Approximate XOR/XNOR-based adders for inexact computing

Abstract

Power dissipation has become a significant issue for integrated circuit design in nanometric CMOS technology. To reduce power consumption, approximate implementations of a circuit have been considered as a potential solution for applications in which strict exactness is not required. In inexact computing, power reduction is achieved through the relaxation of the often demanding requirement of accuracy. In this paper, new approximate adders are proposed for low-power imprecise applications. These adders are based on XOR/XNOR gates with multiplexers implemented by pass transistors. The proposed approximate XOR/XNOR-based adders (AXAs) are evaluated and compared with respect to energy consumption, delay, area and power delay product (PDP) with an accurate full adder. The metric of error distance is used to evaluate the reliability of the approximate designs. Simulation by Cadence's Spectre in TSMC 65nm process has shown that the proposed designs consume less power and have better performance (such as a lower propagation delay) compared to the accurate XOR/XNOR-based adder, while the error distance remains similar or better than other approximate adder designs.