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doc/hw/z80/sms: add Dual EEPROM cartridge design
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doc/hw/z80/sms/at28.txt
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doc/hw/z80/sms/at28.txt
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# 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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doc/hw/z80/sms/img/dual-at28.jpg
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doc/hw/z80/sms/img/dual-at28.jpg
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