16. Partitioning RSFQ Circuits for Current Recycling

RSFQ circuits require a DC bias current to operate properly. The bias current in conventional RSFQ circuits is supplied to each gate, resulting in large current requirements in VLSI complexity SFQ systems, on the order of tens to hundreds of amperes. These high currents are difficult to supply and distribute. Large currents require significant metal and input pin resources. In addition, large currents can inductively couple to sensitive superconductive inductors, degrading circuit operation and producing errors. Current recycling is a well-known technique to reduce these bias currents. RSFQ circuits with similar bias current requirements can be serially biased and placed on separate ground planes. The inputs and outputs of these circuits are galvanically decoupled and require drivers and receivers between connections. In this chapter, a methodology for automated partitioning of complex RSFQ circuits into blocks with similar bias currents is described, where the number of connections among the blocks is minimized. These blocks are biased in series, reducing the total bias current by the number of partitions. The partitioning methodology is intended for use within an automated EDA flow to enable current recycling for arbitrary (nonuniform, irregular) VLSI complexity RSFQ circuits, drastically reducing the overall bias current and input requirements.