Congestion-driven regional re-clustering for low-cost FPGAs

FPGA device area is dominated by a limited amount of interconnect. CAD tools must meet a hard channel-width constraint for a circuit to be successfully mapped to a device. Previous work has shown that if a design cannot be mapped to a device due to insufficient interconnect availability, it is possible to identify regions of high interconnect demand and spread out the logic in this area into surrounding regions. This is done by re-packing logic in the affected regions into an increased number of CLBs. This increases the effective amount of interconnect in these high-demand areas. This methodology has been shown to significantly reduce channel width, at the expense of CLB count and runtime. In this paper, we extend this previous algorithm in two ways: we present novel region selection techniques to optimize the selection of which regions should be depopulated, and we introduce a local channel-width demand model which can be used to more accurately determine the amount of white space insertion at each iteration. Together, these techniques lead to significant run-time improvements and reduce the area of the resulting FPGA implementations. We were able to improve runtime by a factor of up to 5.5 times while reducing area by up to 20% when compared to previous methods.