6502.org

Hello! I'm just in the process of building my very first home brew 6502 computer as a weekend project. I've joined this forum as I'm sure I'm gonna need your help.

One thing I want to do is access a bank of memory alternately between the CPU and a video subsystem so my first question to all those smart people out there is can I just use the low side of PHI2 (I'm using the WDC65C02 by the way).


My project's not very far along at the moment but I have it running some code.

Hi DigitalDunc,

My hardware chops aren't in the same league as many of the other members, but I can tell you that I believe that a certain Mr. Wozniak was doing something quite similar to what you describe in his Apple 2 design, with simple TTL. I'm a bit link-challenged at the moment, but I'm sure there are hundreds of detailed descriptions available on-line, and only a quick search string away! Wozniak's design is at least 38 years old, but offers a surprising amount of functionality, and could be used as a starting point for your own design.

Best of luck,

Mike B.

Welcome, DigitalDunc. I suspect you already know the answer to the question is yes. [Also, Mike just let the cat out of the bag with his post!] However, the hardware skills to implement such a video system are non-trivial. Would you describe yourself as knowledgeable in regard to circuit design & debug? If not, it might be best to shelve the video idea until you've acquired more experience -- learn to walk before you run, and all that. But it's a terrific goal to keep in mind for later.

Can you tell us more about your present experience level, and about your home brew 6502 computer? We'd love to hear more.

cheers,
Jeff

I've never actually built a full video generator before though I did write some Verilog to generate synch pulses and pixel clock a few years ago. There's a start. I've been building electronic doodads and thingummyjigs since I was small ( about 7 ). However I've no formal training. I intend to post pictures when I figure out how to post them in this forum.

Hmmm (scratches head), let me see if this works....20150525_143058.jpg

Ok if this worked then my first ever attempt at a computer (part built) should show up, if not then cue face palm.

OK, great! -- in fact, people that "just grew up building stuff" are often highly talented, IMO.

If you've tangled with Sync pulses then you have some idea how fussy the timing requirements for video are. The other big challenge is generating the sequential addresses, and arranging that they (as well as CPU addresses) can drive the address inputs of the RAM.

When you post your images, be aware you can attach them to your post. IOW, it's unnecessary (actually undesirable) to host them on a third-party server. [Edit: ok, you're one step ahead of me! ]

-- Jeff

Nice looking computer Digitaldunc! Welcome.

One thing I should have done is give my little tower of eightness a name.

Oh and I've been drawing inspiration from the Acorn Electron, Oric 1 and now very recently the Apple II.

Take a look at my SBC-3. I used the PHI2 low side to read the RAM for the video. I used a CPLD to generate the bus timing and video signals.
http://sbc.rictor.org/info3.html

The video was inspired by this design: http://elm-chan.org/works/crtc/report.html

I used the 65816 to allow for more memory access, but you could do this with a 65C02 and some bank switching hardware.

Daryl

Heck, "Tower of Eightness" isn't bad. And, as an acronym, that becomes TOE -- which is entirely fitting, since your project is digital, and toes are digits!

J.

Nice logic Jeff!

I'd just like to say, you guys are inspiring! I did take a look at the SBC-3 and related links and that gave me a further idea. Because I have some dead time in the Borders there's no reason why the memory bandwidth there couldn't be used... But for what. Ahem, I may be getting a little carried away right now. Just getting a picture should be my priority goal and a serial port will be a quick weekend project.

I've got some catching up to make my homebrew PCB's as pretty as the SBC-3.

There are certainly possibilities, but I have to agree with Jeff's first comment. Start small and built it up a piece at a time. You'll have better chances for success that way.

Good luck!

Daryl

My recommendation as well. It'll be easier to debug the hardware and the initial firmware needed to make it run won't have to be as elaborate.

Well, last night I spent a little time making a serial port card for my machine and today I'll try to get hello world out of it. Fingers crossed. I did go the easy route and used a W65C51N so it wouldn't make my life hard.