Building a 6502 based computer

Nameless Visitor · Post by **Nameless Visitor** » Wed Sep 20, 2006 1:33 am

I'm building a 6502 computer, and I'm still mainly looking for parts. I have the 6502 chip, but I need the other parts such as RAM, logic chips, and ROM.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... gory=26206

This LCD display was the simplest and cheapest I could find. I do not know if it could be used. The MICROCHIP 24LC64 seems like a good ROM chip, but I also do not know if that would work. For ram, I would like to know if any of these:
http://search.stores.ebay.com/Vintage-E ... 15QQsofpZ0

would work.

(sorry for the board stretch, URL bb code wasn't working for me)

GARTHWILSON · Post by **GARTHWILSON** » Wed Sep 20, 2006 4:52 am

The display is fine. I've used them many times. You can see a couple of them on my pages at http://www.6502.org/users/garth/projects.php?project=1 and http://www.6502.org/users/garth/projects.php?project=6 . However if you run the processor at more than a pokey 1MHz or so, you'll have to interface the display through an I/O IC like the 6522. Otherwise the display's own interface won't be fast enough. You really only need 6 lines to do it: 4 data, RS (register select), and E (enable). For this minimalist approach, you would just tie the R/W\ line to ground. Getting those things initialized reliably can be a bit of a trick though. Let me know if you need help.

The 24xx ROM is serial, so it cannot be read directly by the processor, let alone at the speed you'll need. You'll probably want a 27xx-family EPROM or 28xx-family EEPROM, and a way to program it.

For RAM, the 62xx family is common, inexpensive and will work well.

I think you might be making the parts acquisition more complicated than it needs to be. Mail-order places like Jameco, Digi-Key, and Mouser sell this stuff all the time at good prices.

Quote:

sorry for the board stretch, URL bb code wasn't working for me

Just uncheck the "Disable BBCode in this post" box under the small window you write your code in.

Nameless Visitor · Post by **Nameless Visitor** » Thu Sep 21, 2006 11:21 pm

Do you mind linking to some of those chips? (I'm really bad at finding things like that

) Probobly the lowest possible type of each of those would work (I'm building an extremely minimalist computer, single board, it will end up being about the size of a 3.5 inch floppy, with only whats neccesary)

Also, does anyone know of a compact, easy to use, 6502 compatible keyboard and the best way to supply five volts of power from a battery, thanks.

GARTHWILSON · Post by **GARTHWILSON** » Fri Sep 22, 2006 3:32 am

Ok, here's the 74HCT00. You'll need one for all the address decoding and glue logic, using the method here. This method allows you up to 16KB of RAM (using half a 32Kx8) and 32KB of EPROM (or vice-versa with a small change) and up to ten 65xx I/O ICs. One of the gates shown is an inverter. Just replace it with one of the NAND sections in the '00. I had a Schmitt-trigger inverter for other things, which is why I used it; but you can do the whole thing with a single quad NAND.

Here's an 8KB SRAM (8Kx8), or if you prefer, a 32KB SRAM (32Kx8). These are 70ns which will be fast enough for getting over 5MHz (which you probably won't be doing initially).

Here's a 32KB EPROM (32Kx8). This is also 70ns. You could go a little slower on EPROM than RAM for the same system, but these are not really considered that fast anymore anyway. There's almost no point in going with an EPROM any smaller than 32Kx8 since the 8Kx8 and 16Kx8 are actually more expensive. You don't have to use the whole thing or even have it show up in the memory map. My EPROM eraser handles four EPROMs at a time, so I like to have at least 5 if I'm in that edit/program/try cycle so every time I need the next EPROM, the next one is ready. That, BTW, is a slow way to develop your program, but you can't do it in RAM until there's some working ROM code in the system. Sure there are simulators (all software) and emulators (part hardware-- you cannot E-mail or download an emulator!) but when you're starting out I think it's good to really get down to the metal. Programmers who start out with nice development software and equipment never seem to get as good a handle on the basics, and tend not to develop such careful programming practices.

kc5tja · Post by **kc5tja** » Fri Sep 22, 2006 2:16 pm

Actually, simulators do come in handy when you're still at the "blank sheet of paper" stage of designing your next computer. In my case, I wrote a simulator for the Kestrel 1 (which I should be publishing on my website this Saturday if I remember), primarily to test my 65816 emulator (itself built around the XGS processor emulation code). It was simple enough to do, though it is not quite cycle accurate. It also doesn't emulate it at real-speed (the emulator runs at full speed, which is, so far as I can tell, in the 45 - 55MHz range based on very informal measurements taken).

I wrote this simulator specifically so I can attempt to have two problems solved:

1) A body of system software that is "known good." Of course, it'll never truely be known good until it runs on raw metal. But it will be "good enough" to allow me to do basic hardware debugging.

2) To figure out exactly what I want to appear in the Kestrel 2 before I plunk down the money to purchase the hardware (although those Xess boards are NICE, they're damn expensive too, at $160 a pop!).

So, basically, I intend on using the simulator as a design test mule, trying and discarding ideas until I (and a few others who are interested in the Kestrel 2) are happy with it. Then it's time to actually procure real-world hardware.

Nameless Visitor · Post by **Nameless Visitor** » Sun Sep 24, 2006 10:16 pm

Thanks!

But do you know of a good EEPROM that I can use instead of that EPROM? I don't really want to use EPROM.

GARTHWILSON · Post by **GARTHWILSON** » Mon Sep 25, 2006 12:22 am

I don't have any experience with parallel EEPROMs, but I think the 28xx family is more or less the EEPROM equivalent of the 27xx family of EPROMs. You can get them from the same places.

coredump · Post by **coredump** » Tue Oct 03, 2006 2:17 pm

Nameless Visitor wrote:

Also, does anyone know of a compact, easy to use, 6502 compatible keyboard and the best way to supply five volts of power from a battery, thanks.

If you add an 8-bit output port, and an 8-bit input port, you can design a software-polled keyboard. That'll allow you to use any matrix layout you like. Or, you could attach the matrix part of a scrap PC keyboard. If you want to connect the keyboard via a smaller cable, you could devise a serial interface using a couple of shift registers (I've done this, and ended up with a 5-wire interface).

As for power, a 7805 regulator will be the easiest way to get up to one amp of power at 5V. But, it won't be very efficient -- the regulator will get rather hot. A more efficient (but more complex) way would be to use a switch-mode or "buck" regulator.

smilingphoenix · Post by **smilingphoenix** » Sat Dec 23, 2006 12:52 pm

By far the easiest way to get a 5volt supply is to use 4 NiCad or NiMh batterys. - This gives 4.8V which is well within the tollerance limits of the 65xx devices.