Logic Optimization, Complex Cell Design, and Retiming of Single Flux Quantum Circuits
Single flux quantum (SFQ) technology has emerged as a promising beyond-CMOS technology with Josephson junctions (JJs) as active devices in a superconducting circuit. In this paper, we (i) describe the algorithms and the methodology we used to synthesize a synchronous SFQ circuit using a CMOS logic synthesis tool by adding new features to the tool and by modifying the existing features; (ii) introduce the concept of two kinds of complex cells: (a) stand-alone cells and (b) interconnected cells along with the advantage gained by these complex cells through the results of synthesized SFQ netlists. Design of stand-alone complex cells presented here includes and and  or gates with more-than-two inputs, high-fanout splitters, and “A+BC” cell. Circuits of decoders, multiplexers, and carrylook-ahead adders are synthesized with complex cells, and the advantage in terms of JJ count and latency is presented. We have also investigated the possibility of SFQ cells supporting multiple fanout drive capability by modifying the input/output interfaces of SFQ standard cells. In this direction, an algorithm to modify the input interface of a cell so that it can be driven along with similar cells by a single splitter output is presented along with the results for a simple clock-distribution line with multiple-fanout capability.