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Oral Defense Abstracts

Date: Tuesday, May 6th, 2003
Time: 9:00am (refreshments 8:45am)
Location: Packard #101

Special University Ph.D. Oral Examination

Measurement, Modeling, and Control of Variable Vdd/Vth CMOS Systems
Vjekoslav Svilan
 Department of Electrical Engineering, Stanford University

The performance of many modern VLSI applications is limited by power constraints rather than maximum achievable performance levels. For many circuit styles, reducing the supply voltage (Vdd) dramatically reduces power dissipation. In order to maintain performance with lower supply voltages, however transistor threshold voltages (Vth) must also to be lowered, resulting in increased leakage power. Interestingly, there exist optimum supply and threshold voltages which satisfy performance demands while minimizing total power dissipation. These optimum voltages vary dynamically and are dependent on parameters such as circuit activity, logic depth, temperature, clock frequency, local and global transistor variations, and other device characteristics.

Traditional, standard CMOS circuits operate at fixed supply and threshold voltages. More recently, variable supply voltage CMOS designs have been introduced. Variable supply, body-bias tunable Vth CMOS technology enables adjustments of both the supply and threshold voltages. In this dissertation we measure and model advantages of variable Vdd, body-bias tunable Vth CMOS systems and design control mechanisms necessary to put those systems into their most optimum energy state.

Comparison of measured energy and performance of an existing 32-bit multiplier design fabricated in both standard CMOS and in a low-Vth body bias tunable CMOS technology provides evidence that low power designs employing variable Vdd and Vth are feasible. In 0.35 um technology, the tunable design can be optimized to achieve a better than 5 times reduction in power dissipation over the standard CMOS implementation, and is able to operate with up to a 38% faster clock rate. In addition, a new modeling tool enables quick design comparisons between various Vdd/Vth techniques. This model compares four design approaches and accounts for various inputs and effects such as average logic depth, circuit activity, local and global variations and temperature. Finally, four alternative control approaches which dynamically adjust both the supply and body bias voltages seek optimum balance of active and leakage power by taking full advantage of variable supply, low Vth body-bias tunable CMOS technology. These four control mechanisms use either a critical path replica or embedded speed sensors to track the circuit speed and either an Ion/Ioff circuit or well voltage perturbation to adjust the transistor threshold voltages. Two of the four achieve near-optimum energy performance. We show that the control mechanism using embedded speed sensors in combination with the well voltage perturbation technique is the most energy efficient, but also the most difficult to implement.

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