Yeah. Sorry about the confusion - I attached the old revision to give an idea of what's involved and what I changed.
GARTHWILSON wrote:
Never fear. The W65C02 is in current production. There's no need to depend on old, pulled 02's.
Using what people already have on hand is plenty of reason
I promise I've read everything you've published about the benefits of using a 65c02. I don't have a problem with supporting WDC, but creating a demand for new IC's when NOS and used IC's are perfectly fine is such a waste in my book. The whole point of this project is using old parts - "reduce, reuse, recycle". When it comes to reliability, speed, power draw, of course a WDC65C02S is better, but it seems I get a lot of masochistic joy out of having the same battles the homebrewers of the late 70's did.. Authentic NMOS pain
..and there's the price.
If I was trying to make a new PC as powerful as possible I probably wouldn't use a 6502 to begin with - and for MCU uses the WDC65C02 can't really compete with modern MCU's anyway. Bang vs $ wise. Not that it doesn't have some niche applications - it's still a contender in the "fewest transistors pr useful instruction" game.
It's funny where people draw the "nostalgia line" about what modern components are allowed and which are not. I try to make this SBC as close "it could conceptually have been made in the late 70's" as possible - even though I cheat with 74HC parts and surplus/used late '90's RAMs and ROMS and I certainly don't hand draw the circuits and take them to the printer. So... For nostalgia reasons it better fits to use old NMOS 6502's - even though it makes a few things harder. If there were still 1-8k ROMS available cheaply I would use them.
I have eliminated two out of four 74hc245's in my new revision with some careful attention to bus contention with a JnotK flip-flop for video sync generation. If I knew I could rely on only 65c02's I could eliminate the last two '245's since the BE pin on the c02 also tristates the address bus - unlike the '02 which always drives it making bus sharing a pain. But - it's just more fun this way.
And just so I don't forget: I owe you a beer, Garth.. or ten more likely! Thank you so much for all the help on your website and this forum! Did you realise your potpourri SBC has the same memory map as suggested in the MCS6500 programming manual on p146?
Back to the slow Ø2:
I don't think it's a defective pullup since it's a problem with all three NMOS 6502's I have on hand - all identical on the scope.. I guess it could be a defective 6522 pulling it down since that's the common denominator or maybe it's a supply issue and the 100nF + 220uF bulk caps aren't enough?
The mislabeled 65c02 I have has a rock solid rail to rail Ø2.
The input clock is already feeding as many things it possibly can but for the bus sharing I can't get it to work reliably without using Ø2/Ø1 timings.
I see some modern dedicated delay lines available but price seems prohibitive ... what's a good alternative? A monostable of some sort? Around 50 ns a 555 probably won't cut it?
For now the external pullup on Ø2 certainly seems like the cheapest solution and I can't think of ways it will damage the old 02's for certain. I hope I'm not wrong?