Functional modeling of RSFQ circuits using Verilog HDL

Abstract

Circuit level simulation is too slow to be used for verification of function and timing of large RSFQ circuits. The alternative, known from semiconductor digital circuit design, is simulating at the logic (gate) instead of the circuit (transistor or junction) level. Using a hardware description language (HDL) such as Verilog, it is possible to write functional model of each of the RSFQ basic gates. A large RSFQ circuit composed of hundreds gates and thousands Josephson junctions can then be simulated using standard semiconductor industry CAD tools. We have developed a library of Verilog models for over 15 basic RSFQ gates. We describe in detail our model for the DRO RSFQ cell. We show how this model can be generalized for other more complex cells. Our library has been verified by employing it in the design of timing for three large RSFQ circuits.