http://ieeexplore.ieee.org TOC Alert for Publication# 5 2010 February 09 98 2 C1 C1 411 98 2 C2 C2 61 98 2 133 134 212 98 2 135 138 252 98 2 139 143 160 In the last couple of decades, handheld wireless devices such as cell phones have become one of the most prolific electronic devices in history. With this has come an exploding demand for performance and features that cover almost every aspect of our digital multimedia interconnected lives including 3-D gaming, still and video cameras, WAN, Bluetooth, high-speed data connections, and so on. As ever increasing features continue to be integrated into these products, there is an ongoing need to develop innovative ways to reduce power consumption and extend battery life. Only through continual process and circuit cooptimization are we able to reap the benefits of technology scaling required to meet the feature and performance demands in the face of increasing process variations and exponentially increasing leakage currents. As a result, SmartReflex power and performance technologies have been developed and applied to 90 nm, 65 nm, and 45 nm system-on-chip (SoC), to achieve optimal power and performance. SmartReflex technologies consist of two major components to optimize SoC power and performance: static and dynamic techniques. Static techniques like power-gating, retention and off-mode are used to lower leakage and allow for extended battery lifetimes for standby times. Dynamic techniques such as dynamic power switching, adaptive voltage scaling, dynamic voltage/frequency scaling with split-rail memories, and adaptive body-biasing address active power and performance challenges. These techniques enable SoC solutions with the performance of the latest process technology and provide the user with advanced multimedia features with orders of magnitude of power reduction. ]]> 98 2 144 159 2561 Ultralow-power dissipation can be achieved by operating digital circuits with scaled supply voltages, albeit with degradation in speed and increased susceptibility to parameter variations. However, operating digital logic and memory circuits in the subthreshold region (supply voltage less than the transistor threshold voltage) for ultralow-power operations requires device, circuit as well as architectural design optimizations, different from the conventional superthreshold design. This paper analyzes such optimizations from energy dissipation point of view and shows that it is feasible to achieve robust operation of ultralow-voltage systems. Operation with power supply as low as 60 mV is demonstrated. Techniques to reduce the impact of process variations on subthreshold circuits are also discussed. In addition, it is shown that subthreshold leakage current can be useful for other applications like thermal sensors. ]]> 98 2 160 190 3780 Energy efficiency of electronic circuits is a critical concern in a wide range of applications from mobile multimedia to biomedical monitoring. An added challenge is that many of these applications have dynamic workloads. To reduce the energy consumption under these variable computation requirements, the underlying circuits must function efficiently over a wide range of supply voltages. This paper presents voltage-scalable circuits such as logic cells, SRAMs, ADCs, and dc–dc converters. Using these circuits as building blocks, two different applications are highlighted. First, we describe an H.264/AVC video decoder that efficiently scales between QCIF and 1080p resolutions, using a supply voltage varying from 0.5 V to 0.85 V. Second, we describe a 0.3 V 16-bit microcontroller with on-chip SRAM, where the supply voltage is generated efficiently by an integrated dc–dc converter. ]]> 98 2 191 214 2987 After decades of continuous scaling, further advancement of silicon microelectronics across the entire spectrum of computing applications is today limited by power dissipation. While the trade-off between power and performance is well-recognized, most recent studies focus on the extreme ends of this balance. By concentrating instead on an intermediate range, an $sim!hbox{8}times$ improvement in power efficiency can be attained without system performance loss in parallelizable applications—those in which such efficiency is most critical. It is argued that power-efficient hardware is fundamentally limited by voltage scaling, which can be achieved only by blurring the boundaries between devices, circuits, and systems and cannot be realized by addressing any one area alone. By simultaneously considering all three perspectives, the major issues involved in improving power efficiency in light of performance and area constraints are identified. Solutions for the critical elements of a practical computing system are discussed, including the underlying logic device, associated cache memory, off-chip interconnect, and power delivery system. The IBM Blue Gene system is then presented as a case study to exemplify several proposed directions. Going forward, further power reduction may demand radical changes in device technologies and computer architecture; hence, a few such promising methods are briefly considered. ]]> 98 2 215 236 2318 Operation in the subthreshold region most often is synonymous to minimum-energy operation. Yet, the penalty in performance is huge. In this paper, we explore how design in the moderate inversion region helps to recover some of that lost performance, while staying quite close to the minimum-energy point. An energy-delay modeling framework that extends over the weak, moderate, and strong inversion regions is developed. The impact of activity and design parameters such as supply voltage and transistor sizing on the energy and performance in this operational region is derived. The quantitative benefits of operating in near-threshold region are established using some simple examples. The paper shows that a 20% increase in energy from the minimum-energy point gives back ten times in performance. Based on these observations, a pass-transistor based logic family that excels in this operational region is introduced. The logic family operates most of its logic in the above-threshold mode (using low-threshold transistors), yet containing leakage to only those in subthreshold. Operation below minimum-energy point of CMOS is demonstrated. In leakage-dominated ultralow-power designs, time-multiplexing will be shown to yield not only area, but also energy reduction due to lower leakage. Finally, the paper demonstrates the use of ultralow-power design techniques in chip synthesis. ]]> 98 2 237 252 1542 Power has become the primary design constraint for chip designers today. While Moore's law continues to provide additional transistors, power budgets have begun to prohibit those devices from actually being used. To reduce energy consumption, voltage scaling techniques have proved a popular technique with subthreshold design representing the endpoint of voltage scaling. Although it is extremely energy efficient, subthreshold design has been relegated to niche markets due to its major performance penalties. This paper defines and explores near-threshold computing (NTC), a design space where the supply voltage is approximately equal to the threshold voltage of the transistors. This region retains much of the energy savings of subthreshold operation with more favorable performance and variability characteristics. This makes it applicable to