Variation Resilient Adaptive Controller for Subthreshold Circuits

Subthreshold logic is showing good promise as a viable ultra-low-power circuit design technique for power-limited applications. For this design technique to gain widespread adoption, one of the most pressing concerns is how to improve the robustness of subthreshold logic to process and temperature variations. We propose a variation resilient adaptive controller for subthreshold circuits with the following novel features: new sensor based on time-to-digital converter for capturing the variations accurately as digital signatures, and an all-digital DC-DC converter incorporating the sensor capable of generating an operating operating Vdd from 0V to 1.2V with a resolution of 18.75mV, suitable for subthreshold circuit operation. The benefits of the proposed controller is reflected with energy improvement of up to 55% compared to when no controller is employed. The detailed implementation and validation of the proposed controller is discussed

Standby Leakage Power Reduction Technique for Nanoscale CMOS VLSI Systems

In this paper, a novel low-power design technique is proposed to minimize the standby leakage power in nanoscale CMOS very large scale integration (VLSI) systems by generating the adaptive optimal reverse body-bias voltage. The adaptive optimal body-bias voltage is generated from the proposed leakage monitoring circuit, which compares the subthreshold current (ISUB) and the band-to-band tunneling (BTBT) current (IBTBT). The proposed circuit was simulated in HSPICE using 32-nm bulk CMOS technology and evaluated using ISCAS85 benchmark circuits at different operating temperatures (ranging from 25°C to 100°C). Analysis of the results shows a maximum of 551 and 1491 times leakage power reduction at 25°C and 100°C, respectively, on a circuit with 546 gates. The proposed approach demonstrates that the optimal body bias reduces a considerable amount of standby leakage power dissipation in nanoscale CMOS integrated circuits. In this approach, the temperature and supply voltage variations are compensated by the proposed feedback loop

A Sub-kT/q Voltage Reference Operating at 150 mV

We propose a subthreshold CMOS voltage reference operating with a minimum supply voltage of only 150 mV, which is three times lower than the minimum value presently reported in the literature. The generated reference voltage is only 17.69 mV. This result has been achieved by introducing a temperature compensation technique that does not require the drain-source voltage of each MOSFET to be larger than 4kT/q. The implemented solution consists in two transistors voltage reference with two MOSFETs of the same threshold-type and exploits the dependence of the threshold voltage on transistor size. Measurements performed over a large sample population of 60 chips from two separate batches show a standard deviation of only 0.29 mV. The mean variation of the reference voltage for VDD ranging from 0.15 to 1.8 V is 359.5 μV/V, whereas the mean variation of VREF in the temperature range from 0°C to 120°C is 26.74 μV/°C. The mean power consumption at 25 °C for VDD = 0.15 V is 26.1 pW. The occupied area is 1200 μm2

A 2x2 bit multiplier using hybrid 13t full adder with vedic mathematics method

Various arithmetic circuits such as multipliers require full adder (FA) as the main block for the circuit to operate. Speed and energy consumption become very vital in design consideration for a low power adder. In this paper, a 2x2 bit Vedic multiplier using hybrid full adder (HFA) with 13 transistors (13T) had been designed successfully. The design was simulated using Synopsys Custom Tools in General Purpose Design Kit (GPDK) 90 nm CMOS technology process. In this design, four AND gates and two hybrid FA (HFAs) are cascaded together and each HFA is constructed from three modules. The cascaded module is arranged in the Vedic mathematics algorithm. This algorithm satisfied the requirement of a fast multiplication operation because of the vertical and crosswise architecture from the Urdhva Triyakbyam Sutra which reduced the number of partial products compared to the conventional multiplication algorithm. With the combination of hybrid full adder and Vedic mathematics, a new combination of multiplier method with low power and low delay is produced. Performance parameters such as power consumption and delay were compared to some of the existing designs. With a 1V voltage supply, the average power consumption of the proposed multiplier was found to be 22.96 µW and a delay of 161 ps

