Exploring architecture parameters for dual-output LUT based FPGAs

Dual-output lookup tables (LUTs) are mainstream in the design of commercial FPGA products. A detailed exploration of architectural parameters of FPGAs based on dualoutput LUTs is presented. Different from traditional single-output LUT based architecture, “shared inputs” between the sub-LUTs is a new parameter specific to dual-output architecture. In this paper, we focus on the effect of ratio of shared inputs on the performance and area-efficiency. First, we study the required cluster inputs and derive a relationship between cluster inputs, LUT size and cluster size under different ratios of shared inputs. Secondly, our evaluation results show that a FPGA with 4-LUTs and a shared input ratio of two thirds is preferred for area-efficiency, while a large LUT size of 9 with no shared inputs achieves best performance. Finally, we determine that a LUT size of 4, a cluster size from 3 to 8, and a shared input ratio between 1/3 and 2/3, provide the best area-delay product for dual-output LUT based FPGAs.