Early Routability Assessment in VLSI Floorplans: A Generalized Routing Model

Bapi Kar, Susmita Sur-Kolay, Chittaranjan Mandal

Multiple design iterations are inevitable in nanometer Integrated Circuit (IC) design flow until desired printability and performance metrics are achieved. This starts with placement optimization aimed at improving routability, wirelength, congestion and timing in the design. Contrarily, no such practice exists on a floorplanned layout, during the early stage of the design flow. Recently, STAIRoute \cite{karb2} aimed to address that by identifying the shortest routing path of a net through a set of routing regions in the floorplan in multiple metal layers. Since the blocks in hierarchical ASIC/SoC designs do not use all the permissible routing layers for the internal routing corresponding to standard cell connectivity, the proposed STAIRoute framework is not an effective for early global routability assessment. This leads to improper utilization of routing area, specifically in higher routing layers with fewer routing blockages, as the lack of placement of standard cells does not facilitates any routing of their interconnections. This paper presents a generalized model for early global routability assessment, HGR, by utilizing the free regions over the blocks beyond certain metal layers. The proposed (hybrid) routing model comprises of (a) the junction graph model in STAIRoute routing through the block boundary regions in lower routing layers, and (ii) the grid graph model for routing in higher layers over the free regions of the blocks. Experiment with the latest floorplanning benchmarks exhibit an average reduction of $4\%$, $54\%$ and $70\%$ in netlength, via count, and congestion respectively when HGR is used over STAIRoute. Further, we conducted another experiment on an industrial design flow targeted for $45nm$ process, and the results are encouraging with $~3$X runtime boost when early global routing is used in conjunction with the existing physical design flow.

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