Dinesh Somasekhar

Models and algorithms for bounds on leakage in CMOS circuits

Subthreshold leakage current in deep submicron MOS transistors is becoming a significant contributor to power dissipation in CMOS circuits as threshold voltages and channel lengths are reduced. Consequently, estimation of leakage current and identification of minimum and maximum leakage conditions are becoming important, especially in low power applications. In this paper we outline methods for estimating leakage at the circuit level and then propose heuristic and exact algorithms to accomplish the same task for random combinational logic. In most cases the heuristic is found to obtain bounds on leakage that are close and often identical to bounds determined by a complete branch and bound search. Methods are also demonstrated to show how estimation accuracy can be traded off against execution time. The proposed algorithms have potential application in power management applications or quiescent current (I/sub D/DQ) testing if one wished to control leakage by application of appropriate input vectors. For a variety of benchmark circuits, leakage was found to vary by as much as a factor of six over the space of possible input vectors.

Adaptive Frequency and Biasing Techniques for Tolerance to Dynamic Temperature-Voltage Variations and Aging

Temperature, voltage, and current sensors monitor the operation of a TCP/IP offload accelerator engine fabricated in 90nm CMOS, and a control unit dynamically changes frequency, voltage, and body bias for optimum performance and energy efficiency. Fast response to droops and temperature changes is enabled by a multi-PLL clocking unit and on-chip body bias. Adaptive techniques are also used to compensate performance degradation due to device aging, reducing the aging guardband.

Leakage control with efficient use of transistor stacks in single threshold CMOS

The state dependence of leakage can be exploited to obtain modest leakage savings in complementary metal-oxide-semiconductor (CMOS) circuits. However, one can modify circuits considering state dependence and achieve larger savings. We identify a low-leakage state and insert leakage-control transistors only where needed. Leakage levels are on the order of 35% to 90% lower than those obtained by state dependence alone. Using a modified standard-cell-design flow, area overhead for combinational logic was found to be on the order of 18%. The proposed technique minimizes performance impact, does not require multiple-threshold voltages, and supports a standard-cell-design flow.

Reducing cache power with low-cost, multi-bit error-correcting codes

Technology advancements have enabled the integration of large on-die embedded DRAM (eDRAM) caches. eDRAM is significantly denser than traditional SRAMs, but must be periodically refreshed to retain data. Like SRAM, eDRAM is susceptible to device variations, which play a role in determining refresh time for eDRAM cells. Refresh power potentially represents a large fraction of overall system power, particularly during low-power states when the CPU is idle. Future designs need to reduce cache power without incurring the high cost of flushing cache data when entering low-power states. In this paper, we show the significant impact of variations on refresh time and cache power consumption for large eDRAM caches. We propose Hi-ECC, a technique that incorporates multi-bit error-correcting codes to significantly reduce refresh rate. Multi-bit error-correcting codes usually have a complex decoder design and high storage cost. Hi-ECC avoids the decoder complexity by using strong ECC codes to identify and disable sections of the cache with multi-bit failures, while providing efficient single-bit error correction for the common case. Hi-ECC includes additional optimizations that allow us to amortize the storage cost of the code over large data words, providing the benefit of multi-bit correction at same storage cost as a single-bit error-correcting (SECDED) code (2% overhead). Our proposal achieves a 93% reduction in refresh power vs. a baseline eDRAM cache without error correcting capability, and a 66% reduction in refresh power vs. a system using SECDED codes.

Design space and scalability exploration of 1T-1STT MTJ memory arrays in the presence of variability and disturbances

This paper presents modeling and analysis of 1T-1MTJ STT RAM memory arrays under process variations and thermal disturbances. Bounds on the magnetic material design space for embedded applications are illustrated. Impact of relaxed timing/area and the effect of scaling for 1T-1MTJ bitcells have been evaluated.

Differential current switch logic: a low power DCVS logic family

We present a new logic family, Differential Current Switch Logic (DCSL) for implementing clocked CMOS circuits. The circuit is in principle a differential cascode voltage switch logic circuit (DCVS). In comparison to other forms of clocked DCVS, DCSL achieves better performance both in terms of power and speed by restricting internal voltage swings in the N tree. Automatic lock-out of inputs on completion of evaluation is a novel feature of the circuit and allows new implementation of logic functions and the possibility of operating with reduced voltage swings. SPICE simulations carried out with the MOSIS 1.2¿ process indicate that DCSL is better than similar clocked DCVS circuits by a factor of two both in terms of power and speed, for moderate tree heights.

Ultra-low voltage circuits and processor in 180nm to 90nm technologies with a swapped-body biasing technique

A low-voltage swapped-body biasing technique where PMOS bodies are connected to ground and NMOS bodies to Vcc is evaluated. Available measurements show more than 2.6x frequency improvement at 0.5V Vcc and the ability to reduce Vcc by 0.2V for the same frequency compared to no body bias in 180 to 90nm CMOS technologies.

A 256-Kb Dual-

This paper addresses the stability problem of SRAM cells used in dense last level caches (LLCs). In order for the LLC not to limit the minimum voltage at which a processor core can run, a dual-VCC 256-Kb SRAM building block is proposed. A fixed high-voltage supply powers the cache which allows the use of the smallest SRAM cell for maximum density, while a separate variable supply is used by the core for ultra-low-voltage operation using dynamic voltage and frequency (DVF). Implemented in a 65-nm bulk CMOS process, the block features low overhead embedded level shifters and an actively clamped sleep transistor for maximum cache leakage power reduction during standby. Measured results show that the proposed block runs at 4.2GHz while consuming 30 mW at 85degC and 1.2V supply. Furthermore, measurements across a wide range of process, voltage, temperature, and aging conditions indicate virtual ground clamping accuracy within a few millivolts of required cache standby VMIN. Extrapolating the 256-Kb block measurement results in a large 64-Mb LLC used in a dual-V CC processor gives 35% reduction in total processor power as compared with a single-VCC processor design running at a high supply voltage

{V}_{\rm CC}

Pulsed wordline (PWL) & pulsed bitline (PBL) techniques to improve SRAM cell stabilities in single-Vcc microprocessor designs are evaluated in 65nm CMOS. At 0.7V Vcc, PWL improves cell failure rate by 15times while incurring <1% area overhead. Both PBL & PWL with read-modify-write (PWL-RMW) provide the best improvements (26times) in cell stability, with significant area overheads (4-8%)

SRAM Building Block in 65-nm CMOS Process With Actively Clamped Sleep Transistor

In this paper, we present a noise-tolerant high-performance static circuit family suitable for low-voltage operation called skewed logic. Skewed logic circuits in comparison with Domino logic have better scalability and are more suitable for low voltage applications because of better noise margins. Skewed logic and its variations have been compared with Domino logic in terms of delay, power, and dynamic noise margin. A design methodology for skewed CMOS pipelined circuits has been developed. To demonstrate the applicability of the proposed logic style, 0.35 /spl mu/m 5.56 ns CMOS 16/spl times/16 bit multipliers have been designed using skewed logic circuits and fabricated through MOSIS. Measurement results show that the multiplier only consumed a power of 195 mW due to its low clock load.