A global-local optimization framework for simultaneous multi-mode multi-corner clock skew variation reduction

As combinations of signoff corners grow in modern SoCs, minimization of clock skew variation across corners is important. Large skew variation can cause difficulties in multi-corner timing closure because fixing violations at one corner can lead to violations at other corners. Such “ping-pong” effects lead to significant power and area overheads and time to signoff. We propose a novel framework encompassing both global and local clock network optimizations to minimize the sum of skew variations across different PVT corners between all sequentially adjacent sink pairs. The global optimization uses linear programming to guide buffer insertion, buffer removal and routing detours. The local optimization is based on machine learning-based predictors of latency change; these are used for iterative optimization with tree surgery, buffer sizing and buffer displacement operators. Our optimization achieves up to 22% total skew variation reduction across multiple testcases implemented in foundry 28nm technology, as compared to a best-practices CTS solution using a leading commercial tool.