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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...). |