Improving Efficiency of Power Gated Circuits through Concurrent Optimization of Power Switch Size and Forward Body Biasing

\u3cp\u3ePower gating (PG) has emerged as an effective technique to reduce standby leakage power in portable devices where battery life time is vital. However, it comes at the cost of timing overhead which is a problem for most of the applications where real-time constraints exist. Designing efficient power gated circuits is very challenging problem due to contrasting requirements in active mode (low timing overhead implying larger power switch size) and standby mode (low standby leakage power implying smaller power switch size). In this work, we show that applying Forward Body Biasing (FBB) to the logic gates in conjunction with power gating (PG + FBB) will provide us with an additional degree of freedom which can be utilized to improve the efficiency of the power gated circuit. We propose an optimization algorithm to find the optimum power switch size and FBB value such that total leakage energy of the design (active + standby) in minimized. Results show that our PG + FBB technique on an average improves the leakage energy savings by 2X-5X as compared to using only power gating. With PG + FBB technique, one can also design a zero delay penalty power gated circuit which is not possible if only power gating is used.\u3c/p\u3

Sub-threshold standard cell sizing methodology and library comparison

Scaling the voltage to the sub-threshold region is a convincing technique to achieve low power in digital circuits. The problem is that process variability severely impacts the performance of circuits operating in the sub-threshold domain. In this paper, we evaluate the sub-threshold sizing methodology of [1,2] on 40 nm and 90 nm standard cell libraries. The concept of the proposed sizing methodology consists of balancing the mean of the sub-threshold current of the equivalent N and P networks. In this paper, the equivalent N and P networks are derived based on the best and worst case transition times. The slack available in the best-case timing arc is reduced by using smaller transistors on that path, while the timing of the worst-case timing arc is improved by using bigger transistors. The optimization is done such that the overall area remains constant with regard to the area before optimization. Two sizing styles are applied, one is based on both transistor width and length tuning, and the other one is based on width tuning only. Compared to super-threshold libraries, at 0.3 V, the proposed libraries achieve 49% and 89% average cell timing improvement and 55% and 31% power delay product improvement at 40 nm and 90 nm respectively. From ITC (International Test Conference 99) benchmark circuit synthesis results, at 0.3 V the proposed library achieves up to 52% timing improvement and 53% power savings in the 40 nm technology node

Library tuning for subthreshould operation

In this paper, the authors extend their work on the balancing-based subthreshold cell sizing methodology. The libraries were benchmarked against a library tuned for super-threshold operation. Two sets of standard cells have been properly sized for operation at 0.3V, and further characterized at different voltages. A super-threshold 90 nm low power library was chosen as a reference library to compare timing, power and voltage scaling ability. The resented libraries show a 31% timing improvement at 0.3 V without area penalty over the conventional library. The comparison of libraries at different voltages shows that with respect to the super-threshold library, the presented library with only transistor channel width tuning has on average 10% better timing from 0.3 V to 1.2 V, and that this library with both transistor channel width and length tuning show timing improvements of 31.4% and 6.9% when the supply voltage increases from 0.3 V to 0.6 V, respectivel

Sub-threshold standard cell sizing methodology and library comparison

\u3cp\u3eScaling the voltage to the sub-threshold region is a convincing technique to achieve low power in digital circuits. The problem is that process variability severely impacts the performance of circuits operating in the sub-threshold domain. In this paper, we evaluate the sub-threshold sizing methodology of [1,2] on 40 nm and 90 nm standard cell libraries. The concept of the proposed sizing methodology consists of balancing the mean of the sub-threshold current of the equivalent N and P networks. In this paper, the equivalent N and P networks are derived based on the best and worst case transition times. The slack available in the best-case timing arc is reduced by using smaller transistors on that path, while the timing of the worst-case timing arc is improved by using bigger transistors. The optimization is done such that the overall area remains constant with regard to the area before optimization. Two sizing styles are applied, one is based on both transistor width and length tuning, and the other one is based on width tuning only. Compared to super-threshold libraries, at 0.3 V, the proposed libraries achieve 49% and 89% average cell timing improvement and 55% and 31% power delay product improvement at 40 nm and 90 nm respectively. From ITC (International Test Conference 99) benchmark circuit synthesis results, at 0.3 V the proposed library achieves up to 52% timing improvement and 53% power savings in the 40 nm technology node.\u3c/p\u3

Standard cell sizing for subthreshold operation

Process variability severely impacts the performance of circuits operating in the subthreshold domain. Among other reasons, this mainly stems from the fact that subthreshold current follows a widely spread Log-Normal distribution. In this paper we introduce a new transistor sizing methodology for standard cells. Our premise relies on balancing the N and P network currents based on statistical formulations. Our approach renders more robust cells. We observe up to 57% better performance and 69% lower energy consumption on a set of ISCAS circuits when they are synthesized with our library as opposed to a commercial library in a CMOS 90nm technology