collapseos/doc/hw/z80/sms/at28.txt

# Writing to a AT28 from a SMS

Writing on the EEPROM that is currently running Collapse OS is
as easy as enabling the WE pin on your hacked up cartridge. How-
ever, this is not practical: If you want to deploy Collapse OS
(or something else) to another machine, or even if you want to
upgrade your current Collapse OS, you will likely want to write
to another EEPROM.

The easiest way to do so is to build yourself a dual EEPROM
cartridge. It's very similar to a simple cartridge, except it
has two AT28 sockets and a '139 decoder to select between the
two.

The design proposed here sacrifices access to the upper 16K of 
your AT28C256 for the sake of simplicity because it uses A14 as
the chip selector. Therefore, addrs 0x0000-0x3fff belong to the
first chip and 0x4000-0x7fff belong to the second.

You can see the schematic in dual-at28.jpg.

The schematic enables WE on both EEPROMs, but in my actual
prototype, I hard-wired the first chip's WE to high because I
never want to write to it, despite bugs I might introduce in
hardware or software (I try a lot of dangerous stuff on my
machines...).

On top of that, you will likely want to add a physical CE-
inhibit jumper (a jumper hard-wired to VCC) on the AT28 socket.
The reason for this is that if the EEPROM you have on your
socket ends with a SEGA TMR signature, it will be a wrong one,
but it will still be picked up by the BIOS and Collapse OS will
refuse to boot. A CE-inhibit switch that you can remove after
boot will solve the problem.