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OCV Webinar: Statistical Timing Finds a Niche

Comments(2)Filed under: Statistical Timing, SSTA, Industry Insights: ARM, webinar, Jacobs, OCV, on-chip, statistical, on-chip variation

Over a year ago I wrote a blog entitled, "Whatever Happened to Statistical Timing?" A Cadence webinar earlier this week provided one answer - it's becoming part of a continuum of capabilities for on-chip variation (OCV) analysis, and will most likely join the mainstream at 32nm and below.

Entitled "Getting back timing margins: Traditional OCV alternatives," the webinar was the first in a series of Digital Implementation and Signoff webinars that will be rolling out over the next few weeks. In this initial webinar Mike Jacobs, senior product manager at Cadence, started with a basic explanation of OCV and then described a range of potential analysis capabilities.

As the name suggests, OCV represents variation across the die itself. This includes random variations (doping fluctuations, gate oxide thickness) and systematic variations (lithography, CMP). OCV is serious business at advanced process nodes. It can cause chips to fail, or result in excessive margins that prevent chips from meeting their performance or power targets. Traditional OCV uses a lumped de-rating factor and is, as Mike said, "inaccurate and pessimistic."

A Range of Solutions

Mike described a range of solutions that might be appropriate at different process nodes (numbers mine):

0 - No OCV at all. Probably okay at 130nm and above.

1 - Traditional OCV with a "lumped" de-rating factor, which typically applies only to clock nets. May work down to 65nm.

2a - Location-based OCV (LOCV). The foundry gives you a set of tables, most likely for stage-based (random) variation, with de-rating factors for different types of paths and cells. LOCV uses logic level, cell complexity, and physical location to select optimal de-rating factors. Good approach for 65nm, possibly applicable down to 28nm.

2b - Advanced stage-based OCV (AOCV). Main difference from LOCV is that you can apply weighting factors to individual cells in a design. Again, 65nm-28nm.

3 - Design-specific statistical OCV. If you have access to a statistical library, and don't want to run a full statistical static timing analysis (SSTA), you can use statistical analysis to generate OCV de-rating factors and then view the same timing reports you'd see with static timing analysis (only more accurate). Most likely use is at 32/28nm.

4 - Statistical static timing analysis. Full SSTA returns statistical distributions rather than best case/worst case absolute numbers, potentially saving dozens or hundreds of corner-case runs. In addition to a highly accurate OCV analysis, you can determine which percentage of chips will yield at a given frequency. While used by a few early adopters today, the "sweet spot" for full SSTA will be at 32nm and below.

So rather than a new "tool" or methodology, SSTA is just an additional capability -- albeit a very powerful one -- for analyzing variability. This may help explain why SSTA, once seen by some observers as the "next big thing" in IC design, has kind of faded from public view in the past few years. The Cadence Encounter Timing System has quietly supported full SSTA for some time, along with the other capabilities listed above.

Upcoming Webinars

Upcoming webinars in the Digital Implementation and Signoff Webinar Series include the following. All take place at 10:00 am Pacific time.

  • Aug. 31 - In-design Signoff: It's All About Getting It Right the First Time
  • Sept. 3 - Should You Design Your Next System With 3D TSVs? Hear from GLOBALFOUNDRIES and Cadence
  • Sept. 7 - Improving Your Yield: What to Watch Out for at 28nm and Beyond
  • Sept. 9 - Maximizing the Performance-per-Watt of Your Next Design

Registration and information is available here.

The webinars will be archived here roughly one week after each live presentation.

Richard Goering



By Daniel Payne on August 26, 2010
I'm curious if gate-based and cell-based SSTA tools will give way to transistor-level SSTA tools because of accuracy concerns. What do you think?

By rgoering on August 26, 2010
Dan -- I think there could be a place for both gate-level and transistor-level SSTA, but I would expect the latter only on a handful of the most critical paths.

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