64KB 2865 eeprom

kernal34 · Post by **kernal34** » Fri Jul 28, 2006 12:16 am

I'm doing a project involving a 6502 and a 2865 eeprom. I only need an 8KB eeprom, but the only one I have available is a 64KB. Can I use the 64KB eeprom, but just use 13 of the 16 address lines to access the 8KB, and will this work fine, or do I have to tie all 16 address lines to the 6502?

Thanks in advance.

8BIT · Post by **8BIT** » Fri Jul 28, 2006 3:51 am

You can use any sized eeprom you want. Just tie the unused upper address lines to ground (logic 0). Be sure to set these same address lines to 0 when you program your eeprom to ensure your code gets read.

Daryl

blackadder · Post by **blackadder** » Fri Jul 28, 2006 5:34 pm

Depending on what you're using the EEPROM for you might also put dip switches or jumpers on the upper address lines (the switch grounds the line when closed, a pullup resistor pulls it high when open) allowing you to select which one of 8 different 8k blocks the computer boots up with.

kernal34 · Post by **kernal34** » Thu Aug 17, 2006 3:12 pm

Thanks a lot, this helps...

kc5tja · Post by **kc5tja** » Thu Aug 17, 2006 9:27 pm

And if you have pins left over on a VIA chip, you can use those to select which ROM bank to use, thus allowing software control over which ROM image to present. The Acorn uses this technique to get metric ***-loads[1] of functionality in its ROM, while still having a wealth of RAM available for user applications.

[1] Note to be confused with imperial ***-loads, which varies from country to country, and conversions between different kinds of loads are arbitrary and difficult to remember.