6. Automatic Synthesis Techniques for Approximate Circuits

Approximate circuits are the basic hardware building blocks of approximate computing platforms. Manual approximate designs of simple arithmetic circuits (adders and multipliers) have demonstrated the potential for efficiency improvements using approximate circuits. However, their broader adoption requires systematic methodologies that can create approximate implementations for any arbitrary circuit. An approximate circuit can be viewed as realizing a logic function that slightly deviates from the original specification, while falling within a specified quality constraint. A design automation methodology should therefore enable the generation of “correct-by-construction” approximate circuits that are guaranteed to satisfy designer-specified quality constraints.

This chapter presents an overview of automatic techniques to design approximate circuits and describes the key principles behind them. The proposed techniques apply to both combinational and sequential circuits and to circuits that contain a mix of control and datapath logic. In many applications, the degree of resilience often varies depending on the application context or the dataset being processed. In such cases, it is desirable to construct quality configurable circuits, or circuits that are capable of reconfiguring to adapt their accuracy at runtime. We describe how approximate circuit design frameworks can be extended to enable automatic synthesis of quality configurable circuits. The synthesized approximate circuits achieve considerable benefits in area and energy with minimal design effort.