Time-Multiplexed FPGA Overlay Architectures

This article presents a comprehensive survey of time-multiplexed (TM) FPGA overlays from the research literature. These overlays are categorized based on their implementation into two groups: processor-based overlays, as their implementation follows that of conventional silicon-based microprocessors, and; CGRA-like overlays, with either an array of interconnected processor-based functional units or medium-grained arithmetic functional units. Time-multiplexing the overlay allows it to change its behavior with a cycle-by-cycle execution of the application kernel, thus allowing better sharing of the limited FPGA hardware resource. However, most TM overlays suffer from large resource overheads, due to either the underlying processor-like architecture (for processor-based overlays) or due to the routing array and instruction storage requirements (for CGRA-like overlays). Reducing the area overhead for CGRA-like overlays, specifically that required for the routing network, and better utilizing the hard macros in the target FPGA are active areas of research.