A Diversion: EEPROM Programming

A project log for Lacey Mk I

A Homebrew 68k Computer for Shits & Giggles

matt-laceyMatt Lacey 05/21/2017 at 04:430 Comments

Having got the CPU freerunning, I was starting to look at what I needed for DTACK etc. and figured that I should really switch to a 5V CPLD, so I ordered one, and have been waiting for several weeks now for a breadboard compatible socket to arrive... ho hum.

In the mean time I decided to look into what I needed in terms of ROM, and ordered a couple of AT28C256 chips from AliExpress, along with a cheapo programmer. Unfortunately when those arrived I discovered I'd been neglectful and the two weren't compatible! I started to look around for a programmer but they're not cheap, and I quickly came across information from Quinn Dunki and others regarding building their own EEPROM programmers. I had an Arduino Uno laying around so I got stuck in. It didn't take long to get my head around using shift registers, but no matter what I tried I couldn't read anything but 0 back from the EEPROM. A couple of weeks later I finally posted on the electronics StackExchange and someone suggested that my data lines were still being driven by a shift register when I was trying to read. I double checked the wiring and sure enough, I'd accidentally grounded the OE (output enable) pin on the data shift register instead of one of the address ones. As soon as that was resolved writing was no problem at all.

A schematic will be forthcoming, and I'm toying with getting this made into an Arduino shield to make things easier down the line. Eventually I think I'll steal Quinn's ideas wholesale and stick a microcontroller into the computer so that EEPROM programming can be performed on board.