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Minimum Energy CMOS Design with Dual Subthreshold Supply and Multiple LogicLevel Gates
"... Abstract—This paper presents a method for minimum energy digital CMOS circuit design using dual subthreshold supply voltages. Stringent energy budget and moderate speed requirements of some ultra low power systems may not be best satisfied just by scaling a single supply voltage. Optimized circuits ..."
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Abstract—This paper presents a method for minimum energy digital CMOS circuit design using dual subthreshold supply voltages. Stringent energy budget and moderate speed requirements of some ultra low power systems may not be best satisfied just by scaling a single supply voltage. Optimized circuits with dual supply voltages provide an opportunity to resolve these demands. The delay penalty of a traditional level converter is unacceptably high when the voltages are in the subthreshold range. In the present work level converters are not used and special multiple logiclevel gates are used only when, after accounting for their cost, they offer advantage. Starting from a lowest per cycle energy design whose single supply voltage is in the subthreshold range, a new mixed integer linear program (MILP) finds a second lower supply voltage optimally assigned to gates with time slack. The MILP accounts for the energy and delay characteristics of logic gates interfacing two different signal levels. New types of linearized AND and OR constraints are used in this MILP. We show energy saving up to 24.5 % over the best available designs of ISCAS’85 benchmark circuits. Keywords — Ultralow power design, Subthreshold circuits, Dual voltage design, Mixed integer linear program.
Subthreshold Voltage Highk CMOS Devices Have Lowest Energy and High Process Tolerance
"... Abstract — Evolving nanometer CMOS technologies provide low power, high performance and higher levels of integration but suffer from increased subthreshold leakage and excessive process variation. The present work examines the 45nm bulk and highk technologies. We evaluate the performance of a 32bi ..."
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Abstract — Evolving nanometer CMOS technologies provide low power, high performance and higher levels of integration but suffer from increased subthreshold leakage and excessive process variation. The present work examines the 45nm bulk and highk technologies. We evaluate the performance of a 32bit ripplecarry adder circuit for the entire range of supply voltages over which it displays correct function. Lowering voltage increases delay, reducing the maximum clock cycle rate. We use the maximum permissible clock rate and the energy per cycle at that clock rate as two performance criteria. The minimum energy per cycle operation occurs at a subthreshold voltage. For minimum energy, the bulk technology has a very low performance (~7 MHz). However, highk technology works at a much higher 250 MHz clock. Faster clock rate reduces the leakage energy making highk almost twice as energy efficient compared to bulk. The energy per cycle versus supply voltage is a Ushaped curve whose bottom, the minimum energy point, provides a stable equilibrium against speed and energy deviations due to process related parametric variations for different technologies. These deviations can be expected to be lower for high k technology compared to those circuits designed in bulk technology that are commonly in use. These deviations are also lower compared to those at higher supply voltages that are commonly in use. Although we expect the clock rate to further improve and energy per cycle to reduce for 32 nm and finer technologies, some projections indicate that energy per cycle could increase with a move towards finer technologies. However, those studies were conducted on bulk technologies and further investigation should ascertain the performance of the highk technology. Keywords – Lowpower circuits, subthreshold voltage operation, nanometer CMOS devices, highk CMOS technology, process variation.
Managing Performance and Efficiency of a Processor
, 2012
"... Voltage scaling has been a very popular low power design methodology in the industry. Having a quadratic relationship with the power consumed, lowering the supply voltage reduces the supply voltage considerably. But this mars the overall performance of the circuit. Optimizing performance and power s ..."
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Voltage scaling has been a very popular low power design methodology in the industry. Having a quadratic relationship with the power consumed, lowering the supply voltage reduces the supply voltage considerably. But this mars the overall performance of the circuit. Optimizing performance and power simultaneously requires a thorough study of the available resources and tradeoffs possible. This project looks into one of the most important areas of contemporary research in electrical and computer engineering: energy efficiency [11, 19, 42]. Power and Performance are two conflicting goals a designer has to achieve [25]. With a number of performance oriented devices emerging with a huge demand of power from a fixed capacity battery, using the battery wisely becomes important. This project investigates the existing and suggests a new metric which can be considered while deciding upon the working conditions of the processor for optimal energy efficiency. Performance of a processor generally refers to its time efficiency. For given architecture, hardware and software, clock frequency (f), i.e., cycles per second (Hz), or cycle rate is the rate of computational work measured in clock cycles done per unit time. In a similar way, we introduce a new measure, cycle efficiency (η) as cycles per joule, or the rate of computational work per unit energy. Characterization of
Dual Voltage Design for Minimum Energy Using
"... Abstract—This paper presents a new slacktime based algorithm for dual Vdd design to achieve maximum energy saving. Although a global optimum is sought computation time is kept low. The slack of a gate is defined as the difference between the critical path delay for the circuit and the delay of the ..."
