Raising FPGA Logic Density Through Synthesis-Inspired Architecture

Abstract

We leverage properties of the logic synthesis netlist to define both a new field-programmable gate-array (FPGA) logic element (function generator) architecture and an associated technology mapping algorithm that together provide improved logic density. We demonstrate that an “extended” logic element with slightly modified K -input lookup tables (LUTs) achieves much of the benefit of an architecture with K+1-input LUTs, while consuming silicon area close to a K-LUT (a K-LUT requires half the area of a K+1-LUT). We introduce the notion of “non-inverting paths” in a circuit's and-inverter graph (AIG) and show their utility in mapping into the proposed logic element architectures. We propose a general family of logic element architectures, and present results showing that they offer a variety of area/performance tradeoffs. One of our key results demonstrates that while circuits mapped to a traditional 5-LUT architecture need 15% more LUTs and have 14% more depth than a 6-LUT architecture, our extended 5-LUT architecture requires only 7% more LUTs and 5% more depth than 6-LUTs, on average. Nearly all of the depth reduction associated with moving from K -input to K+1 -input LUTs can be achieved with considerably less area using extended K-LUTs. We further show that 6-LUT optimal mapping depths can be achieved with a small fraction of the LUTs in hardware being 6-LUTs and the remainder being extended 5-LUTs, suggesting that a heterogeneous logic block architecture may prove to be advantageous.