[sci.electronics] Hardware Equivalent of a Timing Loop

mmm@cup.portal.com (Mark Robert Thorson) (01/19/91)

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).