Planning for local net congestion in global routing

Abstract

Local nets are a major contributing factor to mismatch between the global routing (GR) and detailed routing (DR) stages. A local net has all its terminals inside one global cell (gcell) and is traditionally ignored during global routing. This work offers two contributions in order to estimate and manage the local nets at the GR stage. First, a procedure is given to generate gcells of non-uniform size in order to reduce the number of local nets and thus the cumulative error associated with ignoring or approximating them. Second, we approximate the resource usage of local nets at the GR stage by introducing a capacity for each gcell in the GR graph. With these two complementary approaches, we offer a mathematical model for the congestion-aware GR problem that captures local congestion with non-uniform gcells along with other complicating factors of modern designs including variable wire sizes, routing blockages, and virtual pins. A practical routing procedure is presented based on the mathematical model that can solve large industry instances. This procedure is integrated with the CGRIP congestion analysis tool. In the experiments, we evaluate our techniques in planning for local nets during GR while accounting for other sources of congestion using the ISPD11 benchmarks.