FPGA routing and routability estimation via Boolean satisfiability

Abstract

Guaranteeing or even estimating the routability of a portion of a placed FPGA remains difficult or impossible in most practical applications. In this paper we develop a novel formulation of both routing and routability estimation that relies on a rendering of the routing constraints as a single large Boolean equation. Any satisfying assignment to this equation specifies a complete detailed routing. By representing the equation as a Binary Decision Diagram (BDD), we represent all possible routes for allnets simultaneously. Routability estimation is transformed to Boolean satisfiability, which is trivial for BDDs. We use the technique in the context of a perfect routability estimator for a global router. Experimental results from a standard FPGA benchmark suite suggest the technique is feasible for realistic circuits, but refinements are needed for very large designs.