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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