Architecting Hard Crossbars on FPGAs and Increasing their Area Efficiency with Shadow Clusters

We explore the architecture of on-chip hard crossbars in FPGAs and show that the area efficiency of such FPGAs can be improved when combined with shadow clusters (which are soft-logic LUT-based clusters that are architected to sit "behind" the multiplier), as an exemplar of an application circuit that appears less commonly in the designs targeting FPGAs. The metric that we seek to improve is the "frequency" that the need for hard crossbars must appear in the FPGA's target application suite for the inclusion of the hard crossbar to appear to be area-neutral. For example, we show that this break-even point for a hard 32 full-way crossbar changes from 32% of benchmarks needing to require crossbars to 9% for FPGAs with shadow clusters.