Strategy of one and half layer routing

Abstract

Routing in one custom and one predefined interconnection layer environment represents a specific problem for automatic routers. A proven strategy of one-and-half layer routing is described. The consists of three consecutive phases: an initial routing phase, a completion phase and an optimization phase. This strategy can be easily adapted to more complex routing environments. A maze-type router operating on a general grid-graph model of a routing space plays a key role in all the phases. The quality of a path is measured by a 2-hereditary incremental cost function. This makes it possible to avoid the misuse of predefined underpasses as well as to establish routing direction priorities. A designer can easily influence the performance of the router because the topology of a path generated depends on the cost function definition. The initial routing phase consists of net ordering followed by routing all the nets in one-at-a-time mode. The completion phase is based on re-ordering and re-routing of incomplete and “blockage” nets. The optimization phase consists of the cost function re-definition followed by re-routing of all the nets.