Packing Techniques for Virtex-5 FPGAs

Abstract

Packing is a key step in the FPGA tool flow that straddles the boundaries between synthesis, technology mapping and placement. Packing strongly influences circuit speed, density, and power, and in this article, we consider packing in the commercial FPGA context and examine the area and performance trade-offs associated with packing in a state-of-the-art FPGA---the Xilinx ® Virtex TM -5 FPGA. In addition to look-up-table (LUT)-based logic blocks, modern FPGAs also contain large IP blocks. We discuss packing techniques for both types of blocks. Virtex-5 logic blocks contain dual-output 6-input LUTs. Such LUTs can implement any single logic function of up to 6 inputs, or any two logic functions requiring no more than 5 distinct inputs. The second LUT output has reduced speed, and therefore, must be used judiciously. We present techniques for dual-output LUT packing that lead to improved area-efficiency, with minimal performance degradation. We then describe packing techniques for large IP blocks, namely, block RAMs and DSPs. We pack circuits into the large blocks in a way that leverages the unique block RAM and DSP layout/architecture in Virtex-5, achieving significantly improved design performance.