Static noise margin analysis for CMOS logic cells in near-threshold

The advancement of semiconductor technology enabled the fabrication of devices with faster switching activity and chips with higher integration density. However, these advances are facing new impediments related to energy and power dissipation. Besides, the increasing demand for portable devices leads the circuit design paradigm to prioritize energy efficiency instead of performance. Altogether, this scenario motivates engineers towards reducing the supply voltage to the near and subthreshold regime to increase the lifespan of battery-powered devices. Even though operating in these regime offer interesting energy-frequency trade-offs, it brings challenges concerning noise tolerance. As the supply voltage reduces, the available noise margins decrease, and circuits become more prone to functional failures. In addition, near and subthreshold circuits are more susceptible to manufacturing variability, hence further aggravating noise issues. Other issues, such as wire minimization and gate fan-out, also contribute to the relevance of evaluating the noise margin of circuits early in the design Accordingly, this work investigates how to improve the static noise margin of digital synchronous circuits that will operate at the near/subthreshold regime. This investigation produces a set of three original contributions. The first is an automated tool to estimate the static noise margin of CMOS combinational cells. The second contribution is a realistic static noise margin estimation methodology that considers process-voltage-temperature variations. Results show that the proposed methodology allows to reduce up to 70% of the static noise margin pessimism. Finally, the third contribution is the noise-aware cell design methodology and the inclusion of a noise evaluation of complex circuits during the logic synthesis. The resulting library achieved higher static noise margin (up to 24%) and less spread among different cells (up to 62%).Os avanços na tecnologia de semicondutores possibilitou que se fabricasse dispositivos com atividade de chaveamento mais rápida e com maior capacidade de integração de transistores. Estes avanços, todavia, impuseram novos empecilhos relacionados com a dissipação de potência e energia. Além disso, a crescente demanda por dispositivos portáteis levaram à uma mudança no paradigma de projeto de circuitos para que se priorize energia ao invés de desempenho. Este cenário motivou à reduzir a tensão de alimentação com qual os dispositivos operam para um regime próximo ou abaixo da tensão de limiar, com o objetivo de aumentar sua duração de bateria. Apesar desta abordagem balancear características de performance e energia, ela traz novos desafios com relação a tolerância à ruído. Ao reduzirmos a tensão de alimentação, também reduz-se a margem de ruído disponível e, assim, os circuitos tornam-se mais suscetíveis à falhas funcionais. Somado à este efeito, circuitos com tensões de alimentação nestes regimes são mais sensíveis à variações do processo de fabricação, logo agravando problemas com ruído. Existem também outros aspectos, tais como a miniaturização das interconexões e a relação de fan-out de uma célula digital, que incentivam a avaliação de ruído nas fases iniciais do projeto de circuitos integrados Por estes motivos, este trabalho investiga como aprimorar a margem de ruído estática de circuitos síncronos digitais que irão operar em tensões no regime de tensão próximo ou abaixo do limiar. Esta investigação produz um conjunto de três contribuições originais. A primeira é uma ferramenta capaz de avaliar automaticamente a margem de ruído estática de células CMOS combinacionais. A segunda contribuição é uma metodologia realista para estimar a margem de ruído estática considerando variações de processo, tensão e temperatura. Os resultados obtidos mostram que a metodologia proposta permitiu reduzir até 70% do pessimismo das margens de ruído estática, Por último, a terceira contribuição é um fluxo de projeto de células combinacionais digitais considerando ruído, e uma abordagem para avaliar a margem de ruído estática de circuitos complexos durante a etapa de síntese lógica. A biblioteca de células resultante deste fluxo obteve maior margem de ruído (até 24%) e menor variação entre diferentes células (até 62%)

Subthreshold Dual Mode Logic

In this brief, we introduce a novel low-power dual mode logic (DML) family, designed to operate in the subthreshold region. The proposed logic family can be switched between static and dynamic modes of operation according to system requirements. In static mode, the DML gates feature very low-power dissipation with moderate performance, while in dynamic mode they achieve higher performance, albeit with increased power dissipation. This is achieved with a simple and intuitive design concept. SPICE and Monte Carlo simulations compare performance, power dissipation, and robustness of the proposed DML gates to their CMOS and domino counterparts in the 80-nm process. Measurements of an 80-nm test chip are presented in order to prove the proposed concept

A novel deep submicron bulk planar sizing strategy for low energy subthreshold standard cell libraries

