Two-sided Net Untangling with Internal Detours for Single-layer Bus Routing

Abstract

It is known that one-sided net untangling can be used to untangle the twisted nets inside a bus for single-layer bus routing. However, limited space behind one pin-row may make one-sided net untangling unsuccessful for single-layer bus routing. In this article, the concept of using internal detours on untangled nets can be introduced into two-sided net untangling. Given a set of 2-pin nets inside a bus, based on two one-sided untangling results with internal detours on untangled nets [8], an efficient algorithm first uses a minimal set of internal detours to guarantee that the crossing conditions of the given nets inside the bus can be eliminated in one initial two-sided untangling result with no capacity constraint behind two pin-rows and between two adjacent pins inside any pin-row. Furthermore, based on the maintenance of the non-crossing constraint on any pair of nets and the capacity constraint behind two pin-rows in one initial two-sided untangling result, an iterative rip-up-and-reassign algorithm can be proposed to eliminate the possible capacity violations between two adjacent pins inside two pin-rows to route a maximal set of nets in two-sided net untangling. Compared with Yan's one-sided net untangling [8] for 12 tested examples with different capacity constraints, the experimental results show that our proposed two-sided untangling algorithm can improve 3.5% of routability and use the benefit of more routing space behind two pin-rows to reduce 86.4% of the used internal detours on average in reasonable CPU time. Compared with Yan's two-sided net untangling [9] for 12 tested examples with different capacity constraints, the experimental results show that our proposed two-sided untangling algorithm can improve 2.8% of routability by introducing some internal detours and using iterative rip-up-and-reassign on the average in reasonable CPU time.