Because of numerous circuit resources of FPGAs, there is a performance gap between FPGAs and ASICs. In this paper, we propose a small-memory logic cell, COGRE, to reduce the FPGA area. Our approach is to investigate the appearance ratio of the logic functions in a circuit implementation. Moreover, we group the logic functions on the basis of the NPN-equivalence class. The results of our investigation show that only small portions of the NPN-equivalence class can cover large portions of the logic functions used to implement circuits. Further, we found that NPN-equivalence classes with a high appearance ratio can be implemented by using a small number of AND gates, OR gates, and NOT gates. On the basis of this analysis, we develop COGRE architectures composed of several NAND gates and programmable inverters. The experimental results show that the logic area of 4-COGRE is smaller than that of 4-LUT and 5-LUT by approximately 35.79 and 54.70, respectively. The logic area of 8-COGRE is 75.19 less than that of 8-LUT. Further, the total number of configuration memory bits of 4-COGRE is 8.26 less than the number of configuration memory bits of 4-LUT. The total number of configuration memory bits of 8-COGRE is 68.27 less than the number of configuration memory bits of 8-LUT.
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