Delay Metrics for the Next Fifty Years

Abstract

Fifty years ago, in 1948, W.C. Elmore proposed a simple delay estimation scheme for damped linear circuits that are characterized by monotonic step responses. The Elmore delay, or first moment of the impulse response, is still widely used today as a performance metric during the front-end stages of design, and for timing-driven physical design applications. However, the progress toward deep submicron IC technologies has created some question regarding the efficacy of the Elmore metric. A significant amount of progress has been made on more accurate delay approximations based on moment matching and model order reduction, but these techniques are primarily aimed at solving verification problems on the back-end of the design flow. In this presentation we will analyze the properties of the Elmore delay, and highlight its strengths and weaknesses. We will then propose new directions for delay prediction that are suitable for use in optimization loops and front-end applications, yet are accurate enough to produce results that are strongly correlated with those produced by the back-end algorithms.

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