ZX81 ULA-in-a-CPLD

[McKlaud]

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

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

[PokeMon]

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.

[Andy Rea]

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 )

this is my test zeddy most of the passives have been moved to the back side, and no ear or mic circuit exsists


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.

speedtest reports exact number of frames which is always a good start.

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.

activate special feature... M1NOT yes please


regards Andy

[1024MAK]

@Andy - naughty but nice

Mark

[McKlaud]

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?

[Andy Rea]

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.

[nollkolltroll]

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!

[Andy Rea]

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

[Andy Rea]

some time spent redesigning the PCB and will order some of our favorite purple prototypes for further testing.

So some design outlines,,

Composite 1v p-p video output with back porch, shorting link to select hi-z or 75ohm output. most monitors / composite inputs are 75ohm and will work fine with link removed, hi-z inputs will need link shorting ( it adds a 75ohm load to video circuit )

outline size no bigger than 40pin dip socket slightly higher than original ULA

on board xtal oscillator for accurate frequency control.

original osc pin converted to optional input used to enable M1not.

Tape input pre-amp included for improved tape loading.

Regards Andy

[1024MAK]

Mark