Search found 32 matches

by commie
Fri Dec 16, 2016 7:45 pm
Forum: Development
Topic: 20Mhz ZX81 clone...
Replies: 20
Views: 3377

Re: 20Mhz ZX81 clone...

Hi fella's, So..., anyhow, whats involved in making zeddy go at 20Meg.?, I can think of something straight away, the hsync and vsync will require a 12bit counting chain, but what about any(if any) rom timing constants go? Maybe a requirement to switch between clocks on the fly like from 40MHz to 6.5...
by commie
Wed Dec 07, 2016 5:59 pm
Forum: Development
Topic: ZX81 ULA-in-a-CPLD
Replies: 233
Views: 26470

Re: ZX81 ULA-in-a-CPLD

PokeMon wrote: I assume this is a clock polarity mismatch (falling/rising edge) of the video clock (6.5 MHz).
Correct.
by commie
Wed Dec 07, 2016 3:50 pm
Forum: Development
Topic: ZX81 ULA-in-a-CPLD
Replies: 233
Views: 26470

Re: ZX81 ULA-in-a-CPLD

Hi Fella's, Wow!, I didn't notice the cursor shifted to the right but it is, I did notice that on the chess game the pieces were slightly unsymmetrical anyhow it just so happened it was an easy fix. Thanks for bringing it to my attention. :D In between there was another problem which cropped up, I w...
by commie
Mon Dec 05, 2016 7:27 pm
Forum: Development
Topic: ZX81 ULA-in-a-CPLD
Replies: 233
Views: 26470

Re: ZX81 ULA-in-a-CPLD

Hi All, More good news, I've just loaded my first commercial game CHESS and it is working. :D Retrotechie, about that AND gate with the inverted input?, PokeMon is right, what happens is on the plot screen you get flickerin on the display so you need it. So heres my line up, total chip count =7 and ...
by commie
Mon Dec 05, 2016 6:23 pm
Forum: Development
Topic: ZX81 ULA-in-a-CPLD
Replies: 233
Views: 26470

Re: ZX81 ULA-in-a-CPLD

After much head scratching across the entire weekend and today, I think I have reached my objective.I had to include another d-type flip flop to Retrotechie's design around the line counter reset. http://i571.photobucket.com/albums/ss156/onecommie/IMG_0243_zpsv4nbgq3o.jpg http://i571.photobucket.com...
by commie
Sat Dec 03, 2016 1:28 am
Forum: Development
Topic: ZX81 ULA-in-a-CPLD
Replies: 233
Views: 26470

Re: ZX81 ULA-in-a-CPLD

PokeMon, I love your avatar, it makes me feel like life is in a cartoon and why should it not be with backup, that said, I took heed of your post and went for the hsync generator, starting at 200 counts and ending somewhere around 207 one step at a time. This cleared the zig zag on the top right han...
by commie
Sat Dec 03, 2016 1:05 am
Forum: Development
Topic: ZX81 ULA-in-a-CPLD
Replies: 233
Views: 26470

Re: ZX81 ULA-in-a-CPLD

This is what my zx81 proto actually looks like. :D

Image
by commie
Fri Dec 02, 2016 4:45 pm
Forum: Development
Topic: ZX81 ULA-in-a-CPLD
Replies: 233
Views: 26470

Re: ZX81 ULA-in-a-CPLD

http://i571.photobucket.com/albums/ss156/onecommie/IMG_0237_zpscaommptf.jpg http://i571.photobucket.com/albums/ss156/onecommie/7921726e-fd73-493b-a7f5-150ad2ce4cf1_zpss2bonnua.jpg http://i571.photobucket.com/albums/ss156/onecommie/IMG_0234_zpsnhiu8atw.jpg http://i571.photobucket.com/albums/ss156/on...
by commie
Thu Dec 01, 2016 9:34 pm
Forum: Development
Topic: ZX81 ULA-in-a-CPLD
Replies: 233
Views: 26470

Re: ZX81 ULA-in-a-CPLD

from Wilf Rigter Hi PokeMon, Nice interesting circuit that you have brought to the forum attention, thanks for that. I tested the two bjt latch and it does indeed work very well. Now my zx81 prototype boots straight into slow mode and I can switch from slow to fast mode and back again using just th...
by commie
Wed Nov 30, 2016 11:19 pm
Forum: Development
Topic: ZX81 ULA-in-a-CPLD
Replies: 233
Views: 26470

Re: ZX81 ULA-in-a-CPLD

Are you sure that the transistor actually switches off fast enough? Actually, you are right, with the transistor removed completely from circuit there is no change in screen characteristics in slow mode. :oops: I've tried a very wide range of resistor/cap. values and the best result I can get is wi...