ZX81 ULA-in-a-CPLD

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
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Andy Rea
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Joined: Fri May 09, 2008 2:48 pm

Re: ZX81 ULA-in-a-CPLD

Post by Andy Rea »

Well some further development while i am waiting for parts to hopefully fix the video noise issue, i thought i would turn to tape input. i couldn't get a simple resistor network to work well, even though in my project setting i have pins set to "float" instead of "keeper" it was still sticking at 2.4 volts... very odd behaviour indeed. so a simple single stage bjt amp was built with a gain of around 15 so the output is very square which was exactly the intention , i think the gain is a little high as for the first time ever i have had to use a substantially lower volume on my cassette player than normal :lol:

anyway hopefully with the space freed up by chaging the video circuit i should have enough room for on board transistor amp.

regards.
AFK

McKlaud
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Re: ZX81 ULA-in-a-CPLD

Post by McKlaud »

Hi Andy,

I've faced the same issue when I built the resistor network with the 1.2V bias. It worked well when isolated on a breadboard, but when I plugged the ULA in to the Zeddy's PCB I started reading 2.4-2.5V. Odd. So I gave up with it and just tried to load anything directly with the circuit as it was. I tested with the TZXduino. Everything was fine, software was loaded without errors.
Claudius
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Andy Rea
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Re: ZX81 ULA-in-a-CPLD

Post by Andy Rea »

Thanks for the confirmation that I wasn't going mad, my circuit is very similar indeed, I need to tweak the gain a little but it's all good stuff ready for the next prototype.

Regards Andy
AFK

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Andy Rea
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Re: ZX81 ULA-in-a-CPLD

Post by Andy Rea »

nothing beats the feeling you get when a bodge comes together !

Introducing ... DeadBug sot-23-6

Image

big improvement on the video noise, but still have close to 50mv p-p noise ( spike every 1/2 cpu cycle ) , i want to retain the emmiter follower since as i have a 100ohm load on board, this give compatibility with low-z and high-z input monitor / tv making small improvements.

Thanks go to PokeMon and Claudius for hints and ideas.

i haven't had this much fun since.... the last ULA :lol:

regards Andy
AFK

McKlaud
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Location: St Albans, UK

Re: ZX81 ULA-in-a-CPLD

Post by McKlaud »

"Dead Cockroach Mark II" :lol:
Claudius
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BarryN
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Re: ZX81 ULA-in-a-CPLD

Post by BarryN »

Is slow mode working yet?

McKlaud
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Location: St Albans, UK

Re: ZX81 ULA-in-a-CPLD

Post by McKlaud »

BarryN wrote:
Wed Mar 28, 2018 6:27 pm
Is slow mode working yet?
In my ULA, still work in progress. Unfortunately I had to pack my toys and locked them up for a few weeks. :(

@Andy: Are spikes in your case similar to these in red circlers in the attached picture?
Attachments
zULA_video_out_noise.png
Claudius
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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: ZX81 ULA-in-a-CPLD

Post by 1024MAK »

Andy Rea wrote:
Tue Mar 27, 2018 11:37 pm
nothing beats the feeling you get when a bodge comes together !

Introducing ... DeadBug sot-23-6

Image
In the best Sinclair tradition :lol:

Keep the bodging ticking over guys 8-)

Mark

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Andy Rea
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Re: ZX81 ULA-in-a-CPLD

Post by Andy Rea »

McKlaud wrote:
Wed Mar 28, 2018 6:37 pm


@Andy: Are spikes in your case similar to these in red circlers in the attached picture?
similar as in they are spikes... but way more frequent, i can tell you exactly when, every 1/2 clock cycle
AFK

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Andy Rea
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Re: ZX81 ULA-in-a-CPLD

Post by Andy Rea »

all the following are video signal, probe and ground at the ula pins, noise is worse at modulator socket.

this is white video x10 probe so noise is around 80mv p-p
Image
this is a vertical row of inverse spaces rest of the screen is white, the burst of noise at the left coincides with the force nop

Image

lookd ok as a bigger picture levels are close to perfect but still get that horrible verticle banding,
Image

forwards.... its the only way to go
AFK

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