Hybrid LUT/Multiplexer FPGA Logic Architectures

Abstract

Hybrid configurable logic block architectures for field-programmable gate arrays that contain a mixture of lookup tables and hardened multiplexers are evaluated toward the goal of higher logic density and area reduction. Multiple hybrid configurable logic block architectures, both nonfracturable and fracturable with varying MUX:LUT logic element ratios are evaluated across two benchmark suites (VTR and CHStone) using a custom tool flow consisting of LegUp-HLS, Odin-II front-end synthesis, ABC logic synthesis and technology mapping, and VPR for packing, placement, routing, and architecture exploration. Technology mapping optimizations that target the proposed architectures are also implemented within ABC. Experimentally, we show that for nonfracturable architectures, without any mapper optimizations, we naturally save up to ~8% area postplace and route; both accounting for complex logic block and routing area while maintaining mapping depth. With architecture-aware technology mapper optimizations in ABC, additional area is saved, post-place-and-route. For fracturable architectures, experiments show that only marginal gains are seen after place-and-route up to ~2%. For both nonfracturable and fracturable architectures, we see minimal impact on timing performance for the architectures with best area-efficiency.