( ESNUG 354 Item 12 ) --------------------------------------------- [6/1/00]
Subject: ( ESNUG 353 #5 ) Signal Integrity, Cadence SE-SI, PhysOpt & Yahoo
[ Editor's Note: Again I'm surprized. Usually the Yahoo EDA stock chat
boards are rife with day traders trying to manipulate stock prices. Yet
I accidently found another technical exchange there this week! - John ]
From: [ Tall Thin Dude ]
I have some friends who looked at the supposed Cadence Signal Integrity
offering. Apparently it uses a VERY crude model.
Instead of looking at real agressor slews it averages slews across the whole
design for all agressors losing substantial accuracy. The equation used
once the raw inputs are obtained is also not very good.
Perhaps a bigger limitation is in terms of it's inability to correct
violations. While it may be able to crudely detect some violations the
fix-up will be mostly manual with alot of iterations. The confidence of
detection when you are done will be poor.
It is fundamentally limited for future needs by the primitive SE router.
The only true variable width/spacing support in Cadence tools is through
their IC Craftsman, but this will not help here.
This solution might work for some 200 MHZ designs in 0.25 um and is perhaps
better than nothing at all. You've got to ask yourself one question, do you
feel lucky?
In terms of the Avanti Signal Integrity, this I know the least about. I
have heard it also is fairly primitive and based on their Star-RC flow.
I am looking to the start-ups for solutions here. There are some good
analysis companies coming on line. I plan to check out CADMOS and Moscape.
In terms of fixing the only interesting companies I see here would be those
who offer a full backend solution such as Magma or Monterey. I plan to do
some looking around at the DAC.
- [ Tall Thin Dude ]
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From: [ firfilter ]
Here's my lucky guess from feedback I got:
- Cadence PKS+SE just doesn't cut it
- Avanti neither
- Synopsys PhysOpt improves the Cadence/Avanti flow quite a lot but is
not a full solution (yet)
- Magma has some technology but mainly: a lot of marketing BS
- Monterey: they may have something there, though their product looks
like a pre-pre-beta release
Good luck!
- [ firfilter ]
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From: [ si_wizard ]
> I have some friends who looked at the supposed Cadence Signal Integrity
> offering. Apparently it uses a VERY crude model.
Perhaps you should check out the quality of your friends, or examine the
software yourself.
> Instead of looking at real agressor slews it averages slews across the
> whole design for all agressors losing substantial accuracy. The equation
> used once the raw inputs are obtained is also not very good.
It does this during PLACEMENT, when of course the neighbors are unknown, and
using an average is the best you can do. And using an accurate equation here
is pointless, since the data is only an approximate.
The main idea of this step is to fix the obvious problems early. Note that
PhysOpt, and to my knowledge Avanti, don't do anything at all at the
placement stage to try to reduce downstream Signal Integrity problems.
Once the design is routed, of course, Cadence Signal Integrity uses real
cross coupling caps, accurate slews, timing windows, and an accurate
electrical model.
> Perhaps a bigger limitation is in terms of it's inability to correct
> violations. While it may be able to crudely detect some violations the
> fix-up will be mostly manual with alot of iterations.
In the most recent case of which I have knowledge, the software found 140
potential errors (out of 326K nets) and was able to fix all but 5
automatically.
> It is fundamentally limited for future needs by the primitive SE router.
> The only true variable width/spacing support in Cadence tools is through
> IC Craftsman, but this will not help here.
Note that in general fiddling with width and spacing is the wrong way to
solve Signal Integrity problems. The reason is simple - the big designs
that have these problems are already routing limited. Increasing the width
and/or spacing makes them bigger, or unroutable, or both. However, this
implies the cell utilization is not 100%, so if you can insert buffers or
size cells without affecting the routing you can fix Signal Integrity
problems without a size or speed penalty, in general. This is the approach
used in the example above.
> I am looking to the start-ups for solutions here. There are some good
> analysis companies coming on line. I plan to check out CADMOS & Moscape.
The CADMOS product that was just introduced:
http://www.dacafe.com/DACafe/NEWS/CorpNews2/20000522_fl_cadmos_.html
It does EXACTLY what the Cadence product introduced 1999 does in terms of
timing, and it does not offer nearly the support in terms of automatic
fixing.
> I plan to do some looking around at the DAC.
An excellent idea, especially if you can discard your preconceived notions
and look at the evidence of which tools are working, in production, today.
- [ si_wizard ]
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From: [ Tall Thin Dude ]
Well, si_wizard (or is that Cadence Signal Integrity Mktg. Droid), perhaps
Cadence has refined its crude model slightly by including the individual
slews and driver resistances in the backend.
There are many other aspects to a good model other than just a marketing
"feature checklist" for the basics. It is good to see at least Cadence may
now have the basics. I trust my sources and still have my doubts.
Since there are conceptual limitations with the SE router in supporting more
advanced routing and many known congestion problems with the SE/PBOPT/QPOPT
flow, I can understand si_wizard's limited view on the use of wide wire
spacing and shielding for fixing Signal Integrity violations. From my real
world design experiences I do not agree.
First of all, unlike PhysOpt, PBOPT/QPOPT are severly limited in the
percentage of placement based sizings they can do. Wireloads for initial
sizing and little downsizing of gates means big slews, more wasted area, and
a more poorly Signal Integrity conditioned design to start with.
Until (if ever) PKS becomes usable, the overall noise level of a Cadence
design will be higher to start with than a tool that sizes every gate based
on placement. Perhaps this is the real reason for the large number of
violations being reported in ESNUG. I don't know that I buy that improper
slew story. Lib slews are characterized using SPICE. Unless there is a
problem with bad threshold settings I don't know that this really holds
water.
Concerning the use of wide spacing, I have seen that typically larger nets
will suffer from Signal Integrity problems due to the summation of
individual aggressors. Even though the big chips may have hot spots that
are congested, there will generally be many areas of a problem net where
wide spacing can be used successfully to bring down the noise to acceptable
levels.
Many of these Signal Integrity problem nets tend to also be timing critical
nets. While buffering can be useful in some cases, it increases the risk of
holding timing closure due to the added intrinsic delay of the buffer.
Spacing or shielding on the other hand can be done without negative timing
impact.
In our manual approaches there were many places where buffering would not
have been able to fix problems without making timing worse. For these cases
spacing and shielding were able to do the job with no timing impact. In
fact many cases timing was actually improved from the wide spacing.
Since si_wizard reports only 140 out of about 326K nets that needed
correction, I have a hard time seeing how wide spacing noncongested sections
of this small percentage of wide spaced nets would significantly impact
area. In fact in a gridded routing approach where pitch is typically line
to contact there is alot of spacing that can be done for free just by going
offgrid if the router is capable. The more integrated solutions will
benefit here since they can check the slack on adjacent nets where wires
will be moved closer.
While I have done alot of manual fixing with shielding and spacing in layout
editors, I still believe there is big potential for an automatic tool in
this area. For Cadence to enter this area they will need a more modern P&R
infrastructure.
Since the Cadence Signal Integrity option is only available on their
unstable Genesis database, I question the claims of "production use" of the
tool.
How about giving some names of the customers you claim are using the Cadence
Signal Integrity solution in production flows?
Also be sure to list the combination of tools this Signal Integrity flow
integrates with.
I am sure there will be some folks who will pay the high price for the
option. To me it does not really matter. I like these customers with big
$$$ to spend.
All I can say is read the fine print before you buy, and don't trust anybody
with a Marketing title.
- [ Tall Thin Dude ]
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