Hey everyone- just thought I'd update this to close out the thread. After many hours, I finally got this working. It turns out, in fact, all four of the EEPROMs in the first batch I purchased were bad. Two new ones purchased from another source worked perfectly with my existing circuits. Just goes ...

If these are new devices then the software protection is not the issue.
The datasheet page 4 states...

"The SDP feature may be enabled or disabled by the user; the AT28C256 is
shipped from Atmel with SDP disabled."

In theory, yep, I'm right there with you. Attempting to disable SDP is indeed the ...

Great suggestions, everyone! I think my next step (while waiting for additional chips) is to try and get an ATTiny to bang bits fast enough to meet the page-write spec, and thus write the unlock sequence. I've tried doing it byte-by-byte, both by hand and via the uC, but it seemed to have no effect ...

One thing to note: you're supposed to read back the complement of the last byte you wrote, until the internal read operation is finished. Did you wait long enough (10milliseconds)? (Put another way: if you write any other value, do you get back the complement?)

(Oh, and welcome!)

Cheers
Ed ...

The data sheet says the software data protection comes disabled. I have had at least one eeprom (can't remember the manufactureer) come with the protection enabled.


Yah, that's kinda what I'm afraid of. Could I have four chips all with it enabled? Perhaps. I tried writing the unlock sequence ...

One thing that may help out, look for some Atari arcade game schematics such as Tetris which was 6502 based and used an EEPROM for data storage. So it definitely CAN be done.

Thanks, that's an interesting idea!

Just in case it wasn't clear, the 6502 isn't doing the actual writing. I'm just using ...

Hey everyone! Garth sent me over here, and indicated you folks might be able to help me with something.

I've hit a wall on my "Veronica" 6502 project ( http://quinndunki.com/blondihacks/?p=780 ) , and no amount of googling or researching or experimenting is helping anymore.

I have an Atmel ...