mirror of
https://github.com/hsoft/collapseos.git
synced 2024-12-28 00:18:06 +11:00
35 lines
1.5 KiB
Plaintext
35 lines
1.5 KiB
Plaintext
# 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.
|