22.1 INTRODUCTION

This chapter discusses the impact and optimization of placement on the routing stage. This is commonly referred as congestion-driven placement. Although a placer that produces unroutable designs will be of little use, historically optimization to directly reduce routing congestion has received less attention than wirelength and timing optimization. Often placement papers fail to report any information on congestion and routability. Over the last decade, with design sizes increasing dramatically and limited number of metal layers available for routing of signals and power, routability has become a paramount issue. This has driven the recent research interest in placement techniques to mitigate congestion while optimizing other placement objectives.

Congestion-driven placement techniques can be classified into the following groups: netlistconnectivity-based methods, pin-density-based methods, and routing-estimation-based methods. Netlist-connectivity-based methods use a priori information about the netlist characteristics to influence the placement process. Pin-density-based methods seek to limit the average pin density in local regions to indirectly address the routability concerns. Routing-estimation-based methods are frequently used during and after the placement process when sufficient routing congestion information is available. Global routers or probabilistic route estimators are often used to drive the various congestion mitigation techniques. Other notable techniques for addressing congestion in the design process include congestion-driven logic synthesis and global-placement density control. Several of these techniques are applied separately during global placement and detail placement, the details of each approach change according to the specific context. Often a placement flow will employ...