[90] ROMs

[AndyB]

Can anyone recommend a small (say, 128 -> 512 byte) PROM of some kind that has tri-state output suitable for shoving on a 6502 bus? Ideally I'm looking for something that's easy to program.

[trmelton9]

Jameco Electronics has 74S287 (256 x & 74S472 (512 x PROMS in stock. I'm not sure about programming them. The only tim I used them was for address decoders, many years ago. I had a local electronics destributor program them.

Have you thought of using a serial EEPROM like a 24C02. They're much cheaper & easy to program. You could operate it off of a 6522 or whatever.
Ted Melton

[GARTHWILSON]

Is there any reason it needs to be so small? If you can use a bigger one, availability and price will be better, and there will probably be more programmers that can handle it. Just because you use a bigger one (say, an 8Kx8 27C64)doesn't mean you have to use all of it. You could still have it take only 512 addresses if you wish and leave the rest unused.

[AndyB]

Hello Ted!

TM> Jameco Electronics has 74S287 (256 x & 74S472 (512 x PROMS in stock. I'm not sure about programming them.

Thanks, I'll look those up.

TM> Have you thought of using a serial EEPROM like a 24C02. They're much cheaper & easy to program. You could operate it off of a 6522 or whatever.

I probably couldn't boot from it though, which is what I had in mind.

Regards,

- Andy.

[AndyB]

Hello Garth!

GW> Is there any reason it needs to be so small? If you

GW> can use a bigger one, availability and price will be

GW> better, and there will probably be more programmers

GW> that can handle it. Just because you use a bigger one

GW> (say, an 8Kx8 27C64) doesn't mean you have to use all

GW> of it. You could still have it take only 512 addresses

GW> if you wish and leave the rest unused.

I might end up doing that, (hopefully with something smaller than 8Kb though). It would certainly use up more boardacreage, but I guess I could divide it up into small blocksand use DIP switches to select between them. Thanks for the suggestion.

Regards,
- Andy.

[trmelton9]

Why do you want such a small ROM to boot from ? The size you asked about are in a 16 pin DIP, a 2716 (2K x or 2732 (4K x are in a 24 pin DIP. That's about the shallest for an EPROM. The ones in a 16 pin DIP are OTP (One Time Programmable), a little expensive for developement.
Ted Melton

[AndyB]

Hello Ted!

TM> Why do you want such a small ROM to boot from ?

I should be able to code an appropriate bootstrap in just a few bytes. Digging through my notes though, I'm thinking a
2816 would be appropriate. At 2Kb it's small enough to fitin the 8Kb address range of the 6504 with plenty of room to
spare for RAM and I/O. It's an EEPROM and I think it can be programmed with just a 5V supply. Its pinout is very similar
to that of the 6116 SRAM or 2716 EPROM. It sounds like just the job.

- Andy.

[saipan59]

Yes, I think EEPROM is the way to go.
I've used 2Kx8 devices (like 2816) a couple of times, works great.
For programming, you just treat it like RAM - write a byte to it, then leave it alone for the min amount of time - that's it.

I found several 2816's inside some old modems...

Pete

[GARTHWILSON]

It sounds like board real estate is the only consideration, and that's only if you're putting the parts shoulder-to-shoulder already such that you can't see any board between them. According to the pictures on 6502.org, it looks like very few people put them that close together.

About the address space taken up though, an 8Kx8 can fit in the 512 address spaces if you just ground the highest four address lnputs. As far as cost goes, I designed a product in '94 that had the option of 2Kx8 SRAMS or 8Kx8 just in case the 2Kx8's became hard to get later. As it turned out, the 8Kx8's were already cheaper, so that's what's flying (literally).

Back on board real estate: If you do decide you want to select between blocks on the EPROM, using pin headers with shorting blocks makes for a more compact, more reliable solution than DIP switches.

Someone suggested using a 24cXX serial EEPROM. I used a 24256 32Kx8 in an 8-pin DIP last year and had a devil of a time getting it going. The problem turned out to be that in my "building block" routines, just to be safe, everything was preceeded with a busy poll, which fowls up the address after you set that. After the device is determined to be ready, set the address and then do what you were going to do, without checking again to see if it's busy.