Xref: utzoo comp.arch:20300 sci.electronics:17057
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!zaphod.mps.ohio-state.edu!julius.cs.uiuc.edu!apple!fernwood!portal!cup.portal.com!mmm
From: mmm@cup.portal.com (Mark Robert Thorson)
Newsgroups: comp.arch,sci.electronics
Subject: Hardware Equivalent of a Timing Loop
Message-ID: <38203@cup.portal.com>
Date: 19 Jan 91 03:01:07 GMT
Organization: The Portal System (TM)
Lines: 17

Quoting from the data sheet for the Intel 82311:

"Note that the above names/numbers are frequency independent;  i.e. they
refer to a generic functional VLSI device.  To actually implement for
example, a 20 MHz system, however, requires an 82311-20 Chip Set as opposed
to an 82311-16 Chip Set.  The 25 MHz version of the 82308 (dubbed the
82308HS-25) cannot be used at 16 MHz or 20 MHz."

I speculate that this frequency-dependence is caused by counting off clock
ticks to pulse external signals for a certain duration.  If you change the
clock frequency, the pulse duration changes.  Normally this isn't a problem,
but the 82311 implements an interface to a standard bus (Micro Channel),
so changing the clock frequency can put your bus timing out of spec.

It's like using the execution time of instructions to construct a software
timing loop, with the same result (i.e. the design breaks when run faster
or slower).