6502.org

Hi,

The idea of building a basic 8 bit computer has piqued by interest for quite some time. I'm on the fence on what processor to use. Either the 6502 or the Z80? From my perspective, I'm looking at which would be the most beginner friendly to use and not necessarily looking at which one is faster etc.

I've done research on both of them and still unsure of which one to use.

I'm open to other options for CPU's around the same era. But it seems that the Z80 and 6502 seem the most prevalent among homebrewers.

Hi qwertykeyboard and welcome!

Asking within a 6502 forum whether to start with a Z-80 or a 6502 is funny. Of course a 6502, what else

To be honest, both processors are fine and equal easy or difficult to deal with depending on your skills to use. The question is: can you manage it to build a small computer by your own plus a couple of datasheets or is it likely that you need some assistance?

If you browse through this forum you will see many threads of that kind. People stuck at some point and then ask for (and usually get) help. If you know a Z-80 forum that is vivid like this you can throw a coin to make the decision. If not, well then try the 6502

Regardless how your decision will be: studying Garth Wilsons Primer is always a good starting point!


Cheers
Arne

edit(1) corrected link - sorry

Of course!
Thanks for the plug. The index page for the primer itself is http://wilsonminesco.com/6502primer/ . The 6502 may well be the most documented processor in history.

Does it matter if I go with a 6502 or a 65c02?

Please do yourself a favor and go with the CMOS version, ie, the 65c02. The long list of improvements is summarized at http://wilsonminesco.com/NMOS-CMOSdif/ .

I've also read here:
viewtopic.php?f=1&t=2744

to avoid the WDC version of the 65c02, because it is less tolerant of bad construction techniques. But the problem is that the WDC version is the only one that is still being produced. Where can I find a NOS 65c02?

There are many fine working boards using the WDC chip. It requires some attention as it is a very fast chip. But there are CMOS versions from Rockwell or GTE. As these chips are older (say 1990 or so) they are slower (<= 4 MHz non overclocked). They are easy to use.

As Garth noted, the 65C02 corrects a number of bugs in the NMOS 6502, plus adds useful new instructions. The NMOS 6502 has weak output drive compared to the 65C02, which can actually complicate your design.
The WDC 65C02 produces fast output edges that can give you grief if your construction methods are sloppy. Otherwise, they're no more difficult to use than the older 65C02s produced by Rockwell et al. If you want to use a non-WDC 65C02 you may be able to get one through Jameco Electronics or someone on eBay. Be aware, however, the some eBay sellers are peddling counterfeit devices. As always, caveat emptor!

Could the 6809 be a viable option as well?

Well, it's not a member of the 6502 family. Support may be a problem, not to mention acquiring one...I don't know how many are still around. It's very old silicon.

Welcome qwertykeyboard! If you don't already have a 6502 then a WDC 65C02 is probably easiest to find, and if you take care with construction and get help when necessary you can surely make a working system.

If you feel nervous about that, then any other 65C02 or an NMOS 6502 will be fine - in either case, you're making a tradeoff of advantages. Your first build is probably not going to be your last build, so don't worry about making tradeoffs just once.

As noted upthread, any other 8 bit CPU is also a fine choice, but you won't find quite as much help here for obvious reasons. There might be other places which are equally helpful and experienced, but you'd need to find them. (retrobrewcomputers.org and anycpu.org spring to mind.)

Don't be too nervous of making optimal decisions: whatever you do, there will be some debug investigations, and some compromises. For reasons of progress, and motivation, don't try for a long feature list for your first build. Building an existing design might well be the most time-efficient way to proceed: build your own design after that. Grant Searle's designs spring to mind, but there are several homebrew projects listed here:
http://6502.org/homebuilt

The 6809 is another old NMOS chip with the same circuit-construction problems as an NMOS 6502. It was reasonably powerful in its time, somewhere midway between the 6502 and the 68000.

However, there is a compatible CMOS successor in the Hitachi 6309. It's available in two major variants: one with an internal oscillator circuit to which you only need to attach a ballasted crystal, and another (with an E suffix) to which you need to attach an external oscillator. I'd use the latter, since canned oscillators are easy to get. Like the 65c02, the 6309 has a number of enhancements over the 6809 - only more so; it's a far more capable CPU, which in some cases could outpace a 68000.

There are 6309Es for a few quid on eBay. Whether they're real remains to be seen. Look for the C speed grade if possible; officially they'll do 3.5 MHz, but supposedly they'll normally run fine at 5.

From what I can tell, the 6309 and 65c02 are similar enough in their bus interface, that much of the advice in building a 65c02 based machine will also apply to it. That's most likely because both inherited their bus signalling scheme from the Motorola 6800. The pinouts and some details of control signals will differ.

The 6809 is a nice processor but the 65C02 is a good place to start.

If you are in the UK (or Europe) I have spares of the WDC chips.

If I could throw my $0.02 USD in, here is why I suggest going with 6502 (65C02, actually):

1) This forum. You won't find a bunch of guys more dedicated to an 8-bit CPU than you will find here. Some of these guys actually still use 65C02's in their professional jobs. The knowledge and support is second to none.

2) Going with the modern 65C02 is a great first step to bigger projects. If you decide to branch out and build a 16-bit computer, the modern 65C816 will be a logical step. You will be familiar with the architecture and it has 6502 support built in.

3) Some of the best computers/consoles were built with the 6502 and it's variants. The Commodore 64, Apple II, Nintendo NES, Super Nintendo, etc. This means you have a VAST pool of programming experts even outside this forum. Some of the game coders over on Lemon64 and the VIC-20 forums are legendary IMHO.

4) There are some amazing console/computers built on the Z80 such as the ColecoVision, Master System and ZX Spectrum. However, I don't see a lot of new hardware coming out for those systems (well, the Coleco and ZX Spectrum being exceptions). But I seem to see new hardware coming out for 6502 systems all the time. New hardware for the VIC-20, C64, Apple II, etc. New hardware means new interest which means more support.

Those are some of the reasons why *I* personally prefer the 6502. At the end of the day, you can't go wrong with either the Z80 or the 6502....but those reasons above are why I would recommend the mighty 6502.

Thanks for the generous offer, but I'm based in Canada.