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From: freed@aum.UUCP (Erik Freed)
Newsgroups: net.arch,net.micro.68k
Subject: Re: Asynchronous State machines
Message-ID: <395@aum.UUCP>
Date: Mon, 28-Oct-85 18:40:16 EST
Article-I.D.: aum.395
Posted: Mon Oct 28 18:40:16 1985
Date-Received: Fri, 1-Nov-85 00:18:54 EST
References: <389@aum.UUCP> <6077@utzoo.UUCP>
Organization: The Aurora Systems Bunch
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Xref: linus net.arch:1769 net.micro.68k:1209

> > 	Lots of the designs I see are done in slow sync ways and they
> > could be done in faster and smaller async circuitry. My point is that most
> > engineers would rather plug in a sequencer than a delay line and in alot of 
> > the cases this is a big lose. Once you get used to them they are a blast
> > to use. 
> 
> One possible reason for the preference for sequencers is that sequencer
> timing tolerances can be made quite tight, where delay lines usually have
> fairly sloppy specs.  I know this is why I gave up on delay lines for
> timing dynamic RAMs -- if you believe in worst-case design, the delay
> lines' loose tolerances slow things down seriously.  I admit to not being
> an expert in this area, so perhaps there is something I missed, but all
> the delay-line specs I saw had an awful lot of +- in them.
> -- 
> 				Henry Spencer @ U of Toronto Zoology
> 				{allegra,ihnp4,linus,decvax}!utzoo!henry

I don't think that you got the point, Async circuitry wins in many places.
The lack of syncronizing being one of the major ones. The relatively small
amount of min/max skew in delay lines seems rather dwarfed by these advantages.
I mean we are talking at least 100 -200 ns for the sync problem alone. Try
looking at say a 16R8 versus a 16L8 PAl with the non-registered version you
only have to worry about the skew *BETWEEN* prop times on a single pal. This
allows some very creative fast designs. For the registered version you have
to wait the maximum prop and clock_to_output times. This makes async machines
have much smaller states. and no granularity you get from a master clock.
Signals change when they are ready, not when the clock is ready. 
	I used to feel exactly the same way, but as I got into them I found 
that I was finding extra time all over the place. Try it you'lle like it!
-- 
-------------------------------------------------------------------------------
                           Erik James Freed
			   Aurora Systems
			   San Francisco, CA
			   {dual,ptsfa}!aum!freed