Properly Handing Of Clock Tree Synthesis Specifications

Given today’s design requirements with respect to low power, there is increasing focus on the contribution to total power made by a design’s clock trees. The design decisions made by the front-end team to achieve high performance without wasting power must be conveyed to back-end team. This hand-off must be accurate and complete. A key component of that hand-off is the clock tree synthesis (CTS) constraints.

Let’s look at what can go wrong and how to avoid these pitfalls.The clock trees in chips ten years ago were fairly simple and most chips had only a handful of clock trees. In today’s technologies this has exploded into a forest of clock trees. Sheer volume alone points to the need for automation. But even more daunting are complexities of today’s clock trees. Clock gating has been in use for a while now to aid in reducing power. Included IP blocks will have their own clock requirements. There are generated clocks, overlapping clocks, clock dividers, and on and on. All of this information needs to be packaged by the front-end team into the SDC file and clock specification (clock constraint) file for use by the back-end team.

ICScape’s ClockExplorer tool was developed to provide analysis tools to help both teams understand the entire clock graph being developed. It crosschecks equivalence of constraints generated by front-end and back-end teams. Both teams could use ClockExplorer to analyze and sign-off the netlist and clock constraints. ClockExplorer’s platform checks the clock structure and aids in the generation constraints for a CTS tool, including CTS sequencing for complex situations with multiple SDC files and overlapping clock trees.

If these tasks are done manually by either team, mistakes are much more likely to occur.Beyond the important capabilities of simply generating and checking the constraints, ClockExplorer also optimizes the clock topology to reduce latency. As a visual aid, ClockExplorer also generates a clock schematic, greatly assisting in reviews and discussions between the teams. For a more detailed look at all the analysis features of ClockExplorer, including more details on its SDC constraint checking features, see the white paper.

By using tools such as ICScape’s ClockExplorer, I think that front-end and back-end design teams will be able to cut design errors due to improper understanding of, or generation of, clock tree synthesis constraints. They will have a common view of the clock system, consistent checking and automated generation handling the key aspects of the constraint files. This should make a difficult task much easier and more reliable. Where discrepancies due crop up, the visual aid enabled by the automatic generation of the clock schematics should make debugging and communications between the teams much easier.

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