Interconnect Driver Design for Long Wires in Field-Programmable Gate Arrays

Each new semiconductor technology node brings smaller transistors and wires. Although this makes transistors faster, wires get slower. As FPGA device designers strive to obtain the lowest possible circuit delays from a given technology node, they must take an increasingly interconnect-focused viewpoint in the design process. In particular, for long interconnect wires, signals now require rebuffering somewhere in the middle of the wire. This paper presents a framework for designing and evaluating long, buffered interconnect wires in FPGAs with near-optimal delay performance. Given a target physical wire length, width and spacing, the method determines the number, size, and position of buffers required to obtain the fastest signal velocity for programmable interconnect. A metric introduced during the design is the "path delay profile", or the arrival time of a signal at different points of a long wire. This method is used to design buffering strategies for interconnect based on 0.5mm, 2mm, and 3mm wire lengths in 180nm technology. These interconnect designs are coded into VPR along with an improved timing analyzer which accurately determines the "path delay profile" arrival times. Using VPR, average critical-path delay is reduced by 19% for 0.5mm wires and by up to 46% for 3mm wires over previous designs given in Lemieux et al. (2004)