6502.org

Hi, i'm building a 6502 system with a 5 volt supply. The 6502s I have are all specified to run at 5 volts like usual, but then I have these unusual M27V512 EPROM memory chips that run at 3.3 volts. I have not yet built the circuitry, so I'm wondering if the EPROMs I have (M27V512) are able to operate in read mode under a supply voltage of 5 volts, and that I can integrate it into my system just like a M27C512 (5 volt 32kbyte eprom). The specifications do state that the M27V512 will operate within a vcc range of -2 to 7 volts, but I dunno if it'll be likely that i'll be able to read data off the ERPOM with vcc=5v.

thanks!

The data sheet says the operating voltage is 3.3V ±10%, and higher voltages are only for programming. What's not clear is what would happen if you tried to run it full-time at the higher voltage. I suspect the current would be much higher, and you might slowly go erasing your part or making all bits 0's. Why not just use a 27C512? (These are 64KB EPROMs BTW, not 32KB.)

So I got curious....

I think there's a number of bad things that could happen. My gut feel is that it's probably not going to be a happy thing to treat a 3.3v part as a 5v part.

If you can't find something similar that's 5v (Remember, if you want a 32k ROM and can only find a 64k one, you can always just ground the topmost address pin and not worry overmuch about it) there's actually a proper process to swap stuff over.

A nice, technical piece is at http://focus.ti.com/docs/logic/catalog/ ... e=scea035a. I don't understand half of it myself, but it looks like what you want to do is to have a 5v -> 3.3v and 3.3v -> 5v level translator. There's more stuff at http://www.fairchildsemi.com/an/AN/AN-314.pdf

A 74HCTxx or 74ACTxx buffer will accept 3.3v outputs and convert them to 5v. I'm getting the vauge impression that some buffers may be "overvoltage-tollerant" devices where they can handle 5v on the input... I'm thinking you want a 74AHCxx device. So an 74HCT244 or 74HCT244 to go from 5v down to 3.3v, and a 74AHC244 to bring it back up to 5v.

Remember to take the propigation delay of the two buffers into account to make sure you can still drive it at the proper clock rate.