Issue No.03 - May/June (2005 vol.22)
Published by the IEEE Computer Society
Sachin Sapatnekar , University of Minnesota
DOI Bookmark: http://doi.ieeecomputersociety.org/10.1109/MDT.2005.58
In the old days, when so-called portable phones looked like bricks and weighed as much, power considerations were just an afterthought during design. With the scaling of device technologies and higher levels of integration, power has emerged as a first-class citizen of importance, receiving due deference throughout the design process. This has led to a large volume of research in the past decade, and the authors of Low Voltage, Low Power VLSI Subsystems (McGraw-Hill, 2004, ISBN 0-07-143786-X, 288 pp., $130), Kiat-Seng Yeo and Kaushik Roy, are recognized experts who have been at the vanguard of these efforts.
The book ambitiously attempts to present a picture of design choices right from the device level to the subsystem level, and it is a recommended read for any digital IC designer. It covers material ranging from basic to advanced, and, as with any other book, the reader might be able to skim over some parts more quickly than others, depending on his or her expertise. This is strictly a design book, and although it sensibly does not overreach itself by also covering CAD techniques in detail, even the CAD-oriented reader is likely to find this book useful for the background and problem statements it provides.
The book begins with a comprehensive overview of the fundamentals of power dissipation in circuits and a detailed discussion of power sources. In addition to detailing the effects of dynamic and short-circuit power, which have been the dominant modes in the past decade, the authors place special emphasis on the mechanisms of subthreshold and gate leakage, including channel engineering techniques to reduce these effects. This discussion is particularly relevant to contemporary cutting-edge designs and to future technologies, because trends in sub-100-nm designs show that leakage will become an increasingly large fraction of the overall power dissipation. Next, Chapter 2 describes circuit techniques for building low-power designs, such as transistor stacking, static and dynamic threshold voltage adjustment, and dynamic supply voltage usage. The chapter also provides an overview of techniques for reducing power dissipation in caches.
The next two chapters discuss the design of specific circuit structures, namely adders and multipliers. Chapter 3 describes adder structures such as ripple carry, carry look-ahead, carry skip, and conditional sum, as well as current-mode adders using multilevel logic. In addition to static CMOS, the chapter also discusses domino and pass-transistor styles, and BiCMOS implementations. Chapter 4 presents multiplier architectures such as Braun, Baugh-Wooley, Booth, and Wallace tree. Although both chapters explain the trade-offs between power and speed in the design of these structures, the focus is on dynamic power. Further discussion of leakage issues in these contexts would have been welcome.
The succeeding three chapters describe various aspects of memory design in detail. Chapter 5 presents recipes for building low-voltage, low-power ROMs, discussing the physical principles that underpin the operation of these devices, the large arrays in which these devices are connected, and the techniques used for circuit design. Chapters 6 and 7 describe methods for SRAMs and DRAMs, respectively, with detailed discussions on the transistor-level design of the subblocks that constitute these structures. In a very natural manner, the authors report the evolution of these designs to current-day styles. Finally, Chapter 8 discusses issues in the design of low-power subsystems, using a finite-length impulse response filter as an illustrative example.
In summary, this book is an excellent source for readers, detailing techniques for low-power design through various examples. At the macroscopic as well as the microscopic levels, it "works" because the authors were mindful of a well-knit overarching theme while writing each chapter. For the graduate student or the novice, it forms a valuable learning resource, and for the experienced designer, it presents a view of the new problems that will arise in nanometer-scale designs, along with a set of solutions. The bibliography is extensive, providing the curious reader with a profusion of avenues for further exploration. This book is a very good overview of low-power design, and I consider it a wonderful addition to my bookshelf.