Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site sdcrdcf.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!vax135!petsd!pesnta!hplabs!sdcrdcf!dennisg From: dennisg@sdcrdcf.UUCP (Dennis E. Griesser) Newsgroups: net.micro Subject: Z-80 and DRAMs Message-ID: <2268@sdcrdcf.UUCP> Date: Mon, 19-Aug-85 16:23:31 EDT Article-I.D.: sdcrdcf.2268 Posted: Mon Aug 19 16:23:31 1985 Date-Received: Sat, 24-Aug-85 13:52:26 EDT Reply-To: dennisg@sdcrdcf.UUCP (Dennis E. Griesser) Organization: System Development Corp. R+D, Santa Monica Lines: 29 I am tossing together a micro with a Z-80 and lots of memory (at least 64K). The natural choice is DRAMs. There will be eight sockets and the design should be able to accept 64K or 256K parts. The problem is refresh. To keep parts low, the Z-80's built-in refresh timing will be used. Sadly, the Z-80's internal refresh counter is only 7 bits wide. This will take care of some of the 64K chips on the market, but not all of them. All of the 256K DRAM specs that I have seen require 8 bits of refresh (luckily not 9). My alternatives seem to be as follows: o commercial DRAM controller o self-refreshing RAMs o refresh in software (execute 256 NOPs) o external tri-state 8-bit counter (74xx590) o find Z-80 with 8-bit refresh The commercial DRAM controllers seem like overkill. I would like to avoid self-refreshing RAMs. Software refresh seems risky and would require periodic interrupts to tell the CPU when to refresh. That leaves me two questions: o Is the 74xx590 a real part? I have seen specs from various vendors, but a friend claims that they are impossible to find! o Do later versions or second-sources of the Z-80 have a wider refresh counter? Thank you, in advance! [standard disclaimers apply]