Toward a systematic design methodology for large multigigahertz rapid single flux quantum circuits

Rapid single flux quantum (RSFQ) digital circuits have reached the level of medium- to large-scale of integration. At this level, existing design meth odologies, developed specifically for RSFQ circuits, have become computatio nally inefficient. Applying mature semiconductor methodologies to the desig n of RSFQ circuits, one encounters substantial difficulties originating fro m the differences between both technologies. In this paper, a new design me thodology aimed at large-scale RSFQ circuits is proposed. This methodology is based on a semiconductor semicustom design approach. An established desi gn methodology for small-scale RSFQ digital circuits, based on circuit (jun ction-level) simulation and device parameter optimization, is used for the design of basic RSFQ cells. A library composed of about 20 basic RSFQ cells has been developed based on this approach, A novel design methodology for large-scale circuits, presented in this paper, is based on logic (gate-leve l) simulation and timing optimization. This methodology has been implemente d around the Cadence integrated design environment and used successfully at the University of Rochester for the design of two large-scale digital circ uits.