Engineering andPhysical Science ResearchCouncil (EPSRC) and Arm Ltd for providing funding in the form of grants and studentshipsThis work investigates bulk planar deep submicron semiconductor physics in an attempt to improve standard cell libraries aimed at operation in the subthreshold regime and in Ultra Wide Dynamic Voltage Scaling schemes. The current state of research in the field is examined, with particular emphasis on how subthreshold physical effects degrade robustness, variability and performance. How prevalent these physical effects are in a commercial 65nm library is then investigated by extensive modeling of a BSIM4.5 compact model. Three distinct sizing strategies emerge, cells of each strategy are laid out and post-layout parasitically extracted models simulated to determine the advantages/disadvantages of each. Full custom ring oscillators are designed and manufactured. Measured results reveal a close correlation with the simulated results, with frequency improvements of up to 2.75X/2.43X obs erved for RVT/LVT devices respectively. The experiment provides the first silicon evidence of the improvement capability of the Inverse Narrow Width Effect over a wide supply voltage range, as well as a mechanism of additional temperature stability in the subthreshold regime. A novel sizing strategy is proposed and pursued to determine whether it is able to produce a superior complex circuit design using a commercial digital synthesis flow. Two 128 bit AES cores are synthesized from the novel sizing strategy and compared against a third AES core synthesized from a state-of-the-art subthreshold standard cell library used by ARM. Results show improvements in energy-per-cycle of up to 27.3% and frequency improvements of up to 10.25X. The novel subthreshold sizing strategy proves superior over a temperature range of 0 °C to 85 °C with a nominal (20 °C) improvement in energy-per-cycle of 24% and frequency improvement of 8.65X. A comparison to prior art is then performed. Valid cases are presented where the proposed sizing strategy would be a candidate to produce superior subthreshold circuits

Subthreshold and gate leakage current analysis and reduction in VLSI circuits

CMOS technology has scaled aggressively over the past few decades in an effort to enhance functionality, speed and packing density per chip. As the feature sizes are scaling down to sub-100nm regime, leakage power is increasing significantly and is becoming the dominant component of the total power dissipation. Major contributors to the total leakage current in deep submicron regime are subthreshold and gate tunneling leakage currents. The leakage reduction techniques developed so far were mostly devoted to reducing subthreshold leakage. However, at sub-65nm feature sizes, gate leakage current grows faster and is expected to surpass subthreshold leakage current. In this work, an extensive analysis of the circuit level characteristics of subthreshold and gate leakage currents is performed at 45nm and 32nm feature sizes. The analysis provides several key observations on the interdependency of gate and subthreshold leakage currents. Based on these observations, a new leakage reduction technique is proposed that optimizes both the leakage currents. This technique identifies minimum leakage vectors for a given circuit based on the number of transistors in OFF state and their position in the stack. The effectiveness of the proposed technique is compared to most of the mainstream leakage reduction techniques by implementing them on ISCAS89 benchmark circuits. The proposed leakage reduction technique proved to be more effective in reducing gate leakage current than subthreshold leakage current. However, when combined with dual-threshold and variable-threshold CMOS techniques, substantial subthreshold leakage current reduction was also achieved. A total savings of 53% for subthreshold leakage current and 26% for gate leakage current are reported

