ZX81 ULA-in-a-CPLD

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
McKlaud
Posts: 107
Joined: Tue Dec 19, 2017 10:02 pm
Location: St Albans, UK

Re: ZX81 ULA-in-a-CPLD

Post by McKlaud » Fri Mar 30, 2018 3:01 pm

Well :mrgreen: your scope has been out of calibration for 14+ years ;) So, results as expected :mrgreen: don't blame ULA :mrgreen:

Anyway I decided to use Schmitt-triggers and gave up with the noise. It is as it is.
Claudius
----------

User avatar
PokeMon
Posts: 2224
Joined: Sat Sep 17, 2011 6:48 pm

Re: ZX81 ULA-in-a-CPLD

Post by PokeMon » Sun Apr 01, 2018 3:31 pm

Not sure where you measure but can be a ground problem at the end. What is the signal when you take it directly with GND and VID on your small board (as close as possible) ? I wouldn't take the signal from the modulator or feed it through as this can give more problems. Where do you get your signal or how it is created now ? Is it created from 3.3V ? Is it clean 3.3V ? Perhaps it is better to build a second supply 3.3V for video out to avoid distortions.

By the way - those ground problems may be caused through default grounding of crt monitors and oscilloscopes as well (building a ground loop). So measuring with unconnected TV equipment may give different (better) results. 80 mV noise is quite much - about 10% of full white level (700mV) and well seen on white background.

User avatar
Andy Rea
Posts: 1387
Joined: Fri May 09, 2008 2:48 pm
Location: notts UK

Re: ZX81 ULA-in-a-CPLD

Post by Andy Rea » Sun Apr 15, 2018 10:00 pm

I've been a bit quite since Easter, had a little chop and change and am working out of a temporary lab ( no non zeddy stuff here )
Image
this is my test zeddy most of the passives have been moved to the back side, and no ear or mic circuit exsists
Image
Image
the last time i posted i have way too much noise in the video signal, this is now muchly reduced
with just slight vertical banding showing on my LCD tv using the composite input.
Image
speedtest reports exact number of frames which is always a good start.
Image
Then i had a brainwave... since as i use onboard oscilator i dont use the OSC pin this can be repurposed into an
extra input... memory above 32K check.
Image
activate special feature... M1NOT yes please :D
Image

regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

User avatar
1024MAK
Posts: 1910
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 ULA-in-a-CPLD

Post by 1024MAK » Mon Apr 16, 2018 12:15 am

@Andy - naughty but nice :D

Mark

McKlaud
Posts: 107
Joined: Tue Dec 19, 2017 10:02 pm
Location: St Albans, UK

Re: ZX81 ULA-in-a-CPLD

Post by McKlaud » Mon Apr 16, 2018 7:41 am

Nice and good to hear from you that you made some progress. My unit is in a storage box waiting till I will be back home in a few weeks.

BTW do you have the barebone 32K RAM on your testing PCB?
Claudius
----------

User avatar
Andy Rea
Posts: 1387
Joined: Fri May 09, 2008 2:48 pm
Location: notts UK

Re: ZX81 ULA-in-a-CPLD

Post by Andy Rea » Mon Apr 16, 2018 8:28 am

It only wired for 16k, it was a mistake ordered chip wanted a through hole but got smd by mistake so I made use of it.
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

nollkolltroll
Posts: 234
Joined: Sat Sep 27, 2014 8:02 pm
Location: Stockholm, Sweden

Re: ZX81 ULA-in-a-CPLD

Post by nollkolltroll » Mon Apr 16, 2018 3:34 pm

But without the familiar video-disturbance patterns, how will I be able to debug where the code begins/ends?! I'll be lost forever in an endless loop! ;)
/Adam

User avatar
Andy Rea
Posts: 1387
Joined: Fri May 09, 2008 2:48 pm
Location: notts UK

Re: ZX81 ULA-in-a-CPLD

Post by Andy Rea » Tue Apr 17, 2018 10:18 pm

Well a little backwards step, happy that I had been able to reduce the video noise to an acceptable level I thought I would again try the buffer on a separate supply to see if that had any effect. If the output is took at the buffers itself then yes it is quite clean, but routed out the ula socket to the modulator the noise returns. So I'm not gonna bother with buffers, so just the preamp for the tape input. The circuit I built already the gain is a bit high but it did work.

Regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

Post Reply