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From: krl@jujeh.mlb.semi.harris.com (Ken Lyons)
Newsgroups: comp.arch
Subject: Re: Do chip timing specs mean anything?
Summary: Yes, but probably not what you think.
Message-ID: <1990Aug7.140840.14044@mlb.semi.harris.com>
Date: 7 Aug 90 14:08:40 GMT
References: <1990Aug4.152038.1132@sbcs.sunysb.edu> <712@dg.dg.com>
Sender: Ken Lyons
Distribution: na
Organization: Harris Semiconductor, Melbourne FL
Lines: 61

As an IC designer, it is interesting to see how timing specifications 
are viewed by IC users.  Maybe specs would be less mysterious if the
process by which they were determined was examined.

Specs start out as a number in a product definition that is supposedly 
based on what the market needs.  Usually at this point, this involves 
a lot of guesswork, but nontheless we forge on, and these numbers often 
include several speed grades and percentages of the product that should 
meet them.

In the next step the designer designs the part according to the definition.  
If he is lucky the definition indicates which numbers are most important
so that the designer can make the right trade-offs when he finds some of
the specs cannot be met according to the simulations.

Finally the design becomes actual silicon (well, about 50% of the time :-)
and it is characterized to see what it really does.  At this time,
manufacturing looks at the data and usually insists that some specs be
changed so that they don't have to throw out 80% of the parts they make.

By the time parts are shipped, the specs are quite different from what
they started out as, but they do indicate how good the worst part shipped
out the door will be.  The distribution looks something like this:

                              ***
                       |   ***   ***
                       |***         ***
                    ***|               ***
              Rejects  |  Good parts

The problem with using parts out of spec is that the distribution changes
over time.  Most of the time this is for the better as manufacturing
technology improves.  Sometimes, however, the manufacturing process runs
amuck and the rejects hit 90% and the good parts are very close to the
specs.  Obviously, this is very expensive for the manufacturer, but it
also produces phone calls from angry customers who have designed to some
bogus spec that they have made up.  Though it is a reality that customers
need to design beyond specs to be competitive with others doing so, they
need to be aware that there is a small but finite risk in doing so.

There is another problem with doing this, which is much more rare. 
Ocasionally, a customer will design a system that depends on a part being
arbitrarily close to the spec.  A typical example of this is output enable
times: the spec says 10ns, the parts start out typically at 30ns and the
customer designs for 20ns.  A few years into production, the parts get
faster, say 15ns, and the customer's system starts having bus contention. 
The added noise on the power bus causes intermittant failures and the
customer determines that the old parts work but the new ones don't. 
Meanwhile the customer has gotten military approval for their system and
cannot change the design without going through another long and expensive
approvel cycle.  This is something greatly to be avoided.

Just how great the risks of using parts beyond their specs is hard to say. 
It is there, however; I have worked with customers who had these problems. 
I think that if you must take these risks, be aware of them, only take
them if necessary, and, if possible, redesign the risks out as soon as
possible, before they bite you.

Ken Lyons
These are my opinions.  If my company has opinions, they don't tell me
about them.