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KrzysioCartMMC3 - famicom flash cart for mmc3 mapper
http://forums.nesdev.com/viewtopic.php?f=9&t=14768
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Author:  krzysiobal [ Thu Sep 01, 2016 4:17 pm ]
Post subject:  KrzysioCartMMC3 - famicom flash cart for mmc3 mapper

I got AX5202P DIL40 chip (which is pirate MMC3), so I thought of making super-duper MMC3-dev-cart with PRG-ROM (512 kB) + WRAM (8 kB) with battery backup + CHR-ROM (256 kB) + CHR-RAM (8kB). Change between CHR-RAM and CHR-ROM is done with jumper. The main goal was to make it ready for uploading games via KrzysioKazzo.
To make it, I connected
famicom !ROMSEL to PRG-ROM-!CS
famicom CPU_R/!W to PRG-ROM-!WE
MMC ROM_CS to PRG-ROM-!OE
Thanks to that, writes to $8000-$FFFF goes also to flash-rom. Minor disadvantage is that writing to mapper registers (which sits at $8000-$FFFF) also goes to memory, however that is no problem because to program one byte, four write cycles need to be done (with alternating $555/$2AA pattern).

The AX5202P I got is quite buggy - in some games (eg. Super Mario Bros 3), the scrollbar takes more that half of a screen. I think scanline counter is broken. However, this only applies when the cartridge is in PAL famicom. In NTSC, everything works fine.
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https://www.youtube.com/watch?v=2FGD0-E4xkM

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