Clock frequency is a major attribute of any microprocessor design. Early on, during product definition, it constitutes a major business or marketing decision and it is usually the result of a trade-off among customer needs, competitive landscape, and time-to-market. As soon as the frequency target is handed down the food chain to silicon implementation, it will affect all project design aspects from the day that the project is kicked off until it tapes out (and in most cases well beyond this point too).

It is not surprising therefore that the job of generating, distributing, and analyzing the clocks in complex chips is considered to be an important and visible assignment. Clock design has traditionally been an area of innovation and has been in the spotlight in technical conferences and journals. Why is clock frequency such an important microprocessor aspect? For a number of applications it is only loosely correlated with performance with other design aspects such as memory system, parallelism, and hardware acceleration being equally or even more effective. Nevertheless, it is a single number that is widely understood by both technical and nontechnical audiences and in certain situations has strong correlation with single-thread performance.

Clock frequency, although very important, is only one aspect of clock design. Other aspects include power dissipation, efficient clock signal distribution in large and complex chips, coping with variation and uncertainty, managing multiple clock domains in the context of highly integrated system-on-a-chip (SoC) designs, and multicore integration, providing good voltage/frequency scalability to support a wide product roadmap, tuning capabilities for yield enhancement and postsilicon optimization, and sophisticated active power management features. The purpose of this book is to introduce a designer to important aspects of stateof-the-art clock design by exposing methodology steps and analytical modelling techniques, providing design examples and case studies and enumerating a long list of references for further study.