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Abstract—This paper presents a new slacktime based algorithm for dual Vdd design to achieve maximum energy saving. Although a global optimum is sought computation time is kept low. The slack of a gate is defined as the difference between the critical path delay for the circuit and the delay of the longest path through that gate. A lineartime algorithm is used for computing slacks for all gates of the circuit. Positive nonzero slack gates are classified into two groups, one in which all gates can be unconditionally assigned low voltage and the other where only a selected subset can be assigned low voltage without violating the positive nonzero slack requirement. Multiple voltage boundaries are given special consideration. The overall complexity of this power optimization algorithm is linear in number of gates as compared to a previously published exponentialtime exact algorithm using mixed integer linear program (MILP). We apply the new algorithm to optimize ISCAS’85 benchmark circuits and compare the results with those from MILP. We avoid the use of level converters at multiple voltage boundaries. Energy savings from the new slacktime based algorithm is very closed to those from MILP. For c880, the energy saving is 22 % for subthreshold voltage operation and 50 % for nominal operation in PTM CMOS 90nm. For c2670 nominal voltage design, time of dual voltage optimization is reduced 44X compared to the MILP method. This new algorithm is beneficial for a large circuits with many large positive slack paths that would require enormous time for optimization by the MILP approach. I.
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, 2011
"... With transistor sizes being reduced to sub 45nm ranges, we have seen an improvement in speed, better performance, and deeper integration of digital circuits. However, there has been a corresponding increase in power consumption, along with greater energy dissipation. The reason is because of increas ..."
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With transistor sizes being reduced to sub 45nm ranges, we have seen an improvement in speed, better performance, and deeper integration of digital circuits. However, there has been a corresponding increase in power consumption, along with greater energy dissipation. The reason is because of increased leakage current in the channel. A proposed solution is a shift towards highk materials and metal gate from polysilicon gate of yesteryear. Reduced feature sizes also suffer from greater parametric process variations during lithography and cause identical circuits to behave differently. With highk technology overshadowing bulk technology ever since transistor sizes hit 45nm, a greater understanding of how the properties of highk technology will affect digital devices especially their speed, power consumption, and energy dissipated upon voltage scaling is needed. Also, a better estimation of effects of parametric variations on circuits designed in highk technology can provide valuable information which can be used to improve current designs.
Ultra Low Energy CMOS Logic Using BelowThreshold DualVoltage Supply
, 2011
"... This paper investigates subthreshold voltage operation of digital circuits. Starting from the previously known single supply voltage for minimum energy per cycle, we further lower the energy consumption by using dual subthreshold supplies. Level converters, commonly used in the above threshold desig ..."
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This paper investigates subthreshold voltage operation of digital circuits. Starting from the previously known single supply voltage for minimum energy per cycle, we further lower the energy consumption by using dual subthreshold supplies. Level converters, commonly used in the above threshold design, are found to be unacceptably slow for subthreshold voltage operation. Therefore, special constraints are used to eliminate level converters. We give a new mixed integer linear program (MILP) that automatically and optimally assigns gate voltages, avoids the use of level converters, and holds the minimum critical path delay, while minimizing the total energy per cycle. Using examples of a 16bit ripplecarry adder and a 4 × 4 multiplier we show energy savings of 23 % and 5%, respectively. The latter is a worst case example because most paths are critical. Alternatively, for the same energy as that of single belowthreshold supply, an optimized dual voltage design can operate at 3 to 4 times higher clock rate. Also, we show energy saving up to 22.2 % from the minimum energy point over ISCAS’85 benchmark circuits. The MILP optimization with special consideration for level converters is general and applicable to any supply voltage range.