Mohab Anis

Design and optimization of multithreshold CMOS (MTCMOS) circuits

Reducing power dissipation is one of the most important issues in very large scale integration design today. Scaling causes subthreshold leakage currents to become a large component of total power dissipation. Multithreshold technology has emerged as a promising technique to reduce leakage power. This paper presents several heuristic techniques for efficient gate clustering in multithreshold CMOS circuits by modeling the problem via bin-packing (BP) and set-partitioning (SP) techniques. The SP technique takes the circuits routing complexity into consideration which is critical for deep submicron (DSM) implementations. By applying the techniques to six benchmarks to verify functionality, results obtained indicate that our proposed techniques can achieve on average 84% savings for leakage power and 12% savings for dynamic power. Furthermore, four hybrid clustering techniques that combine the BP and SP techniques to produce a more efficient solution are also devised. Ground bounce was also taken as a design parameter in the optimization problem. While accounting for noise, the proposed hybrid solution achieves on average 9% savings for dynamic power and 72% savings for leakage power dissipation at sufficient speeds and adequate noise margins.

MOS current mode circuits: analysis, design, and variability

The interest in MOS current-mode logic (MCML) is increasing because of its ability to dissipate less power than conventional CMOS circuits at high frequencies, while providing an analog friendly environment. Moreover, automated design methodologies are gaining attention by circuit designers to provide shorter design cycles and faster time to market. This paper provides designers with an insight to the different tradeoffs involved in the design of MCML circuits to efficiently and systematically design MCML circuits. A comprehensive analytical formulation for the design parameters of MCML circuits using the BSIM3v3 model is introduced. In addition, a closed-form expression for the noise margin of two-level MCML circuits is derived. In order to verify the validity of the analytical formulations, an automated design methodology for MCML circuits is proposed to overcome the complexities of the design process. The effectiveness of the design methodology and the accuracy of the analytical formulations are tested by designing several MCML benchmarks built in a 0.18-/spl mu/m CMOS technology. The error in the required performance in the designed circuits is within 11% when compared to HSPICE simulations. A worst case parameter variations modeling is presented to investigate the impact of variations on MCML circuits as well as designing MCML circuits for variability. Finally, the impact of variations on MCML circuits is investigated with technology scaling and different circuit architectures.

Energy-efficient noise-tolerant dynamic styles for scaled-down CMOS and MTCMOS technologies

A new high-speed domino circuit, called HS-Domino has been developed. HS-Domino resolves the tradeoff between performance and reliability in conventional CD-domino logic while dissipating low dynamic power with minimal area overhead. HS-Domino, therefore, extends dominos operation in the deep submicron regime. A multithreshold implementation of HS-Domino is also devised to achieve substantially low leakage values during standby, while maintaining high performance and low power during the active mode. Furthermore, the generic multithreshold scheme is applied to differential cascode voltage switch (DDCVS) logic.

High-speed dynamic logic styles for scaled-down CMOS and MTCMOS technologies

A new high-speed domino circuit, called HS-Domino is developed. HS-Domino resolves the trade-off between performance and noise margins in conventional CD-Domino logic while dissipating low dynamic power with minimal area overhead. A dual-threshold (MTCMOS) implementation of HS-Domino and DDCVS logic is also devised. This implementation achieves low leakage values during standby, while maintaining high performance and low dynamic power during the active mode.

Impact of technology scaling on CMOS logic styles

In this paper, the main challenges of technology scaling are reviewed in depth. Five popular logic families namely; conventional CMOS, complementary pass logic, Domino, differential cascode voltage switch logic, and current mode logic are presented, highlighting their advantages and drawbacks. The behavior of each logic style in deep submicrometer technologies is analyzed and predicted for future technology generations. To verify the qualitative analysis, simulations were performed on the basic logic gates, full adder and a 16-bit carry look ahead adder. The circuits were implemented in 0.8-, 0.6-, 0.35-, and 0.25-/spl mu/m CMOS technologies, and optimized for minimum energy-delay product.

Statistical Design of the 6T SRAM Bit Cell

In this paper, a method for the statistical design of the static-random-access-memory bit cell is proposed to ensure a high memory yield while meeting design specifications for performance, stability, area, and leakage. The method generates the nominal design parameters, i.e., the widths and lengths of the bit-cell transistors, which provide maximum immunity to the variations in a transistors dimensions and intrinsic threshold-voltage fluctuations. Moreover, the need to deviate from the conventional bit-cell sizing strategy to obtain a high-yield low-leakage design in the nanometer regime is demonstrated.

