Techniques for scalable and effective routability evaluation

Abstract

Routing congestion has become a critical layout challenge in nanoscale circuits since it is a critical factor in determining the routability of a design. An unroutable design is not useful even though it closes on all other design metrics. Fast design closure can only be achieved by accurately evaluating whether a design is routable or not early in the design cycle. Lately, it has become common to use a “light mode” version of a global router to quickly evaluate the routability of a given placement. This approach suffers from three weaknesses: (i) it does not adequately model local routing resources, which can cause incorrect routability predictions that are only detected late, during detailed routing; (ii) the congestion maps obtained by it tend to have isolated hotspots surrounded by noncongested spots, called “noisy hotspots”, which further affects the accuracy in routability evaluation; and (iii) the metrics used to represent congestion may yield numbers that do not provide sufficient intuition to the designer, and moreover, they may often fail to predict the routability accurately. This article presents solutions to these issues. First, we propose three approaches to model local routing resources. Second, we propose a smoothing technique to reduce the number of noisy hotspots and obtain a more accurate routability evaluation result. Finally, we develop a new metric which represents congestion maps with higher fidelity. We apply the proposed techniques to several industrial circuits and demonstrate that one can better predict and evaluate design routability and that congestion mitigation tools can perform much better to improve the design routability.