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35 lines
1.5 KiB
Plaintext
35 lines
1.5 KiB
Plaintext
# Writing to a AT28 from a SMS
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Writing on the EEPROM that is currently running Collapse OS is
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as easy as enabling the WE pin on your hacked up cartridge. How-
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ever, this is not practical: If you want to deploy Collapse OS
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(or something else) to another machine, or even if you want to
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upgrade your current Collapse OS, you will likely want to write
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to another EEPROM.
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The easiest way to do so is to build yourself a dual EEPROM
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cartridge. It's very similar to a simple cartridge, except it
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has two AT28 sockets and a '139 decoder to select between the
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two.
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The design proposed here sacrifices access to the upper 16K of
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your AT28C256 for the sake of simplicity because it uses A14 as
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the chip selector. Therefore, addrs 0x0000-0x3fff belong to the
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first chip and 0x4000-0x7fff belong to the second.
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You can see the schematic in dual-at28.jpg.
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The schematic enables WE on both EEPROMs, but in my actual
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prototype, I hard-wired the first chip's WE to high because I
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never want to write to it, despite bugs I might introduce in
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hardware or software (I try a lot of dangerous stuff on my
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machines...).
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On top of that, you will likely want to add a physical CE-
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inhibit jumper (a jumper hard-wired to VCC) on the AT28 socket.
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The reason for this is that if the EEPROM you have on your
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socket ends with a SEGA TMR signature, it will be a wrong one,
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but it will still be picked up by the BIOS and Collapse OS will
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refuse to boot. A CE-inhibit switch that you can remove after
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boot will solve the problem.
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