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Multi Fanout Routing Approaches for Effective Wire Resource Utilization

IP.com Disclosure Number: IPCOM000240557D
Publication Date: 2015-Feb-09
Document File: 3 page(s) / 127K

Publishing Venue

The IP.com Prior Art Database

Abstract

This article covers a method to effectively route critical multi-fanout nets on higher metal layers to improve routability of the design.

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Multi Fanout Routing Approaches for Effective Wire Resource Utilization

This document presents a system to utilize routing resources effectively by limiting higher layer allocation only for critical sinks of multi-fanout nets.

Technology scaling and the move to non-planar FETs allows for a super-linear (near-exponential) increase in the device density per unit area. This can theoretically enable increased on-chip functionality in a multi-core, multi-thread design environment, it also poses significant routing challenges. Routing congestion has become a major problem in current designs, and drives an increase in the number of metal layers needed to successfully route a chip. This results in greater manufacturing cost and variability. With wires scaling at an inferior rate to device performance, interconnects are becoming a major bottleneck in pushing the performance limits of current technologies, and hence a greater percentage of nets need to be routed on higher metal layers to meet the frequency requirements.

Additionally, with the increasing complexity of designs and advancement of tools, the functional interconnectivity of various nets in the designs is greater than before. This manifests itself in a greater percentage of nets in the design having more than one sink, creating a dense inter-wined interconnect structure.

Routability of a design can be improved through a judicious use of available routing resources. This may include proper understanding and intelligent allocation of available metal resources and upfront planning of wire resources in terms of preroutes. The scope of the work presented here involves proposing a different approach with focus specifically on multi-fanout routing which will help in effective utilization of metal resources, with no significant impact on timing. This method uses a system of tagging critical nets that allows for use of expensive, and limited higher metal layers only for critical sinks of multi-fanout nets.

As per current methodology, a net that is tagged as critical (under a metric of interest,
e.g timing or reliability) is considered 'completely' critical, i.e critical to all of its sinks. All routing approaches or subsequent tools respect this net criticality uniformly across the entire physical reach of the net. However, in the case of a multi fanout net, the criticality of the net may be driven by a subset of its sinks, with a majority of its sinks being non-critical. These critical sinks will typically be placed closer to the source, (as per placement engines), while other non-critical sinks may be further away.

Figure 1 is an example of a multi-fanout net where only 1 of the 6 sinks of the net is timing critical....