a broad range of power-constrained computing segments from sensors to high performance servers. This paper explores the barriers to the widespread adoption of NTC and describes current work aimed at overcoming these obstacles. ]]> 98 2 253 266 1115 Subthreshold digital circuits minimize energy per operation and are thus ideal for ultralow-power (ULP) applications with low performance requirements. However, a large range of ULP applications continue to face performance constraints at certain times that exceed the capabilities of subthreshold operation. In this paper, we give two different examples to show that designing flexibility into ULP systems across the architecture and circuit levels can meet both the ULP requirements and the performance demands. Specifically, we first present a method that expands on ultradynamic voltage scaling (UDVS) to combine multiple supply voltages with component level power switches to provide more efficient operation at any energy-delay point and low overhead switching between points. This system supports operation across the space from maximum performance, when necessary, to minimum energy, when possible. It thus combines the benefits of single-${V}_{rm DD}$ , multi-${V}_{rm DD}$, and dynamic voltage scaling (DVS) while improving on them all. Second, we propose that reconfigurable subthreshold circuits can increase applicability for ULP embedded systems. Since ULP devices conventionally require custom circuit design but the manufacturing volume for many ULP applications is low, a subthreshold field programmable gate array (FPGA) offers a cost-effective custom solution with hardware flexibility that makes it applicable across a wide range of applications. We describe the design of a subthreshold FPGA to support ULP operation and identify key challenges to this effort. ]]> 98 2 267 282 1936 Discrete and embedded nonvolatile memory (NVM) technologies have been an integral part of electronic systems for the past 25 years. In recent years, the proliferation of personal media devices such as multimedia-enabled cell phones, personal music players, and digital cameras has accelerated the adoption of silicon-based solid state storage cards in consumer markets. Despite the expanded use of nonvolatile memory technologies in a variety of integrated systems, little has changed with respect to the core technology and cells that hold the data when power has been turned off. Today, floating gate (FG) or oxide-nitride-oxide trapped charge (ONO) cell structures dominate as the core technology behind all NVM devices and embedded blocks. All of the nonvolatile memory devices in production today based on these technologies require high voltage in excess of 5–8 V to operate primarily due to the fundamental nature of core cells and the physics of charge storage mechanisms. These are huge overvoltage requirements considering that the transistors in the logic block require substantially lower voltages (e.g., sub-65 nm logic CMOS operate at less than 1 V). Integrating such high-voltage operation in advanced logic processes such as 65 nm or below logic CMOS process is yet another challenge limiting the exploitation of NVM for low-power embedded applications. The high voltage requirement for operation of these core cells has put strains on the continued scaling of today's discrete and embedded NVM technologies. Furthermore, future ultralow-power and subthreshold CMOS applications such as energy starved electronics require operations at sub-500 mV which clearly set forth significant challenges in integrating today's NVM technologies as nonvolatile storage elements for such systems. Several emerging technologies are competing to become the building blocks of next-generation nonvolatile memory solutions. Each of these emerging technologies has unique characteristi- cs in terms of physical scaling, voltage scaling, cost, performance, and power features which differ from today's FG and ONO based technologies. This paper reviews the fundamental characteristics of current nonvolatile memory technologies as well as several promising emerging technologies from energy and power perspectives and specifically discusses the suitability of each one for use in ultralow-power and subthreshold CMOS applications. ]]> 98 2 283 298 2368 Energy performance requirements are causing designers of next-generation systems to explore approaches to lowest possible power consumption. Subthreshold operation is being examined to stretch low-power circuit designs beyond the normal modes of operation, with the potential for large energy savings. Some of the challenges to be overcome, like 10–100$times$ performance penalties, are being addressed by research into parallelism. However, the uncertainty in timing generated by operating in subthreshold represents a major challenge to overcome. In this paper, first, we will introduce some background information on digital logic subthreshold operation, then provide some background on clockless logic design approaches giving a brief overview of some of the characteristics of the different design styles and focusing on NULL convention logic. Next, we will examine the application of that clockless logic approach to a military system, reviewing the background of the experiment, factors considered in the comparison, and then summarizing the results of the comparisons. Finally, an overview of additional research and development that will be needed to make the technique available to subthreshold designers is presented. ]]> 98 2 299 314 1632 Since the first demonstration of a comparator-based switched-capacitor circuit, analog-to-digital (A/D) converters based on virtual ground detection have made steady and significant progress. Comparators have been replaced by zero-crossing detectors, leading to the development of zero-crossing based circuits for faster speed and lower power. All facets of performance including the sampling rate, effective number of bits, noise floor, and figure-of-merit have improved substantially. This paper focuses on recent implementations of zero-crossing based A/D converters and discusses the technical issues unique to these A/D converters as well as solutions that have been developed to improve their performance and practicality. A series of prototype designs whose performance ranges from 8 bit, 200 MS/s to 12 bit, 50 MS/s are described. The ultimate low power potentials of these A/D converters are compared with various different types of complementary metal–oxide–semiconductor A/D converters from a fundamental thermal noise standpoint. ]]> 98 2 315 332 3503 Ultralow-power electronics will expand the technological capability of handheld and wireless devices by dramatically improving battery life and portability. In addition to innovative low-power design techniques, a complementary process technology is required to enable the highest performance devices possible while maintaining extremely low power consumption. Transistors optimized for subthreshold operation at 0.3 V may achieve a 97% reduction in switching energy compared to conventional transistors. The process technology described in this article takes advantage of the capacitance and performance benefits of thin-body silicon-on-insulator devices, combined with a workfunction engineered mid-gap metal gate. ]]> 98 2 333 342 1302 98 2 343 345 513 98 2 346 347 118 98 2 348 348 324 98 2 C3 C3 488 98 2 C4 C4 415