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Path: utzoo!aesat!bmw
From: bmw@aesat.UUCP (Bruce Walker)
Newsgroups: net.micro
Subject: Re: (HD64180 question)
Message-ID: <468@aesat.UUCP>
Date: Tue, 24-Sep-85 08:14:12 EDT
Article-I.D.: aesat.468
Posted: Tue Sep 24 08:14:12 1985
Date-Received: Tue, 24-Sep-85 08:36:30 EDT
References: <207@ittral.UUCP> <53400071@trsvax> <2957@ut-sally.UUCP>
Reply-To: bmw@aesat.UUCP (Bruce Walker)
Organization: AES Data Inc., Mississauga Ont., Canada
Lines: 41
Keywords: refresh
Summary: 

[This message was made from %100 recycled characters.]

In article <2957@ut-sally.UUCP> crandell@sally.UUCP (Jim Crandell) writes:
>
> > Their Z80 is called the HD64180.  Apart from adding on-chip MMU,
> > ... Hitachi shaved the t-states on many of the old-opcodes.
> > Even the NOP executes in fewer t-states.
>
> Really?  Not a very good idea.  The Z-80 nop is 4 states -- standard
> opcode fetch.  Considering the tightness of the read and refresh
> cycles in an opcode fetch, a 3-clock nop would absolutely necessitate
> eliminating the refresh cycle.  Now, suppose you ran the following code:
>
> [... a long NOP loop ...]
> The loop iteration time at 4 MHz is 95.75 uS, giving a refresh interval
> for row 7F of over 12 mS, which is out of spec for every dynamic RAM I've
> ever heard of.  Furthermore ...

The Z80 nop is 4 states.  The Hitachi designers took a page from Zilog's book
    ^^^
and copied the Z8000 refresh timer concept.  On the 64180, refresh cycles are
not inserted after every opcode fetch, rather, a counter watches machine cycles
and calls for a refresh request to be inserted after every N t-states, where N
is programmable.  So, no matter what combination of instructions is occurring,
two or three (also programmable) cycle refresh operations will occur at a
rate sufficient to keep your drams alive.

If you have a system using only static rams and have no need for refresh, you
can suppress the refresh cycles altogether and avoid paying the time penalty,
something you could not do with the good ol' Z80.


Bruce Walker     {allegra,ihnp4,linus,decvax}!utzoo!aesat!bmw

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