionality approx 75 made
5. W.I.P.... Wait what.... DIP 40 sized direct replacement 2 small CPLDS working together uses own clock so garantees oscillator will work unlike the previous one that occasionally needed tweaking.

I think I still have the code for the gals. It was a mixture of 20v8 and 16v8 I think the ttl chip was the paralell load serial out 8 bit shift register, ( don't remember the part no.)

Regards Andy

Which CPLD did you use ?...(Xilinx, Altera etc) and how many macrocells, logic blocks did it take ? I have some EPM7128s,..EPM9320s & EPM7512s (Surface mount) so I'd like to pop the VHDL code onto one of these !

Thanks

[Andy Rea]

Hi Mdave,

the cpld version that is out ther uses an Altera EPM7128SQC100-7 i was able at the time to get hold of these for a good price, rgw dwsign is in verilog i can let you have a copy of the design files if you wish, there is no schematic ! i never bothered to draw one,

this design does something frowned upon by all, it uses a cascade of flops as a delay, this was done to provide a 6.5Mhz clean clock from a not so clean 6.5Mhz almost sawtooth clock.

regards Andy

[mrtinb]

And Andy, you also made a version with the XC9572 CPLD.

[Andy Rea]

Hi Martin, i never did a version on a xilinx part, that was RetroTechie and i beleieve my mate Phill did an early version on a xilinx chip way back when i started with the altera design around about the time that I discovered the hsync counter was only reset by the INT-ACK and not the Vsync as used in so many clones before.

Regards Andy

[mrtinb]

Sorry. There are so many great tinkerers in these forums.

[gozzo]

Andy, have you ever released publicly the details of the 6 gal 1 ttl version? i know of the all ttl thing and of course, the cpld module?

[Andy Rea]

its here in this thread viewtopic.php?f=6&t=445 however the images, shcematics seem to have gone awol, the source files for the gal are still there.

regards Andy