- Joined: 12 Jul 2005
- Posts: 55
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SMS homemade ROM mapper- block 1 picture. The register address block
 Posted: Tue Sep 05, 2000 2:37 am
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On this first block I did this :
U1 does decode the Z80 memory frames. From three of these 4 frame signals I generate a special chip enable signal for the
ROM/EPROM chip using 3 AND ports (for inverted signals they acts as OR ports.). This gives me the ROM mapped at the
address range from 0000 to BFFF. (/CS generates a global chip select. Then I should do FULL addressing decode. The /CS_X
is low when the slot is selected in port 3E and /MREQ is low (Memory request from Z80).
U2 and U3 generates the strobes for addresses from FFF8 to FFFF. These strobes are both for read or write.
U5A disables the ROM in frame 2 only, if the line KILL ROM is high. This signal is used by the SRAM circuit to swap
the rom for the 16kb RAM window.
U5D generates the write strobe for the SRAM control circuit. (/WR_FFFC)
Personal comments :
I hope you guys understand this block. Also, I think this part of the circuit fits into a GAL16V8 easily ...
(Exept for the BIG NAND port which should be external.)
If you look at my address circuit, you will notice that the SEGA mappers DO use the very same BUS signals I did.
Also, I was using /EXM2 for as A15 until this morning (This seems to fuck up the SRAM access. 315-5235 doesn't
use /EXM2 also. But all one chip mappers do use it.)
This part of the project was the easiest to design, but it's a critical part of the circuit. One error on this block and the mapper
will not work at all. If you want to assembly this "wire-up monster" , test each block separated before joining them.
If you're going to make a board for this thing, mail me (I might want a board or two of it ..) . ;)
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