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Read strobe?

patroclus02 - May 11, 2006

		patroclus02		May 11, 2006
			Ok, I'm working on my expansion port cart (I'll post pictures and data soon). So far, it has a ROM mapped $000000-$03FFFF (256Kb) Now I want to map a device $040000-$3FFFFF But this I need to read and write. As I'll be using only low byte accesses, I'll do byte accesses to odd addresses (or word accesses to even address, I supose it is the same, as it won't respond to high bytes). I plan to use /LWR as write strobe. Can I use /ASEL for read? According to 68000 docs, /ASEL tells us about valid address and data in the bus, so it should work as read strobe. But when writing... will /LWR assert BEFORE /ASEL ?? If not, a slightly short read could be done! (according to M68000 docs, /AS asserts before /LDS, and so, before /LWR. Maybe /CAS-0 works as /OE?) Did I explain myself?

		ExCyber		May 11, 2006
			I'm not sure what you're trying to do. Mapping to an area whose size is not a power of two is kind of ugly. Anyway, the "/CAS0" on the schematics is basically the same thing as /OE. Have you seen this thread...? I thought you had posted in it but I guess my memory is failing me. <_<

		patroclus02		May 11, 2006
			Really, it will be mapped at $040000-$07FFFF, and then mirrored. So it is power of 2. I'll have a look with a logic analyzer at /CAS-0, and see if it asserts only for read access, then it will be great.

		ExCyber		May 11, 2006
			Wait... if you already have a ROM connected, what are you using for /OE for that?

		patroclus02		May 11, 2006
			/CAS-0 for ROM /OE /ROM for ROM /CS I plan to enable the new device when /ROM = 1 /ASEL = 0 This means valid data in the bus, but above $03FFFF. ROM will be disabled.

		ExCyber		May 11, 2006
			How will the ROM be disabled? As far as I know $03FFFF-3FFFFF is still in the ROM area, $400000-7FFFFF is the cart slot (when booting from the expansion slot), and above that is either internal hardware or requires external generation of /DTACK (I think). Why not just use a decoder?

		patroclus02		May 13, 2006
			As far as I know, and Charles McDonnald confirmed me, /ROM signal in expansion port is active only in range $000000-$03FFFF... Umm.. I should check this with logic analyzer to make sure, but I think this scheme I'm thinking of to address spaces above $03FFFF is the one used by Sega for addressing its SegaCD RAM It makes sense.. remember that in Expansion port there're only address lines to access 256Kb