Designing a robust clock tree structure

Amol Agarwal and Priyanka Garg, Freescale semiconductor
EETimes (8/6/2012 10:39 AM EDT)

Clock tree synthesis (CTS) is at the heart of ASIC design and clock tree network robustness is one of the most important quality metrics of SoC design. With technology advancement happened over the past one and half decade, clock tree robustness has become an even more critical factor affecting SoC performance. Conventionally, engineers focus on designing a symmetrical clock tree with minimum latency and skew. However, with the current complex design needs, this is not enough.

Today, SoCs are designed to support multiple features. They have multiple clock sources and user modes which makes the clock tree architecture complex. Merging test clocking with functional clocking and lower technology nodes adds to this complexity. Due to the increase in derate numbers and additional timing signoff corners, timing margins are shrinking.

To meet the current requirements, designs that are timing friendly are needed and provide minimum power dissipation. This article describes the factors which a designer should consider while defining clock tree architecture. It presents some real design examples that illustrate how current EDA tools or conventional methodologies to design clock trees are not sufficient in all cases. A designer has to understanding the nitty -gritty of clock tree architecture to be able to guide an EDA tool to build a more efficient clock tree. First, the basics of CTS and requirements for good clock tree are presented.

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