Date : Wed, 26 Jul 1989 13:10:39 GMT
From : mcvax!hp4nl!phigate!nlgvax!geertj@uunet.uu.net (Geert Jan de Groot)
Subject: Uh-oh.... (was 7-bit Refresh)
[In various articles, people proposed all kinds of solutions for the
128-cycle Z80 refresh problem. I still had my answer to the original
writer lying around and thought it might be useful to others, so here
it goes:]
In article <398@uop.uop.EDU> he wrote:
>I need 64Kx1 dynamic RAM chips, like 4164's. They need to be 100 ns.
>They need to work with 7 bit (128 cycle) refresh.
The most 64K DRAMs are 7 bit refresh. From memory, Hitachi, NEC, Fujitsu
are all good. Texas Instruments is not. 128 or 256 bit refresh is clearly
marked in the databook. 90% of the 64K RAMs (usually those from
The Land Of The Rising Yen) are OK.
If you still only get access to the remaining 10%, there is another
trick using some extra circuitry. Get an extra 4040, connect the clock
of the 4040 to RFSH of the Z80, and use Q7 as the extra refresh address
you were missing. Use a 74153 as multiplexer for the extra addr pin:
---------- ----------
| | +---|I0 |
| | | | |
Z80 RFSH ---|clk Q7|-----------+---|I1 Q|--- new A7 4164's
| | | |
GND ---|reset | Z80 A14 ---|I2 |
| | | |
| | Z80 A15 ---|I3 |
| | | |
| | | |
| HEF 4040 | ADDR MUX ---|A0 |
---------- | |
Z80 RFSH ---|A1 |
| |
| 74LS153 |
----------
During R/W operation, A14/15 is fed to the RAM's, which you needed to
build anyway because the old 4116's didn't have A7. During refresh,
Q7 is fed to the RAMs. Because Q7 toggles each 128 refresh periods,
it refreshes all columns.
I hope this is clear enough. Good luck!
>Am I the only one in the world who understands the difference between
>7 and 8 bit refresh??? Am I going to have to buy 256K RAM chips and
>waste 192K of RAM to make this project go???
256K chips are even worse. They have 8/9 bit refresh, so you have 2
address bits to add. You can use Q8 of the 4040 and the other half
of the 74153 to do this if you like, and have 256K available
(with the multi-task, multi-user Z80 system I built with a group,
we even have 384K available, enough to support 6 CP/M user simultaniously.
How 384K is addressed with a Z80 is another story, available on request).
>Nick Sayer | nsayer@uop.edu | ...ucbvax!ucdavis!uop!nsayer
--8<--nip-nip---------------------------------------------------------------
Geert Jan de Groot, Email: geertj@nlgvax.pcg.philips.nl
Philips TDS-ICA, ..!mcvax!nlgvax!geertj
Innovation Centre Aachen, Ham: PE1HZG
Weisshausstrasse, "MS-DOS is just a bootstrap" - me
5100 Aachen, Germany.
phone: +49 241 6003 714 [Standard disclaimers apply]
End of INFO-CPM Digest V89 Issue #156
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