Siva G. Narendra

Parameter variations and impact on circuits and microarchitecture

Parameter variation in scaled technologies beyond 90nm will pose a major challenge for design of future high performance microprocessors. In this paper, we discuss process, voltage and temperature variations; and their impact on circuit and microarchitecture. Possible solutions to reduce the impact of parameter variations and to achieve higher frequency bins are also presented.

Adaptive body bias for reducing impacts of die-to-die and within-die parameter variations on microprocessor frequency and leakage

Measurements on a 150 nm CMOS test chip show that on-chip bidirectional adaptive body biasing compensates effectively for die-to-die parameter variation to meet both frequency and leakage requirements. An enhancement of this technique to correct for within-die variations triples the accepted die count in the highest frequency bin.

Dynamic-sleep transistor and body bias for active leakage power control of microprocessors

Sleep transistors and body bias are used to control active leakage for a 32b integer execution core implemented in a 100nm dual V, CMOS technology. A PMOS sleep transistor degrades performance by 4% but offers 20/spl times/ leakage reduction which is further improved with body bias. Time constants for leakage convergence range from 30ns to 300ns allowing 9-44% power savings for idle periods greater than 100 clock cycles.

Scaling of stack effect and its application for leakage reduction

Technology scaling demands a decrease in both V/sub dd/ and V/sub t/ to sustain historical delay reduction, while restraining active power dissipation. Scaling of V/sub t/ however leads to substantial increase in the sub-threshold leakage power and is expected to become a considerable constituent of the total dissipated power. It has been observed that the stacking of two off devices has smaller leakage current than one off device. In this paper we present a model that predicts the scaling nature of this leakage reduction effect. Device measurements are presented to prove the models accuracy. Use of stack effect for leakage reduction and other implications of this effect are discussed.

Subthreshold leakage modeling and reduction techniques

As technology scales, subthreshold leakage currents grow exponentially and become an increasingly large component of total power dissipation. CAD tools to help model and manage subthreshold leakage currents will be needed for developing ultra low power and high performance integrated circuits. This paper gives an overview of current research to control leakage currents, with an emphasis on areas where CAD improvements will be needed. The first part of the paper explores techniques to model subthreshold leakage currents at the device, circuit, and system levels. Next, circuit techniques such as source biasing, dual Vt partitioning, MTCMOS, and VTCMOS are described. These techniques reduce leakage currents during standby states and minimize power consumption. This paper also explores ways to reduce total active power by limiting leakage currents and optimally trading off between dynamic and leakage power components.

MTCMOS hierarchical sizing based on mutual exclusive discharge patterns

Multi-threshold CMOS is a popular circuit style that will provide high performance and low power operation. Optimally sizing the gating sleep transistor to provide adequate performance is difficult because the overall delay characteristics are strongly dependent on the discharge patterns of internal gates. This paper proposes a methodology for sizing the sleep transistor for a large module based on mutual exclusive discharge patterns of internal blocks. This algorithm can be applied at all levels of a circuit hierarchy, where the internal blocks can represent transistors, cells within an array, or entire modules. This methodology will give an upper bound for the sleep transistor size required to meet any performance constraint.

Comparative delay and energy of single edge-triggered and dual edge-triggered pulsed flip-flops for high-performance microprocessors

Flip-flops and latches are crucial elements of a design from both a delay and energy standpoint. We compare several styles of single edge-triggered flip-flops, including semidynamic and static with both implicit and explicit pulse generation. We present an implicit-pulsed, semidynamic flip-flop (ip-DCO) which has the fastest delay of any flip-flop considered, along with a large amount of negative setup time. However, an explicit-pulsed static flip-flop (ep-SFF) is the most energy-efficient and is ideal for the majority of critical paths in the design. In order to further reduce the power consumption, dual edge-triggered flip-flops are evaluated. It is shown that classic dual edge-triggered designs suffer from a large area penalty and reduced performance, prohibiting their use in critical paths. A new explicit-pulsed dual edge-triggered flip-flop is presented which provides the same performance as the single edge-triggered version with significantly less energy consumption in the flip-flop as well as in the clock distribution network.

Effectiveness of reverse body bias for leakage control in scaled dual Vt CMOS ICs

Examines the effectiveness of opportunistic use of reverse body bias (RBB) to reduce leakage power during active operation, burn-in, and standby in 0.18 /spl mu/m single-V/sub t/ and 0.13 /spl mu/m dual-V/sub t/ logic process technologies. Investigates its dependencies on channel length, target V/sub t/, temperature and technology generation. Shows that RBB becomes less effective for leakage reduction at shorter channel lengths and lower V/sub t/ at both high and room temperatures, especially when target intrinsic leakage currents are high. RBB effectiveness also diminishes with technology scaling primarily because of worsening short-channel effects (SCE), particularly when target V/sub t/ values are low. A model is given that relates different transistor leakage components to full-chip leakage current, and is validated through test-chip measurements across a range of RBB values.

Forward body bias for microprocessors in 130-nm technology generation and beyond

Device and test chip measurements show that forward body bias (FBB) can be used effectively to improve performance and reduce complexity of a 130nm dual-V/sub T/ technology, reduce leakage power during burn-in and standby, improve circuit delay and robustness, and reduce active power. FBB allows performance advantages of low temperature operation to be realized fully without requiring transistor redesign, and also improves V/sub T/ variations, mismatch, and g/sub m/ /spl times/ r/sub 0/ product.

Effectiveness of adaptive supply voltage and body bias for reducing impact of parameter variations in low power and high performance microprocessors

Test chip measurements show that adaptive V/sub CC/ is useful for reducing impacts of parameter variations on frequency, active power and leakage power of microprocessors. Using adaptive V/sub CC/ together with adaptive V/sub BS/ or WID-V/sub BS/ is much more effective than using any of them individually.