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U. Glaeser

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Two generations of clock-powered microprocessors were presented in this article and compared against<br />

an equivalent standard CMOS design. For both processors, the results indicated that a small percentage<br />

of nodes (i.e., 5–10%) contributed most of the dynamic power dissipation (i.e., 80–90%) when operated<br />

in conventional mode. Compared to the standard CMOS design, the improved second-generation clockpowered<br />

microprocessor would dissipate approximately 40% less energy per cycle, assuming 85% efficiency<br />

of the clock driver.<br />

DC1 is powered from a single supply voltage. Typically, microprocessors contain a few circuit critical<br />

paths. If such a system is powered from a single supply voltage, voltage scaling cannot provide a nearoptimum<br />

dissipation versus speed trade-off, because the voltage level is determined by the few critical<br />

paths. The noncritical paths would switch faster than they absolutely need to. If a second dc supply<br />

voltage was used to power the high-capacitance nodes in conjunction with low-voltage-swing drivers<br />

[33], the DC1 dissipation would be decreased at the expense of reducing its clock frequency; however,<br />

clock-powered logic is inherently a multiple-supply-voltage system. As the driver experiment showed in<br />

section 21.6, energy dissipation for clock-powered nodes scales better than a dual-supply-voltage<br />

approach, since both the clock voltage swing and the switching time can be scaled. Another low-power<br />

approach is to dynamically adjust the system dc supply voltage and clock frequency based on performance<br />

demands [34]. Such a system resembles clock-powered logic. Voltage is dynamically varied to different<br />

constant dc levels, whereas in clock-powered logic, the supply voltage itself is statically time-varying, i.e.,<br />

every cycle, it switches between 0 and V ϕ.<br />

Clock-powered logic is a low-power approach that does not rely solely on low-voltage operation.<br />

Therefore, it can be applied in CMOS processes without the need of low-threshold transistors that result<br />

in excessive leakage dissipation. Furthermore, the availability of high-energy signaling in clock-powered<br />

logic offers better noise immunity compared to low-voltage approaches.<br />

To summarize, three conditions must be satisfied for applying clock-powered logic to a low-power<br />

system. First, the system should contain a small percentage of high-capacitance nodes with moderate to<br />

high switching activity. Second, the system should contain a few critical and many time-relaxed circuit<br />

paths. Third, the switching time of clock-powered nodes should be longer than the minimum obtainable<br />

switching time from the technology process. Controlling the speed of the energy transport to clockpowered<br />

nodes results in less energy dissipation. If the longer switching time of clock-powered nodes is<br />

made to be an explicit system-design consideration, conventional switching techniques can be used for<br />

nodes in critical paths while the other high-capacitance nodes are clock-powered.<br />

Acknowledgments<br />

This work would not have been completed without contributions from many individuals from the<br />

ACMOS group at University of Southern California Information Sciences Institute over the last decade.<br />

Drs. Svensson, Koller, and Peterson helped in numerous aspects of the project. X.-Y. Jiang, H. Li, P. Wang,<br />

and W.-C. Liu contributed to the physical design of AC-1. Dr. Mao, W.-C. Liu, R. Lal, K. Chong, and<br />

J.-S. Moon were also members of the MD1 and DC1 design teams. The author is grateful to Fujitsu<br />

Laboratories of America for providing the time to write this manuscript and to B. Walker for reviewing it.<br />

PowerMill and Design Compiler were provided by the Synopsys Corporation, HSPICE was provided<br />

by Avant!, and the Epoch place and route synthesis tool was provided by Duet Technologies.<br />

References<br />

1. Chandrakasan, A. P. and Brodersen, R. W., Low Power Digital CMOS Design, Kluwer Academic<br />

Publishers, Norwell, 1995.<br />

2. Gelsinger, P. P., Microprocessors for the new millenium: challenges, opportunities, and new frontiers,<br />

in Proc. Int. Solid-State Circuits Conference, San Francisco, 2001, 22.<br />

3. Athas, W. C., Svensson, L. J., Koller, J. G., Tzartzanis, N., and Chou, E., Low-power digital systems<br />

based on adiabatic-switching principles, IEEE Transactions on VLSI Systems, 2, 398, 1994.<br />

© 2002 by CRC Press LLC

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