Wotan: A tool for rapid evaluation of FPGA architecture routability without benchmarks

Abstract

FPGA routing architectures consist of routing wires and programmable switches which together account for a significant portion of the fabric delay and area. Routing architectures have traditionally been evaluated using a full CAD flow with a suite of benchmark circuits. While the results of such a flow can be accurate, CAD tools are often tuned to a specific architecture type and can take a long time to run which prohibits quick exploration of different architectures early in the design process. In this paper we present an alternative approach that quickly estimates routability for a wide range of architectures without the use of benchmark circuits. Our new routability predictor first assigns congestion probabilities to the architecture's routing resources based on demand estimates found via efficient path enumeration through the routing graph. Next, we compute the probabilities of successfully routing different source/sink connections and finally we combine them to assign an overall routability score. We describe our predictor and present routability estimates for a range of 6-LUT and 4-LUT architectures, showing reasonable agreement with routability results from the full VPR CAD flow in much less CPU time.