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doc/hw/sms: adjust ROM hacking instructions
Previous instructions would only work with an AT28C64. Now, it also work with an AT28C256.
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@ -8,8 +8,9 @@ had a z80 system for compatibility!
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This makes this platform *very* scavenge-friendly and worth
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working on.
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SMS Power[1] is an awesome technical resource to develop for this
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platform and this is where most of my information comes from.
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SMS Power[1] is an awesome technical resource to develop for
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this platform and this is where most of my information comes
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from.
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This platform is tight on RAM. It has 8k of it. However, if you
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have extra RAM, you can put it on your cartridge.
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@ -18,8 +19,8 @@ have extra RAM, you can put it on your cartridge.
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* A Sega Master System or a MegaDrive (Genesis).
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* A Megadrive D-pad controller.
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* A way to get an arbitrary ROM to run on the SMS. Either through a writable
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ROM cartridge or an Everdrive[2].
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* A way to get an arbitrary ROM to run on the SMS. Either
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through a writable ROM cartridge or an Everdrive[2].
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# Hacking up a ROM cart
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@ -36,13 +37,20 @@ SMS-related stuff, I recommend the 32K version instead of the 8K
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one because fitting Collapse OS with fonts in 8K is really
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tight.
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The ROM cartridge follow regular ROM pinout, which means that
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A14 are just under VCC, where WE is on the AT28. We need WE to
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be perma-disabled and A14 to be properly connected.
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1. De-solder the ROM
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2. Take a 28 pins IC socket
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3. Cut off its WE pin (the one just under VCC), leaving a tiny
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bit of metal.
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4. Hard-wire it to VCC so that WE is never enabled.
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5. Solder your socket where the ROM was.
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6. Insert Collapse OS-filled EEPROM in socket.
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6. With a cutter, cut the trace leading to A14.
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7. Wire A14 to the trace just under WE (which doesn't actually
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touch WE because we've cut the IC socket's pin).
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8. Insert Collapse OS-filled EEPROM in socket.
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As simple as this! (Note that this has only been tested on a SMS
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so far. I haven't explored whether this can run on a megadrive).
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