6502.org

Ok, I'm not really ready to tackle a video system for my build, but I do want to study the subject in advance preparation.

First question, what format should I study? I presume the two likely choices are composite and VGA outputs. I would lean towards VGA, because I have a ready source of free LCD monitors that support that.

I know zilch about VGA from a hardware standpoint, and only the barest about composite.

I do want to do some sort of color graphics down the road.

Also, concerning character fonts. I assume there are bitmap collections out there that I can download so I don't have to create my own from scratch.

I wold love to do this from scratch, but I know folks are doing this sorta thing with microcontrollers, and I am open to that notion. Call it a strong option B. But I love the idea of doing this the old fashioned way. My end goal is education, and feeling of accomplishment, so I'm fine with some difficulty/early failure to do this from scratch.

I do have one random book on the matter, "The Cheap Video Cookbook" by Don Lancaster. Looks to have been published in '78.

Anyway, any thoughts?

That book seems to have quite a following. I haven't seen it, but I gather that it's a very good book. The last half of the displays page in the 6502 primer has a lot of suggestions for video. One forum topic you'll probably like (out of possibly quite a few) is "Building Don Lancaster's TVT-6."

If you look into it, it's quite remarkable how much it took to drive the video of the older classic '02 machines, both in terms of just raw memory, and CPU bandwidth. And those were just pushing 40 bytes per line. There was a lot of cycle stealing going on out there as well.

There's a better chance with modern CPUs at higher clock rates, but at the 1MHz-2MHz area, the CPU is quite busy just keeping the screens up.

Well, how many square feet of breadboards do you have ?

Part of me feels like it is cheating, but for my build I'm looking at using this:

http://www.batsocks.co.uk/products/Othe ... 20Tiny.htm

I have two on order. I plan to have a dedicated serial port (separate from the "console"). The programmable fonts is cool. (It is done differently depending on how much horsepower the processor used for the TellyMate has.) It is tiny, cheap, and easy to interface. Hopefully I'll have one on the bench in a week or two to tinker with.

Thanks,
Jim W4JBM

A product that I am hoping to soon have assembled and ready for test is Geoff Graham's ASCII terminal. If I'm satisfied with the results it will be integrated into POC V3 and become the console. All that is needed to make it work, other than power, is a standard PC keyboard and a standard VGA monitor.

If you don't mind something modern, the Parallax Propeller chip makes a good video chip for composite or VGA video, and there's plenty of firmware available so no programming is required. It's used by many homebrew computer enthusiasts. For example the RetroBrew I/O board uses it for their CP/M system, Vince Briel uses it in his PockeTerm, and Jac Goudsmit uses it in his L-Star.

Summed up, there are several ways to skin this cat without having to use a pile of 74xx logic devices or laboriously work out how to generate an accurate VGA signal.

I'm not opposed to a modern solution like the propeller to get things going, but honestly, (laboriously working out VGA, or a pile of 7400 logic actually sounds like fun long term! You've definitely given me some roads to explore, which was my goal with the question!

Don't get me wrong. I revel in complexity and I generally don't take the expedient way out with this stuff (you should see my large-scale model locomotive). However, there are only so many hours in the day and at the end of the metaphoric day I like to have working hardware. That's why I would never considered developing a roll-your-own video output. Doing so will consume a huge amount of time that will not be justified by the result.

I totally get that, and will probably go a less "roll yer own" route early on. But this whole build is basically a "see if I can" type of thing. It would be pretty rad to see a screen image I concocted on my own. This is a "down the road" goal though.

I have a couple of 15cm (6") color graphic LCDs I'll give to anyone who can make use of them. They're roughly the size of a 3.5" floppy-disc drive. They're 20 years old but have never been used, and one still has the protective film over the viewable portion. They're Sharp model LQ6NC02, serial numbers 380541 and 423388. Each has a dozen tiny trimmers on the back for things like tint, brightness, contrast, horiz pos, etc.. They need a flex-circuit ribbon connector which I have only one of, which says on it, "AWM STYLE 20566 105°C, PARLEX CORP. 2193". They're about .9" thick over most of the area, and 1.25" at the thickest point where there's a small extra board on the back which I assume is a power supply for the back lighting which appears to be tiny fluorescent tubes. I hope someone can make good use of these. Someone gave them to me several years ago but I'll never use them.

There are also a lot of color & B&W graphic LCD modules of different sizes and resolutions on the market, many using a parallel or SPI interface so you could write your own driver software and not need separate video hardware. They take care of the strobing internally plus various features like, scrolling and character generation. I don't know if any have sprites.

Ok, yer idea of an SPI driven display is intriguing. I'd have to get SPI going on my build, but that should be not to difficult from what I've been reading. I could probably bit bang that. Done it before on MCU's

Hmmm. As for the screens you have, if no one else pipes in, I'm interested, but I'd rather wait and see if someone has a specific need before I take them just as a whim.

The graphic LCD I show about 40% of the way down the page at http://wilsonminesco.com/6502primer/displays.html is a small 64x128 SPI-interfaced one from Displaytech, which I bit-banged on my 5MHz 65c02 workbench computer in the short video demo there. There's a link to my code there, which is partly in assembly language and partly in Forth. (The assembly is what's in the Forth primitives.) I might go for a bigger one, like 128x256. I probably won't go any bigger. My interest is in technical workbench stuff like oscillograms and spectrum analysis, not games or flowery consumer-type stuff. I'll still mostly use an intelligent character (text-only) display for text, although the graphics LCD can do text as well (just at a higher software overhead cost).

Good call. It's probably best not to attempt everything all at once.

It's true, as BDD says, that there are only so many hours in a day. OTOH video may be the toy with which you choose to fill your play time!

Perhaps your first build should have no video at all. Instead it could rely on serial I/O to an external terminal or terminal emulator. Then later you can consider rolling your own video.

I won't attempt to steer you toward or away from Don Lancaster's Cheap Video approach, but certainly it can be a fun challenge and good learning experience. The essence of the idea is tricking the microprocessor into behaving like a 16-bit counter for generating sequential memory addresses -- and within that context a great deal of variation is possible. I've implemented Cheap Video three times, and you'll find some of the details here.

ETA: SPI to an LCD with its own controller will bypass the rather tricky timing requirements posed by Cheap Video. But managing timing challenges is a skill that can be valuable elsewhere. OTOH learning about SPI is valuable, too.

-- Jeff