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doc/hw/sms: adjust ROM hacking instructions

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Previous instructions would only work with an AT28C64. Now, it also
work with an AT28C256.
This commit is contained in:
Virgil Dupras 2020-11-03 16:54:30 -05:00
parent beddb6a375
commit 29f3bf1e70
1 changed files with 13 additions and 5 deletions
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18
doc/hw/z80/sms.txt
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@ -8,8 +8,9 @@ had a z80 system for compatibility!
This makes this platform *very* scavenge-friendly and worth
working on.
SMS Power[1] is an awesome technical resource to develop for this
platform and this is where most of my information comes from.
SMS Power[1] is an awesome technical resource to develop for
this platform and this is where most of my information comes
from.
This platform is tight on RAM. It has 8k of it. However, if you
have extra RAM, you can put it on your cartridge.
@ -18,8 +19,8 @@ have extra RAM, you can put it on your cartridge.
* A Sega Master System or a MegaDrive (Genesis).
* A Megadrive D-pad controller.
* A way to get an arbitrary ROM to run on the SMS. Either through a writable
ROM cartridge or an Everdrive[2].
* A way to get an arbitrary ROM to run on the SMS. Either
through a writable ROM cartridge or an Everdrive[2].
# Hacking up a ROM cart
@ -36,13 +37,20 @@ SMS-related stuff, I recommend the 32K version instead of the 8K
one because fitting Collapse OS with fonts in 8K is really
tight.
The ROM cartridge follow regular ROM pinout, which means that
A14 are just under VCC, where WE is on the AT28. We need WE to
be perma-disabled and A14 to be properly connected.
1. De-solder the ROM
2. Take a 28 pins IC socket
3. Cut off its WE pin (the one just under VCC), leaving a tiny
bit of metal.
4. Hard-wire it to VCC so that WE is never enabled.
5. Solder your socket where the ROM was.
6. Insert Collapse OS-filled EEPROM in socket.
6. With a cutter, cut the trace leading to A14.
7. Wire A14 to the trace just under WE (which doesn't actually
touch WE because we've cut the IC socket's pin).
8. Insert Collapse OS-filled EEPROM in socket.
As simple as this! (Note that this has only been tested on a SMS
so far. I haven't explored whether this can run on a megadrive).