20Mhz ZX81 clone...

[Andy Rea]

Well the board is now populated, mostly everything is working, implemented wilf rigter Why wait, increasing speed a little more go the AY sorted with auto wait state insertion if running in turbo mode, so far my tests with the sound chip have been successful and i have not heard any playback faults. gone with the mic/ear on one stereo socket, and the AY sound on the other stereo socket. got an annoting buzz on the audio (not very loud but you can hear it when no sound from the ay is playing) i think it's probably cause by the switching noise on the main 5V rail, will try with a separate regulated supply tomorrow.

Andy

[1024MAK]

The noise on the audio from the AY sound chip could also be ground line noise.
If possible, take the ground line for the output connector (and any components that connect between the audio and ground) from the ground pin of the AY sound chip.

Also use a filter circuit for the +5V supply to the AY sound chip. I would try a series 47R or 100R resistor followed by 100uF and 100nF decoupling capacitors (which should be as close as possible to the supply pins of the AY sound chip).

Try to route the audio (and audio ground) signals away from any fast switching digital circuits (including the traces/tracks or wires)

Mark

[sirmorris]

by Andy Rea » Sun Sep 30, 2012 3:49 am

Picture of Andy at the time of posting the above:

I think someone was in the zone..!

[PokeMon]

Well sometimes I like to work in the night, too.
At 2 am or 3 am you can concentrate, no email, no ringing and only shit in TV.

[Andy Rea]

seems like i got it working and then posted nothing more typical of me, no documentation or anything. anyway

it has 20Mhz Z80
128K static ram ( split into 8K pages )
the full 64K can be RAM
0-16K is fully pageable
16-48K is always pages 2,3,4,5
48-64K is fully pageable ( but during video pages 2,3 )

other feature include
turbo (20Mhz)
8-16K ram wrx/udg switchable
m1not switchable

ZxPand-a-like (very very alike Thanks SirMorris) possible to page zxpand rom (once loaded to ram by special boot program) in and out

erm think thats about it... oh runs Sponzy slightly faster than original 48K spectrum !

regards Andy

[1024MAK]

Does it still exist?

If yes, pictures please

Mark

[Andy Rea]

You bet it still exsists, it's my goto Zeddy it's not pretty, but hey it works.

Top left clock wise, Z80, 40Mhz osc, 128K ram, 8K eeprom, 2 x AY sound chips piggy backed, just to the right ( tiny chip ) audio op-amp, then PIC for ZxPand, jtag header and finally CPLD on separate breakout board


Spaghetti underside


side view


other side (reset switch)


zxpand LED above keyboard


Regards Andy

[RWAP]

What ever happened to this project - would love to see it available - even if made possible via the ZXMore....

[PokeMon]

There is a chance as I made some modifications to the second revision of ZXmore having two different clocks but this has to be tested first. If it is working, the new one will work with 8 MHz instead of just 6.5 MHz and option to be increased later. So never give up hope.

[commie]

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 6.5MHz and back again.

Cheers for now
Commie