In article <4067nl$55b@saba.info.ucla.edu>,
Mike Loving <loving@plover.cs.ucla.edu> wrote:
>Ralph Mason (ralph.mason@liffe.com) wrote:
>: Has anyone here ever built a board that uses a software refresh DRAM
>: scheme.  What I'm thinking of trying is generating an interrupt using
>: a timer and then refreshing x rows every time.
>
>: I've not seen this done before ( but then I'm fairly new in this game
>: ) but I can't see any reason it wouldn't work. I would appreciate any
>: details of anyones past experiences.
>
>
>I've done it, it works.  Even on a 2.5MHz Z-80.  I built a printer buffer
>in about 1985 that did this.  I used a z80 (which has an built-in "refresh"
>scheme, but that only puts out 7 bits of address and I needed 8).

You should have used the Z80 built in refresh.  If you need 8 bits all you
need is to interrupt at half the maximum refresh interval and invert the
upper bit of the refresh register.  Your interrupt routine looks like this:

        PUSH    AF
        LD      A,R
        XOR     80H
        LD      R,A
        POP     AF
        EI
        RETI

If your DRAM needs 256 refreshes every 4ms, you just need an interrupt every
2ms.  If you're using only a 2.5MHz clock, this would cost you .003% of your
Z80 capacity (assuming 75 clocks to handle the interrupt).