picoprom VS nanoprom

The details are on GitHub here: https://github.com/gfoot/picoprom

You might be interested in my NanoProm project: https://github.com/gfoot/nanoprom - or the Pico equivalent which is in github too. They're for programming EEPROMs without glue logic, feel free to use any of the code that you find useful.

It is actually possible to program the Atmel ones with a regular Arduino, or even a Nano, without needing shift registers, because the address pins are latched at the start of the write operation. This is how my NanoProm worked: https://github.com/gfoot/nanoprom

If you remove the shift registers and connect the pins directly to the EEPROM then it can be done just using PORT writes. This worked well for me in NanoPROM, see here: https://github.com/gfoot/nanoprom

I have an alternative setup here for Nano and Uno that you might want to try - it deals with SDP, writes much more quickly overall, and doesn't require external components: https://github.com/gfoot/nanoprom

I also made an Arduino Nano version here: https://github.com/gfoot/nanoprom

If you're interested, I have code here that does all the page writes, unlock codes, etc correctly: https://github.com/gfoot/nanoprom It only supports writing, not reading, and doesn't use shift registers like yours does, so you can't just switch to it, but maybe seeing the code can help with the timing issues.