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Observing sonic's game gear cart
http://www.smspower.org/uploads/Development/DIY_SEGA_Game_Gear_512Kb_Flash_Cartridge.pdf
the only control signals going to the chip (mask rom+mapper) are ce and wr.
When does ce go down? only when both z80's mreq and rd go down?
or does it go down also when mreq and wr go down?

Quote

When does ce go down? only when both z80's mreq and rd go down? or does it go down also when mreq and wr go down?

CE follows MREQ, except for when the BIOS is enabled and then it is disabled for the first 1K or so. It isn't affected by RD or WR.

Sega games often connect the ROMs without using RD, because the mapper hardware assumes if WR isn't low, then RD must be. So it's sufficiently to only have WR connected.

If you are making your own cartridge/mapper then you probably don't want to follow this arrangement, it's just something unique to Sega's carts.

I see, thank you!

Author	Message
kamillebidan Joined: 25 Feb 2013 Posts: 384 Location: Osaka	Game gear CE Posted: Fri Mar 30, 2018 6:03 am
	Observing sonic's game gear cart http://www.smspower.org/uploads/Development/DIY_SEGA_Game_Gear_512Kb_Flash_Cartridge.pdf the only control signals going to the chip (mask rom+mapper) are ce and wr. When does ce go down? only when both z80's mreq and rd go down? or does it go down also when mreq and wr go down?

Charles MacDonald Joined: 28 Sep 1999 Posts: 1197	Posted: Sat Mar 31, 2018 5:08 am
Charles MacDonald Joined: 28 Sep 1999 Posts: 1197	Quote When does ce go down? only when both z80's mreq and rd go down? or does it go down also when mreq and wr go down? CE follows MREQ, except for when the BIOS is enabled and then it is disabled for the first 1K or so. It isn't affected by RD or WR. Sega games often connect the ROMs without using RD, because the mapper hardware assumes if WR isn't low, then RD must be. So it's sufficiently to only have WR connected. If you are making your own cartridge/mapper then you probably don't want to follow this arrangement, it's just something unique to Sega's carts.

kamillebidan Joined: 25 Feb 2013 Posts: 384 Location: Osaka	Posted: Sun Apr 01, 2018 10:16 pm
	I see, thank you!

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