Wire synthesizable global routing for timing closure

Abstract

Despite remarkable progress in the area of global routing, the burdens imposed by modern physical synthesis flows are far greater than those expected or anticipated by available (academic) routing engines. As interconnects dominate the path delay, physical synthesis such as buffer insertion and gate sizing has to integrate with layer assignment. Layer directives -commonly generated during wire synthesis to meet tight frequency targets -play a critical role in reducing interconnect delay of smaller technology nodes. Unfortunately, they are not presently understood or honored by leading global routers, nor do existing techniques trivially extend toward their resolution. The shortcomings contribute to a dangerous blindspot in optimization and timing closure, leading to unroutable and/or underperforming designs. In this paper, we aim to resolve the layer compliance problem in routing congestion evaluation and global routing, which is very critical for timing closure with physical synthesis. We propose a method of progressive projection to account for wire tags and layer directives, in which classes of nets are successively applied and locked while performing partial aggregation. The method effectively models the resource contention of layer constraints by faithfully accumulating capacity of bounded layer ranges, enabling three-dimensional assignment to subsequently achieve complete directive compliance. The approach is general, and can piggyback on existing interfaces used to communicate with popular academic engines. Empirical results on the IC-CAD 2009 benchmarks demonstrate that our approach successfully routes many designs that are otherwise unroutable with existing techniques and naïve approaches.

CATALYST: Planning Layer Directives for Effective Design Closure

“…Several methods have been proposed to address layer directives in global routing [13]- [15]. We use the progressive projection method in [15] due to the flexibility it affords. In this method, the nets are first partitioned to several sets according to their directives.…”

“…Therefore, we must reroute some nets, and we do so using the progressive projection method. As suggested by [15], we should route the nets in the order: DM → DM−1· · · → D1. However, we find this order is not appropriate for the purposes of directive refinement.…”

“…To evaluate the effects of assigned layer directives on routing congestion, a global router with ability to obey the layer directives, such as [13]- [15], can be used to evaluate the congestion. We should try to keep the congestion with newly generated directives similar to that without any directives.…”

“…In this work, a 2D-routing engine adapted from MaizeRouter [6] is used but the maze routing in MaizeRouter is replaced by extreme edge shifting (refer to [6] for details about this technique) to improve the speed; however, it should be pointed out our flow can work with any other 2D routing engine. To deal with directives in 2D routing, we use the progressive projection method in [15] as discussed in Sec. II-B.…”

“…While it is certainly necessary to consider layer assignment during routing, the timing gain in these works is limited since routing is performed after all optimizations are completed, or at least after a majority of buffers are placed. Recent papers [13]- [15] focus on how to obey the given layer assignment constraints from the prior synthesis stage in the routing algorithm, but do not discuss the process of generating these constraints.…”