Adaptive Body Bias for Reducing the Impacts of NBTI and Process Variations on 6T SRAM Cells

Reliability and variability have become big design challenges facing submicrometer SRAM designers. A low area overhead adaptive body bias (ABB) circuit is proposed in this paper to compensate for NBTI aging and process variations to improve the SRAM reliability and yield. The proposed ABB circuit consists of a threshold voltage sensing circuit and an on-chip analog controller. Postlayout simulation results, referring to an industrial hardware-calibrated STMicroelectronics 65 nm CMOS technology transistor model, are presented. The transistor model contains process variations and NBTI aging model cards, which are declared by STMicroelectronics to be silicon verified. Cadence RelXpert, Virtuoso Spectre, and Virtuoso UltraSim tools are used to estimate the NBTI aging and process variations impacts on the SRAM array. These results show that the proposed ABB compensates effectively for NBTI aging and process variations. For example, the proposed ABB reduces the read failure probability from 0.32% to 0.05% and the SNM degradation from 10.9% to 2.6% at 10 years aging time. In addition, the proposed ABB enhances the soft errors immunity of the SRAM cell by reducing the critical charge degradation from 12.7% to 3.4% at 10 years aging time.

Self-aligned double-patterning (SADP) friendly detailed routing

Amongst the possible double patterning strategies for sub 32nm processes, self-aligned double patterning (SADP) has moved from Flash-only processes to more general purpose devices. The reason is that while litho-etch- litho-etc (LELE) process was originally preferred due to its simplicity and relative low cost, its sensitivity to overlay error has prompted the search for other methods. Although the basic SADP process is fairly robust against the overlay error, the robustness of 2D SADP method strongly depends on layout and decomposition styles and decomposability compliance. In this paper, we first discuss different printability challenges for SADP method. Afterward, we propose a SADP-aware detailed routing method, by applying a correct-by-construction approach, to provide SADP-friendly layouts. This method performs detailed routing and layout decomposition concurrently to prevent litho-limited layout configurations. Experimental results show that, compared with a SADP-blind detailed router, the proposed method achieves considerable robustness against lithography imperfection in expense of tolerable wire length overhead.

Dynamic Standby Prediction for Leakage Tolerant Microprocessor Functional Units

Leakage power is projected to comprise approximately 50% of the processors power for sub 65 nm technologies. Much of this power is consumed in the processors functional units. Accordingly, leakage control techniques are employed to reduce leakage in these functional units. Many of these techniques are dynamic and are based on an input sleep signal to initiate a low leakage mode. However, since most of these leakage control techniques are based on circuit level schemes, such techniques inherently lack information about the operational profile of the functional units they manage. This limitation is usually handled statically by using a fixed length counter that generates the sleep signal when the functional unit is idle for a specified number of cycles. In this paper, the limitations of the static sleep signal generation approach are identified, and the use of a dynamic alternative that is capable of adopting the counter length to the running application is proposed. In order to assess the accuracy of the proposed dynamic sleep signal generator, the length of the sleep period following the sleep signal generation is used as a metric to identify the usefulness of utilizing the dynamic approach. Experimental results for the dynamic alternative shows up to 98% accuracy in predicting the length of the standby period compared to an average of 40-60% in the static case, which translates into increased leakage savings. This is achieved while consuming 360 muW of overhead power at 1 GHz

A Statistical Design-Oriented Delay Variation Model Accounting for Within-Die Variations

The increase of statistical variations in advanced nanometer CMOS technologies poses a major challenge for digital circuit design. In this paper, we study the impact of random variations on the delay variability of a gate and derive simple and scalable statistical models to effectively evaluate delay variations in the presence of within-die variations. The derived models are verified and compared to Monte Carlo SPICE simulations using industrial 90-nm technology. This paper provides new design insight and highlights the importance of accounting for the effect of input slew on delay variations, particularly at lower supply voltages. We also show that, for a given supply voltage, there is an optimum input slew that minimizes the relative delay variation of the gate. We present conditions to achieve this minimum. The derived analytical models account for the impact of supply voltage and output loading and can be used in early design cycle. These results are particularly important for variation-tolerant design in nanometer technologies, particularly in low-power and low-voltage operation.