|
- # 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.
|
