A lower bound computation method for evaluation of statistical design techniques

Abstract

Increase in variability in the nanometer era has contributed to pessimistic guardbands for conventional circuit design techniques that optimize at worst-case process corners. Smart deterministic approaches have been proposed that employ statistical timing analysis to reduce pessimism in the guardbands while retaining the deterministic nature of the algorithms. Other statistical optimization techniques focus on algorithms to maximize robustness of design while being aware of variability. It is not clear how much improvement can be gained using the latter set of approaches over more simple deterministic approaches. This work presents a new lower bound to evaluate these statistical optimization techniques, drawing inspiration from recent advances in sampling based SSTA. We prove that the presented lower bound gives the minimum possible area that can be achieved for a design while meeting a particular timing yield, which is the percentage of die that meeting a specified timing constraint. We then compare several statistical design optimization approaches, including one proposed in this paper called SLOP, against the computed lower bound. We show that even the simplest statistical optimization approaches produce area results which are, on average, within 9.6% of the lower bound while the best ones performed only marginally better, reaching within 3.7% of the bound. This demonstrates that the proposed bound is a close bound. In addition, it also shows that the existing optimization methods have nearly exhausted the obtainable improvement from being statistically aware and mostly provide trade-offs in runtime speed.