A Survey on Low-Power Approximate Unsigned Multipliers
Approximate circuits have been considered for applications that can tolerate some loss of accuracy with improved performance and/or energy efficiency. Multipliers are key arithmetic circuits in many of these applications including digital signal processing (DSP). In this paper, a novel approximate multiplier with a low power consumption and a short critical path is proposed for high-performance DSP applications. This multiplier leverages a newly designed approximate adder that limits its carry propagation to the nearest neighbors for fast partial product accumulation. Different levels of accuracy can be achieved by using either OR gates or the proposed approximate adder in a configurable error recovery circuit. The approximate multipliers using these two error reduction strategies are referred to as AM1 and AM2, respectively. Both AM1 and AM2 have a low mean error distance, i.e., most of the errors are not significant in magnitude. Compared with a Wallace multiplier optimized for speed, an 8×8 AM1 using four most significant bits for error reduction shows a 60% reduction in delay (when optimized for delay) and a 42% reduction in power dissipation (when optimized for area). In a 16×16 design, half of the least significant partial products are truncated for AM1 and AM2, which are thus denoted as TAM1 and TAM2, respectively. Compared with the Wallace multiplier, TAM1 and TAM2 save from 50% to 66% in power, when optimized for area. Compared with existing approximate multipliers, AM1, AM2, TAM1, and TAM2 show significant advantages in accuracy with a low power-delay product. AM2 has a better accuracy compared with AM1 but with a longer delay and higher power consumption. Image processing applications, including image sharpening and smoothing, are considered to show the quality of the approximate multipliers in error-tolerant applications. By utilizing an appropriate error recovery scheme, the proposed approximate multipliers achieve similar processing accuracy as exact multipliers, but with significant improvement in power.