Low energy digital circuits in advanced nanometer technologies

The demand for portable devices and the continuing trend towards the Internet ofThings (IoT) have made of energy consumption one of the main concerns in the industry and researchers. The most efficient way of reducing the energy consump-tion of digital circuits is decreasing the supply voltage (Vdd) since the dynamicenergy quadratically depends onVdd. Several works have shown that an optimumsupply voltage exists that minimizes the energy consumption of digital circuits. This optimum supply voltage is usually around 200 mV and 400 mV dependingon the circuit and technology used. To obtain these low supply voltages, on-chipdc-dc converters with high efficiency are needed.This thesis focuses on the study of subthreshold digital systems in advancednanometer technologies. These systems usually can be divided into a Power Man-agement Unit (PMU) and a digital circuit operating at the subthreshold regime.In particular, while considering the PMU, one of the key circuits is the dc-dcconverter. This block converts the voltage from the power source (battery, supercapacitor or wireless power transfer link) to a voltage between 200 mV and 400mV in order to power the digital circuit. In this thesis, we developed two chargerecycling techniques in order to improve the efficiency of switched capacitors dc-dcconverters. The first one is based on a technique used in adiabatic circuits calledstepwise charging. This technique was used in circuits and applications wherethe switching consumption of a big capacitance is very important. We analyzedthe possibility of using this technique in switched capacitor dc-dc converters withintegrated capacitors. We showed through measurements that a 29% reductionin the gate drive losses can be obtained with this technique. The second one isa simplification of stepwise charging which can be applied in some architecturesof switched capacitors dc-dc converters. We also fabricated and tested a dc-dcconverter with this technique and obtained a 25% energy reduction in the drivingof the switches that implement the converter.Furthermore, we studied the digital circuit working in the subthreshold regime,in particular, operating at the minimum energy point. We studied different modelsfor circuits working in these conditions and improved them by considering thedifferences between the NMOS and PMOS transistors. We obtained an optimumNMOS/PMOS leakage current imbalance that minimizes the total leakage energy per operation. This optimum depends on the architecture of the digital circuitand the input data. However, we also showed that important energy reductionscan be obtained by operating at a mean optimum imbalance. We proposed two techniques to achieve the optimum imbalance. We used aFully Depleted Silicon on Insulator (FD-SOI) 28 nm technology for most of the simulations, but we also show that these techniques can be applied in traditionalbulk CMOS technologies. The first one consists in using the back plane voltage of the transistors (or bulk voltage in traditional CMOS) to adjust independently theleakage current of the NMOS and PMOS transistor to work under the optimum NMOS/PMOS leakage current imbalance. We called this approach the OptimumBack Plane Biasing (OBB). A second technique consists of using the length of the transistors to adjust this leakage current imbalance. In the subthreshold regimeand in advanced nanometer technologies a moderate increase in the length has little impact in the output capacitance of the gates and thus in the dynamic energy.We called this approach an Asymmetric Length Biasing (ALB). Finally, we use these techniques in some basic circuits such as adders. We show that around 50% energy reduction can be obtained, in a wide range of frequency while working near the minimum energy point and using these techniques. The main contributions of this thesis are: • Analysis of the stepwise charging technique in small capacitances. •Implementation of stepwise charging technique as a charge recycling tech-nique for efficiency improvement in switched capacitor dc-dc converters. • Development of a charge sharing technique for efficiency improvement inswitched capacitor dc-dc converters. • Analysis of minimum operating voltage of digital circuits due to intrinsicnoise and the impact of technology scaling in this minimum. • Improvement in the modeling of the minimum energy point while considering NMOS and PMOS transistors difference. • Demonstration of the existence of an optimum leakage current imbalance be-tween the NMOS and PMOS transistors that minimizes energy consumptionin the subthreshold regiion. • Development of a back plane (bulk) voltage strategy for working in this optimum.