ZX81 ULA-in-a-CPLD

[gozzo]

whoah!!!!

[1024MAK]

I don't know, you wait ages for a ULA replacement to come along, then two turn up following one another!

Well done to both of you

Mark

[McKlaud]

Mark,

Andy is miles ahead of me. His ULA is working perfectly fine My version is nearly there.

[Andy Rea]

Mark,

Andy is miles ahead of me. His ULA is working perfectly fine My version is nearly there.

Not really miles ahead... I need to sort out tape input, i guess i have some experience because I already did some ula's before. The video out has some noise in it, which needs addressing.

Anyway what problems are you having ? Pretty sure myself or mark can help out.

[McKlaud]

It is look like that I should assemble the board again, because in this one I've got some synch issues. When I press the board in the socket hard I am getting the "K" prompt on the screen. There is some noise on the video out too. I've been thinking about different type of a NPN transistor there. At the moment I use BC847B one.

What about the tape input? Maybe a bias would help.

[Andy Rea]

Ah yes i have a motherboard like that, pretty sure a trace is broken somewhere but try as I might i can't find it so it only works when pressure is applied at just the right spot...

Yes bias is required I have provisioned on the PCB a spot for 2 resistors one to rail the other to ground currently they are unpopulated as i was more interested in getting other things working first. I also use npn transistor as emmiter follower bc817 gain is a non issue in emmiter follower mode but current handling can be because of the relatively low load ( 75ohm ) input of most composite inputs, I also have a 1k load on the board so that high impedance composite input monitors can be used without additional work ( a lesson from my altera based solution ) I have a 0.1uf cap right next to the transistor between collector (+5 ) and ground. I was thinking about either a small smd inductor immediately before the collector. I also put an extra 33pf between base and ground this helps a little.

[superfo]

Nice work for both, congratulation !

Now I have to dig out my board and continue working on it

viewtopic.php?f=6&t=445&p=3796&hilit=superfo#p3796

[McKlaud]

Superfo: welcome to the club

Andy:

The small smd inductor (e.g. 1uH) and than a 0.1uF capacitor between supply rail and GND might be a good idea. I will give it a try.

[Andy Rea]

Mine has had a bit of a torture test today, had to go out to see my mum of course left the house at 10.30am and also left a test zeddy with my ula installed running maxdemo... Got back at 6.15pm ( 7hours 45minutes later ) and......

It's still running !

I'm gonna pat myself on the back and call that stable...

Now then where did I leave the trumpet?

Andy

[McKlaud]

Congratulation Andy!!!

My ULA was also tortured and I did a few minor mods earlier today. Now Zeddy boots up every time to the 'K' prompt. Screen and border inversion works as it should. Anyway, still works in progress.