( ESNUG 446 Item 2 ) -------------------------------------------- [09/01/05]
From: [ Uncle Fester of the Addams Family ]
Subject: Designer Thinks the New Magma Clock Tree Generation Blows Chunks
Hi John,
I just attended the Magma clock tree generation seminar. I think I would
like to remain anonymous on this since I didn't think much of their tool.
If the Magma clock generator tool could actually do what it claimed, it
would be very useful to a backend designer trying to turn a bad design into
something useful. Unfortunately the basic design of its clock trees is
wrong. I have personally designed several clock trees so I have thought
about the issues quite a bit. The basic flaw in the Magma scheme is the
concept that you can analyze any geometry and get accurate timing numbers.
That was never a good idea which is why people used H trees and the problem
is getting much worse with current geometries.
Magma has already noticed problems in that the new release lets you specify
several process corners instead of just one. Unless you know which process
points, and worse, what OCV is going to be in the worst places for your
clock tree, you can't plug them in.
Magma was talking about ways that you can tune clocks and insertion delays
relative to each other. I wouldn't let automatic tools loose on this kind
of a problem. I have done full custom designs where I purposly delayed a
clock, but those were under very controlled situations, like after a
multiplier. When I tried to ask the Magma people about how they derived
their delay numbers I was given the "let me put you in touch with the
salesman" run around. Their demo showed single numbers for delays so unless
you guess the right process point and switching conditions to input into
their analysis, it can't possibly generate the worst numbers. Of course
there is the other problem that the best way to analyze this is some kind
of SPICE -- not any sort of RC.
I feel sorry for anyone who believes their story and tries to use it as it
is advertised. Those people need help. There was some idiot in the
audience who was suggesting shorting the lowest level of clocks, nearest
the flops, to minimize skew. That does keep down the skew, the DEC ALPHA
famously did it, but it causes quite a bit of fighting if the clock trees
are not properly balanced which is impossible to do without an H tree like
structure over process. The guy from Magma said he would take that back
to the development team. Yikes.
As I said I don't think I want to be a lightening rod so if you could keep
me anonymous, I would appreciate it. Keep up the good work.
- [ Uncle Fester of the Addams Family ]
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