• Development of a sizing strategy for working in the aforementioned optimum. • Analysis of the impact of architecture and input data on the optimum im-balance. The thesis is based on the publications [1–8]. During the Ph.D. program, other publications were generated [9–16] that are partially related with the thesis butwere not included in it.La constante demanda de dispositivos portables y los avances hacia la Internet de las Cosas han hecho del consumo de energía uno de los mayores desafíos y preocupación en la industria y la academia. La forma más eficiente de reducir el consumo de energía de los circuitos digitales es reduciendo su voltaje de alimentación ya que la energía dinámica depende de manera cuadrática con dicho voltaje. Varios trabajos demostraron que existe un voltaje de alimentación óptimo, que minimiza la energía consumida para realizar cierta operación en un circuito digital, llamado punto de mínima energía. Este óptimo voltaje se encuentra usualmente entre 200 mV y 400 mV dependiendo del circuito y de la tecnología utilizada. Para obtener estos voltajes de alimentación de la fuente de energía, se necesitan conversores dc-dc integrados con alta eficiencia. Esta tesis se concentra en el estudio de sistemas digitales trabajando en la región sub umbral diseñados en tecnologías nanométricas avanzadas (28 nm). Estos sistemas se pueden dividir usualmente en dos bloques, uno llamado bloque de manejo de potencia, y el segundo, el circuito digital operando en la region sub umbral. En particular, en lo que corresponde al bloque de manejo de potencia, el circuito más crítico es en general el conversor dc-dc. Este circuito convierte el voltaje de una batería (o super capacitor o enlace de transferencia inalámbrica de energía o unidad de cosechado de energía) en un voltaje entre 200 mV y 400 mV para alimentar el circuito digital en su voltaje óptimo. En esta tesis desarrollamos dos técnicas que, mediante el reciclado de carga, mejoran la eficiencia de los conversores dc-dc a capacitores conmutados. La primera es basada en una técnica utilizada en circuitos adiabáticos que se llama carga gradual o a pasos. Esta técnica se ha utilizado en circuitos y aplicaciones en donde el consumo por la carga y descarga de una capacidad grande es dominante. Nosotros analizamos la posibilidad de utilizar esta técnica en conversores dc-dc a capacitores conmutados con capacitores integrados. Se demostró a través de medidas que se puede reducir en un 29% el consumo debido al encendido y apagado de las llaves que implementan el conversor dc-dc. La segunda técnica, es una simplificación de la primera, la cual puede ser aplicada en ciertas arquitecturas de conversores dc-dc a capacitores conmutados. También se fabricó y midió un conversor con esta técnica y se obtuvo una reducción del 25% en la energía consumida por el manejo de las llaves del conversor. Por otro lado, estudiamos los circuitos digitales operando en la región sub umbral y en particular cerca del punto de mínima energía. Estudiamos diferentes modelos para circuitos operando en estas condiciones y los mejoramos considerando las diferencias entre los transistores NMOS y PMOS. Mediante este modelo demostramos que existe un óptimo en la relación entre las corrientes de fuga de ambos transistores que minimiza la energía de fuga consumida por operación. Este óptimo depende de la arquitectura del circuito digital y ademas de los datos de entrada del circuito. Sin embargo, demostramos que se puede reducir el consumo de manera considerable al operar en un óptimo promedio. Propusimos dos técnicas para alcanzar la relación óptima. Utilizamos una tecnología FD-SOI de 28nm para la mayoría de las simulaciones, pero también mostramos que estas técnicas pueden ser utilizadas en tecnologías bulk convencionales. La primer técnica, consiste en utilizar el voltaje de la puerta trasera (o sustrato en CMOS convencional) para ajustar de manera independiente las corrientes del NMOS y PMOS para que el circuito trabaje en el óptimo de la relación de corrientes. Esta técnica la llamamos polarización de voltaje de puerta trasera óptimo. La segunda técnica, consiste en utilizar los largos de los transistores para ajustar las corrientes de fugas de cada transistor y obtener la relación óptima. Trabajando en la región sub umbral y en tecnologías avanzadas, incrementar moderadamente el largo del transistor tiene poco impacto en la energía dinámica y es por eso que se puede utilizar. Finalmente, utilizamos estas técnicas en circuitos básicos como sumadores y mostramos que se puede obtener una reducción de la energía consumida de aproximadamente 50%, en un amplio rango de frecuencias, mientras estos circuitos trabajan cerca del punto de energía mínima. Las principales contribuciones de la tesis son: • Análisis de la técnica de carga gradual o a pasos en capacidades pequeñas. • Implementación de la técnica de carga gradual para la mejora de eficiencia de conversores dc-dc a capacitores conmutados. • Simplificación de la técnica de carga gradual para mejora de la eficiencia en algunas arquitecturas de conversores dc-dc de capacitores conmutados. • Análisis del mínimo voltaje de operación en circuitos digitales debido al ruido intrínseco del dispositivo y el impacto del escalado de las tecnologías en el mismo. • Mejoras en el modelado del punto de energía mínima de operación de un circuito digital en el cual se consideran las diferencias entre el transistor PMOS y NMOS. • Demostración de la existencia de un óptimo en la relación entre las corrientes de fuga entre el NMOS y PMOS que minimiza la energía de fugas consumida en la región sub umbral. • Desarrollo de una estrategia de polarización del voltaje de puerta trasera para que el circuito digital trabaje en el óptimo antes mencionado. • Desarrollo de una estrategia para el dimensionado de los transistores que componen las compuertas digitales que permite al circuito digital operar en el óptimo antes mencionado. • Análisis del impacto de la arquitectura del circuito y de los datos de entrada del mismo en el óptimo antes mencionado