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From: jesup@cbmvax.commodore.com (Randell Jesup)
Newsgroups: comp.arch
Subject: Re: Do chip timing specs mean anything?
Message-ID: <13666@cbmvax.commodore.com>
Date: 6 Aug 90 22:52:39 GMT
References: <1990Aug4.152038.1132@sbcs.sunysb.edu>
Reply-To: jesup@cbmvax (Randell Jesup)
Distribution: na
Organization: Commodore, West Chester, PA
Lines: 35

In article <1990Aug4.152038.1132@sbcs.sunysb.edu> owen@sbcs.sunysb.edu (Owen Kaser) writes:
>Do timing specs for microprocessors and their support chips bear
>any relation to reality, and do industrial designers 
>(need to) pay any heed to them?  Here's the background for my question:
....
>What is the moral of this story?  Usually perhaps 30% better performance
>than indicated was required, and the chips delivered it.  Was it because
>the chips had been around long enough that the manufacturers had improved
>them and not updated their data books?  Was it that the specs were good
>until 70 degrees C, and we never rose above 55 degrees C?  Were we
>just lucky, and will the units stop working after the chips age
>(electromigration etc)?

	Probably a combination of things.  Normally, chips are rated at
70 C, 4.5 volts instead of 5, and since you want most of the parts to pass
at that level, you add in a safety factor (which may be up to 30% or more).
Also, age of chip, lot variation, etc may affect timing.  As you said, it
may be that they migrated to a newer, faster process but didn't re-engineer
the design to figure out the new speed parameters.

	For example, the rpm-40 was speced at 40Mhz, but no one expected to
run that except in a high-speed pcb.  When first silicon came out, it
was dropped into the (careful) wire-wrap board that was expected to handle
maybe 10 or 15 Mhz.  I'm told it ran at 40Mhz in the wire-wrap (but at
nominal temp and voltage).  It had been designed with a _lot_ of leeway
everywhere.

	You're right to be leary of violating them, though.  It's the sort of
thing that will come back an bite in all sorts of flakey, annoying ways.

Disclaimer: I'm a software guy who kibitzes on hardware guys.
-- 
Randell Jesup, Keeper of AmigaDos, Commodore Engineering.
{uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.cbm.commodore.com  BIX: rjesup  
Common phrase heard at Amiga Devcon '89: "It's in there!"