Tortoise Versus Hare … or How to Improve Your Time to Tapeout Using In-Design Signoff

Now that Wei Lii Tan has helped you with your New Year’s resolution to “create a chip that is so compelling …” in his previous blog, I would like to help you understand how Cadence is using our signoff qualified engines during the design implementation flow to reduce your time to tapeout.

Anyone remember the story of the tortoise and the hare from your childhood? The moral of the story is that simply being fast isn’t always better, and in a way that is very much like using signoff quality engines during the design implementation flow. Let me explain …

Traditional design methodologies that have been established over the years were optimized for runtime, mostly by trading off accuracy. By reducing runtimes at points in the design where accuracy wasn’t needed, a design could progress more rapidly to signoff. On the surface, this seems like a very logical approach to take. However, deeper analysis shows that this might not always enable the fastest time to tapeout.

While the runtime for individual parts of the flow can be improved by trading off accuracy for increased performance, many of Cadence’s customers have already reflected that the bigger problem is repeating implementation steps to address violations that are only found during late-stage signoff.

In fact, such customers have already experienced that using signoff quality engines during the design implementation flow actually improves time to tapeout by enabling better convergence in the solution. While it does take slightly longer to run the signoff engines in the flow, the additional time is quite small compared to having to repeat steps through the back-end design methodology to fix problems.

OK, let’s get even more specific with an example.

Consider that the routing engine of an ASIC solution does not manage all of the additional complex rules required at 28nm and below. After the routing is finally completed, often following many optimization loops to address timing violations, the design meets the timing requirements and is ready for final signoff. Unfortunately, signoff DRC checking now flags that the router didn’t meet all of the design rules, forcing the design team to fix the routing … but the router isn’t aware of the complex rules that have failed, and so an automated fix is impossible. Instead, the design team must manually fix the additional DRC violations, and that’s ECO hell!!!

Now consider the alternative solution. The router is enabled with signoff quality DRC checking and so avoids creating DRC violations … admittedly, the router takes slightly longer to run, but once timing closure is attained, the design team can have confidence that no additional DRC violations will be found at signoff.

While the above example explains how time to tapeout can be improved for DRC checking, consider that all signoff components could result in similar issues. At Cadence, I am proud that we have a very comprehensive suite of signoff solutions that can execute separate standalone signoff, or enable our signoff engines integrated within our Virtuoso custom IC and Encounter digital platforms.

Included in the signoff suite are products that enable the following functionality (Cadence product names are in parentheses)

•        Parasitic extraction (Cadence QRC Extraction)
•    Physical verification (Cadence PVS Design Rule Checker and Cadence PVS Layout vs Schematic Checker)
•        Static timing and SI analysis (Encounter Timing System)
•         Power integrity analysis (Encounter Power System)
•         Litho hotspot detection and analysis (Cadence Litho Electrical Analyzer and Cadence Litho Physical Analyzer)
•         CMP analysis (Cadence CMP Predictor).

So, just like the tortoise and hare story, faster runtimes might sound like the best option, but faster time to tapeout requires a different approach.

Pete McCrorie