Massimo Alioto

Ultra-Low Power VLSI Circuit Design Demystified and Explained: A Tutorial

In this paper, the state of the art in ultra-low power (ULP) VLSI design is presented within a unitary framework for the first time. A few general principles are first introduced to gain an insight into the design issues and the approaches that are specific to ULP systems, as well as to better understand the challenges that have to be faced in the foreseeable future. Intuitive understanding is accompanied by rigorous analysis for each key concept. The analysis ranges from the circuit to the micro-architectural level, and reference is given to process, physical and system levels when necessary. Among the main goals of this paper, it is shown that many paradigms and approaches borrowed from traditional above-threshold low-power VLSI design are actually incorrect. Accordingly, common misconceptions in the ULP domain are debunked and replaced with technically sound explanations.

Analysis and comparison on full adder block in submicron technology

In this paper the main topologies of one-bit full adders, including the most interesting of those recently proposed, are analyzed and compared for speed, power consumption, and power-delay product. The comparison has been performed on two classes of circuits, the former with minimum transistor size to minimize power consumption, the latter with optimized transistor dimension to minimize power-delay product. The investigation has been carried out with properly defined simulation runs on a Cadence environment using a 0.35-/spl mu/m process, also including the parasitics derived from layout. Performance has been also compared for different supply voltage values. Thus design guidelines have been derived to select the most suitable topology for the design features required. This paper also proposes a novel figure of merit to realistically compare n-bit adders implemented as a chain of one-bit full adders. The results differ from those previously published both for the more realistic simulations carried out and the more appropriate figure of merit used. They show that, except for short chains of blocks or for cases where minimum power consumption is desired, topologies with only pass transistors or transmission gates are not attractive. In contrast, the most interesting implementations in terms of trade off between power and delay are the traditional CMOS and mirror topologies. Moreover, the dual-rail domino and the CPL allow the best speed performance.

Understanding DC Behavior of Subthreshold CMOS Logic Through Closed-Form Analysis

In this paper, the DC behavior of subthreshold CMOS logic is analyzed in a closed form for the first time in the literature. To this aim, simplified large-signal and small-signal models of MOS transistors in subthreshold region are first developed. After replacing transistors with these equivalent models, analysis of the main DC parameters of CMOS logic gates is presented. In particular, the change in the DC characteristics shape due to operation at ultra-low voltages is analyzed in detail, evaluating analytically the degradation in the logic swing, the symmetry and the steepness of the transition region, as well as the change in the unity-gain points position. The resulting expressions permit to gain an insight into the basic dependence of DC behavior on design and device parameters. The noise margin is explicitly evaluated and modeled with a very simple expression. Interestingly, analysis shows that the noise margin deviates from the ideal half-swing value by an amount that linearly depends on the logarithm of the pn -ratio. Analysis permits to evaluate the minimum supply voltage that ensures correct operation of CMOS logic (i.e., positive noise margin). Previously proposed rule of thumbs to evaluate minimum voltage are also theoretically justified. Moreover, the impact of pMOS/nMOS unbalancing on DC characteristics is analyzed from a design perspective. Considerations on the impact of process/voltage/temperature variations are also introduced. Results are validated through extensive simulations in a 65-nm CMOS technology.

Understanding the Effect of Process Variations on the Delay of Static and Domino Logic

In this paper, the effect of process variations on delay is analyzed in depth for both static and dynamic CMOS logic styles. Analysis allows for gaining an insight into the delay dependence on fan-in, fan-out, and sizing in sub-100-nm technologies. Simple but reasonably accurate models are derived to capture the basic dependences. The effect of process variations in transistor stacks is analytically modeled and analyzed in detail. The impact of both interdie and intradie variations is evaluated and discussed. Interestingly, the input capacitance of static and dynamic logic is shown to be rather insensitive to variations. The delay variability was also shown to be a weak function of the input rise/fall time and load. Analysis shows that domino logic circuits suffer from a doubled variability as compared to the static CMOS logic style. The positive feedback associated with the keeper transistor is shown to be responsible for the variability increase, which, in turn, limits the speed performance. This adds to the well-known speed degradation due to the current contention associated with the keeper transistor. Monte Carlo simulations on a 90-nm technology, including layout parasitics, are performed to validate the results.

Design strategies for source coupled logic gates

In this paper, a strategy for the design of source-coupled logic (SCL) gates both with and without an output buffer is proposed. Closed-form design equations to size bias currents and transistor aspect ratios to meet assigned specifications are derived from a simple SCL gate analytical delay model, shown to be sufficiently accurate by extensive simulations. The design criteria proposed are simple and provide the designer with a more profound understanding of the tradeoff between delay and power consumption. More specifically, design criteria are derived to consciously manage this tradeoff in practical design cases, i.e., when either high performance or an optimum balance with power dissipation is needed. Therefore, the strategy proposed is useful right from the early design phases, and avoids tedious simulation iterations.

