Improving FPGA Performance with a S44 LUT Structure

FPGA performance depends in part on the choice of basic logic cell. Previous work dating back to 1999-2005 found that the best look-up table (LUT) sizes for area-delay product are 4-6, with 4 better for area and 6 for performance. Since that time several things have changed. A new 'LUT structure' mapping technique can target cells with a larger number of inputs (cut size) without assuming that the cell implements all possible functions of those inputs. We consider in particular a 7-input function composed of two tightly-coupled 4-input LUTs. Changes in process technology have increased the relative importance of wiring delay and configuration memory area. Finally, modern benchmark applications include carry chains, math and memory blocks. Due to these changes, we show that mapping to a 7-input LUT structure can approach the performance of 6-input LUTs while retaining the area and static power advantage of 4-input LUTs.