Sinclair ZX80 / ZX81 / Z88 Forums

(..) and a lot of the cutting edge research all revolves around 8 bit technology. Don't know where you get that idea... maybe there's some interesting projects that happen to use 8 bit CPU's (easier to construct?), but other than that you might as well say that a lot of cutting edge research revolv...

Then i started poking around with the scope and when monitoring ROMCS (rom side of R28) and suddenly 1k hi-rez was working, take the probe away and it goes back to garbage. Nothing strange there, Andy: the scope probe will add some capacitance to the signal. In combination with the series resistor ...

Ah I see... :idea: that would mean /ROMCS is only extended in forced-NOP cycles, and not in regular ROM reads / instruction fetches? If so, I'd consider changing my design accordingly. But perhaps I'll just leave this as it is, as long as there's no overlap with /RAMCS there's no problem, right? Ano...

No analog delays here, as I wrote I did quite some cleanup! /ROMCS is done as follows: Goes low with /MREQ low and A14 low. But when /MREQ returns high, /ROMCS follows at next low-to-high transition of CPU clock . In a forced NOP cycle, that is end of T4 / start of T1 in next instruction cycle, and ...

Excellent progress, i don't understand why your 'stretching' the /ROMCS (my scope shows /romcs switches approx 25ns after /mreq) When /MREQ returns high, pattern data isn't yet clocked into video shift register - that's why /ROMCS extends some beyond /MREQ going high (check Grant's scope pix). I'm ...

Things are nicely progressing here! I've changed my hardware setup a bit such that all equipment (keyboard / TV / development PC / programming cable) is connected at all times, this enables much quicker design -> test cycles. Did significant cleanup: Got rid of all analog timing-related parts (RC de...

In case of doubt: increase # of copies (and indeed, try to store physically apart, if possible in different buildings). Disk space (read: drives) is too cheap these days compared to the headaches of losing a lot of important / irreplaceable data (family photo's, self-developed software, etc). Person...

I counted the pins, and there aren't enough IOs with the 44-pin Xilinx devices. Nope, only 34 I/O's on those. The alternative was to off-load the keyboard read to a seperate chip (as on the ZX80), and just have a kb read enable o/p on the CPLD. This would bring it within the 34 IOs but would need 2...

Yeah, I've been looking (glazed over a bit) at bodo's sourcecode. Yeah, couldn't make much sense of it either... ;) then again, I've started reading snippets of Verilog & VHDL here & there, but not really dived into it. The small projects I've done so far, were done using schematic entry. Since you...

You may be interested to know that a circuit exists for the ZX Spectrum to increase the number of usable I/O ports to 128. Not really... I have designed & built many logic circuits like these in the past, so I'd know what to do & how to do it, no need for an example. In my current design, such a ch...