Leakage Power Analysis Attacks: A Novel Class of Attacks to Nanometer Cryptographic Circuits

In this paper, a novel class of power analysis attacks to cryptographic circuits is presented. These attacks aim at recovering the secret key of a cryptographic core from measurements of its static (leakage) power. These attacks exploit the dependence of the leakage current of CMOS integrated circuits on their inputs (including the secret key of the cryptographic algorithm that they implement), as opposite to traditional power analysis attacks that are focused on the dynamic power. For this reason, this novel class of attacks is named ¿leakage power analysis¿ (LPA). Since the leakage power increases much faster than the dynamic power at each new technology generation, LPA attacks are a serious threat to the information security of cryptographic circuits in sub-100-nm technologies. For the first time in the literature, a well-defined procedure to perform LPA attacks that is based on a solid theoretical background is presented. Advantages and measurement issues are also analyzed in comparison with traditional power analysis attacks based on dynamic power measurements. Examples are provided for various circuits, and an experimental attack to a register is performed for the first time. An analytical model of the LPA attack result is also provided to better understand the effectiveness of this technique. The impact of technology scaling is explicitly addressed by means of a simple analytical model and Monte Carlo simulations. Simulations on a 65- and 90-nm technology and experimental results are presented to justify the assumptions and validate the leakage power models that are adopted.

Analysis and Comparison in the Energy-Delay-Area Domain of Nanometer CMOS Flip-Flops: Part I—Methodology and Design Strategies

In this paper (split into Parts I and II), an extensive comparison of existing flip-flop (FF) classes and topologies is carried out. In contrast to previous works, analysis explicitly accounts for effects that arise in nanometer technologies and affect the energy-delay-area tradeoff (e.g., leakage and the impact of layout and interconnects). Compared to previous papers on FFs comparison, the analysis involves a significantly wider range of FF classes and topologies. In particular, in this Part I, the comparison strategy, which includes the simulation setup, the energy-delay estimation methodology, and an overview of an optimum design strategy, together with the introduction of the analyzed FF classes and topologies, are reported.

A Class of Maximum-Period Nonlinear Congruential Generators Derived From the Rényi Chaotic Map

In this paper, a family of nonlinear congruential generators (NLCGs) based on the digitized Reacutenyi map is considered for the definition of hardware-efficient pseudorandom number generators (PRNGs), and a theoretical framework for their study is presented. The authors investigate how the nonlinear structure of these systems eliminates some of the statistical regularities spoiling the randomness of sequences generated with linear techniques. In detail, in this paper, a necessary condition that the considered NLCGs must satisfy to have maximum period length is given, and a list of such maximum period PRNGs for period lengths up to 231-1 is provided. Referring to the NIST800-22 statistical test suite, two PRNG examples are presented and compared to well-known PRNGs based on linear recurrencies requiring a similar amount of resources for their implementation

General Strategies to Design Nanometer Flip-Flops in the Energy-Delay Space

In this paper, a general and complete design flow for nanometer flip-flops (FFs) is presented. The proposed design methodology permits to optimize FFs under constraints within the energy-delay space through extensive adoption of the Logical Effort method, which also allows for defining the bounds in the design space search. Transistors sizing is rigorously discussed by referring to cases that occur in practical designs. Appropriate metrics with clear physical meaning are proposed and various interesting properties are derived from circuit analysis. A well-defined design procedure is derived that can be easily automated with commercial CAD tools. In contrast to previous works, the impact of local interconnections is explicitly accounted for in the design loop, as is required in nanometer CMOS technologies. A case study is discussed in detail to exemplify the application of the proposed methodology. Extensive simulations for a typical FF in a 65-nm CMOS technology are presented to show the whole design procedure and validate the underlying assumptions.

Leakage–Delay Tradeoff in FinFET Logic Circuits: A Comparative Analysis With Bulk Technology

In this paper, we study the advantages offered by multi-gate fin FETs (FinFETs) over traditional bulk MOSFETs when low standby power circuit techniques are implemented. More precisely, we simulated various vehicle circuits, ranging from ring oscillators to mirror full adders, to investigate the effectiveness of back biasing and transistor-stacking in both FinFETs and bulk MOSFETs. The opportunity to separate the gates of FinFETs and to operate them independently has been systematically analyzed; mixed connected- and independent-gate circuits have also been evaluated. The study spans over the device, the layout, and the circuit level of abstraction and appropriate figures of merit are introduced to quantify the potential advantage of different schemes. Our results show that, thanks to a larger threshold voltage sensitivity to back biasing, the FinFET technology is able to offer a more favorable compromise between standby power consumption and dynamic performance and is well suited for implementing fast and energy-efficient adaptive back-biasing strategies.