An Early Clock Based Multi Tap Flexible H-Tree for High Performance CPU Design for Achieving Best Cross Corner Scaling

Abstract

The High-Performance Designs need a robust Clock Tree which achieves a low skew, insertion delay and minimum power. A Conventional CTS does not provide an optimal solution to these. The Proposed methodology enables an early clock Based flow for a Multi-Tap Flexible H Tree Clock Tree Generation. The Early Clock Flow will build a preliminary Clock Tree at placement helping in achieving a clock gate enable timing estimation and better placement. The Multi Tap Flexible H Tree helped in best cross corner scaling as well as electrically and geometrically symmetry.

Introduction

The paper discusses Customized Clock Tree Synthesis (CTS) for high-frequency CPU designs, emphasizing Flexible H-Tree and Multi-Tap CTS techniques to achieve low skew, balanced wire lengths, and electrically symmetric buffering. Unlike conventional CTS, Flexible H-Trees relax geometric symmetry requirements while maintaining electrical symmetry, allowing automated synthesis even in constrained floorplans. Multi-Tap CTS integrates with Flexible H-Trees to provide local buffering and balance between the top-level tree and clock sinks.

Early Clock Flow (ECF) is employed during placement to insert a preliminary clock tree, improving congestion estimation, timing accuracy, and enabling pre-CTS optimizations like skewing, critical path adjustments, and clock gating analysis. Iterative optimization—including placement, timing, congestion, and power adjustments—further refines the design.

The study applied these methods to a CPU design with 5 million gates at 1.2?GHz using Cadence Innovus, showing that the Multi-Tap Flexible H-Tree approach reduces CTS runtime, achieves better electrical symmetry, maintains low skew across corners, and outperforms conventional H-Tree CTS for high-frequency designs.

Conclusion

Balanced CTS optimization is really a great challenge. Based on unique insight into Clock skew and Latency , this novel ethodology is proposed to optimize the CTS , which can achieve better clock latency and clock power compared with the results from Conventional Method of CTS build. This Proposed CTS Methodology can also improve TNS of setup as well hold timing, for the impact of OCV is reduced from the cross-corner scaling. This approach is useful for a better leakage power reduction. Further enhancements can be made by implementing this strategy on designs with more congestion and multiple power domains.

References

[1] Kwangsoo Han, Andrew B. Kahng, Jiajia Li, “Generalized H-Tree Topology and Buffering for High-Performance and Low-Power Clock Distribution” DOI 10.1109/TCAD.2018.2889756, IEEE Transactions on Computer- Aided Design of Integrated Circuits and Systems2018. [2] Wei-Khee Loo, Kok-Siang Tan and Ying-Khai The, “A study and design of CMOS H-Tree clock distribution network in system-onchip”.2009 IEEE 8th International Conference on ASIC. doi:10.1109/asicon.2009.5351254. [3] Chuan Yean Tan†, Rickard Ewetz‡, and Cheng-Kok Koh, “Clustering of flip-flops for useful-skew clock tree synthesis”. 2018 23rd Asia and South Pacific Design Automation Conferences (ASP-DAC). doi:10.1109/aspdac.2018.8297374.” [4] Meng Liu, Zhiwei Zhang, Wenqin Sun, Donglin Wang, “Liu, M.,Zhang, Z., Sun, W., & Wang, D. (2017). Obstacle-aware symmetrical clock tree construction. 2017 IEEE 60th International Midwest Symposium on Circuits and Systems (MWSCAS). doi:10.1109/mwscas.2017.8052973”. [5] Q. K. Zhu, C. E. Dike, “High-Speed Clock Network Design” Kluwer Academic Publishers, 2003. [6] T. Lu and A. Srivastava, “Gated low-power clock tree synthesis for 3D-ICs”, in Proc. ISLPED, pp.319-322, Aug. 2014. [7] Chao Deng, Yici Cai, Qiang Zhou, Zhuwei Chen, “An efficient buffer sizing algorithm for clock trees considering process variatons” in 6th asia symposium on quality electronic design, Aug,2015. [8] Minghao Lin, Heming Sun and Shinji Kimura, “Power- Efficient and Slew-Aware Three-Dimensional Gated Clock Tree Synthesis”, 2016 IFIP/IEEE International Conference on Very Large-Scale Integration (VLSI-SoC). [9] Pinaki Chakrabarti, “Clock Tree Skew Minimization with Structured Routing”, 2012 25th International Conference on VLSI Design. [10] Anand Rajaram, Member, IEEE, and David Z. Pan, Senior Member, IEEE, “Robust Chip-Level Clock Tree Synthesis”, IEEE transactions on computer-aided design of integrated circuits and systems, vol. 30, no. 6, June 2011. [11] Cadence Innovus User Guide, http://www.cadence.com

Copyright

Copyright © 2025 Mukul Anand, Nikunj Tripathi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.