Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site hcrvx1.UUCP
Path: utzoo!hcrvax!hcrvx1!hugh
From: hugh@hcrvx1.UUCP (Hugh Redelmeier)
Newsgroups: net.micro.68k,net.arch
Subject: Glitch Phenomenon (Re: Multiple 68020's on VME ?)
Message-ID: <1258@hcrvx1.UUCP>
Date: Thu, 3-Oct-85 12:41:29 EDT
Article-I.D.: hcrvx1.1258
Posted: Thu Oct  3 12:41:29 1985
Date-Received: Fri, 4-Oct-85 00:29:00 EDT
References: <442@rna.UUCP> <1192@vax1.fluke.UUCP>
Reply-To: hugh@hcrvx1.UUCP (Hugh Redelmeier)
Organization: Human Computing Resources, Toronto
Lines: 34
Summary: it's gonna get yah!

About 10 years ago, I first learned of what was called the Glitch
Phenomenon.  It was described (if I remember correctly) in a tech report
from MIT.  They showed theoretically that asynchronous systems could not
be synchronized in *any* bounded amount of time!  They then showed some
practical examples with real TTL and oscilloscope traces.  If I remember
correctly, it is possible to build a circuit that syncronizes, and says
when it has done so (after an unbounded but short amount of time).  I
also seem to remember that a project attempting to build a machine that
was internally very asynchronous ended up having to invent equivalents
for TTL MSI so they wouldn't get bit by the glitch (perhaps Al Davis's
Data-Driven machine).

In article <1192@vax1.fluke.UUCP> witters@fluke.UUCP (John Witters) writes:
>I'd suggest reading the August 1st 1985 issue of Computer Design before you
>rush off and do this.  The article of interest is titled "Metastability haunts
>VME bus and Multibus II system designers" on page 29. ...
>Because the
>arbiter makes its arbitration decisions in about 20ns, the output of its
>synchronizer has only 20 ns to settle to a stable state, but needs at least 50
>ns to ensure reliable operation.  

Theoretically, any finite bound is not good enough.  Perhaps the
probability of metastbility extending past 50ns should be calculated
*and stated*.  Of course, maybe the journal article did (I don't have
access to it), but even the net article should qualify these bald
numbers. The danger at 50ns might well be acceptably unlikely (the
probability exponentially decreases with time) but it depends very much
on the circuit technology and design -- not too nice for an interface
standard.  As a software-type, I like things to be right or wrong,
but I understand engineers live in another universe (perhaps the real
one).

Hugh Redelmeier (416) 922-1937
{utzoo, ihnp4, decvax}!hcr!hugh


Brought to you by Super Global Mega Corp .com