( ESNUG 380 Item 8 ) -------------------------------------------- [10/25/01]
Subject: ( ESNUG 379 #7 ) The Cadence CTGEN vs. SPC Benchmark Was Pre-Route
> I think that the slew & skew figures given in this benchmark are generated
> by the clocktree generation tools right after clock tree generation. ...
> In general, skew analysis done with a neutral STA tool like PrimeTime
> would be more acceptable. This is because, CTGEN and First Encounter may
> have different timing engines. ... If the person who did the benchmark can
> do such post-route analysis also, the benchmark would be more realistic.
>
> - Donepudi Narasayya
> STMicroelectronics
From: [ Rinse, Lather, Repeat ]
Please keep me anonymous again on the CTGEN comments.
I was the one who originally started this thread and I admit, the numbers
were pre-route. I can say that in the case of CTGEN the skew actually got
worse after final routing and extraction. We don't have a router in house
so I don't know if I will be able to provide final First Encounter numbers
unless we do purchase the tool. However, SPC does use estimated routes that
are better than what other tools seem to be using (ie: not Steiner routes),
so I think its likely SPC's skew numbers are not far off from reality,
especially given what I have heard others say about this tool and in
particular, the clock tree generation capabilities.
On CTGEN again, I gave it a problem where a large macro effectively split a
clock domain into 2 chunks of flops, one on the side of the macro and one on
the bottom. I don't think CTGEN considers obstructions at all so when it
tried to generate a clock tree on this domain it went haywire and came up
with a skew of 25 ns and over 10,000 buffers. I ended up getting good
results by splitting the 2 chunks of flops into 2 trees, generating the
trees for each chunk, and then unifying them with a simple "top" tree. This
gave good results but since I saw similar problems on other blocks with
macros I came to the conclusion that CTGEN is in no way suited to SOC types
of problems as it does not consider obstructions or routing or seeminly
anything in any decent way.
I don't see how anyone doing large chips can seriously consider continuing
to use CTGEN unless they like heavily assisted flows.
- [ Rinse, Lather, Repeat ]
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