SegaXtreme
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? 🙂
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? 🙂