A Fast, Iterative Clock Skew Scheduling Algorithm with Dynamic Sequential Graph Extraction

Clock skew scheduling (CSS) is a well-known technique that improves design timing slack by adjusting clock latency to flipflops. CSS requires obtaining timing path information between sequential elements (including flip-flops and I/O ports), known as sequential graph extraction, which is the most time-consuming part of advanced CSS. In this paper, to quickly identify the potential of clock skew in slack optimization, we propose an iterative CSS algorithm that leverages timing propagation to facilitate sequential graph extraction. Then, we provide a comprehensive skew calculation method that considers multiple clock latency constraints, obtaining the target latency of each flip-flop. Finally, we present slack optimization techniques to achieve the target latencies. Our algorithm achieves a $49.11 \times$ speedup compared to the advanced CSS algorithm based on partial graph extraction, reducing 90.05% of the extracted edges. Compared to a state-of-the-art CSS-based slack optimization methodology, our algorithm delivers a $27.01 \times$ speedup with superior slack improvement.