Effect of the prefabricated routing track distribution on FPGA area-efficiency

In most commercial field programmable gate arrays (FPGA's) the number of wiring tracks in each channel is the same across the entire chip. A long-standing open question for both FPGA's and channeled gate arrays is whether or not some uneven distribution of routing tracks across the chip would lead to an area benefit. For example, many circuit designers intuitively believe that most congestion occurs near the center of a chip, and hence expect that having wider routing channels near the chip center would be beneficial. In this paper, we determine the relative area-efficiency of several different routing track distributions. We first investigate FPGA's in which horizontal and vertical channels contain different numbers of tracks in order to determine if such a directional bias provides a density advantage. Second, we examine routing track distributions in which the track capacities vary from channel to channel. We compare the area efficiency of these nonuniform routing architectures to that of an FPGA with uniform channel capacities across the entire chip. The main result is that the most area-efficient global routing architecture is one with uniform (or very nearly uniform) channel capacities across the entire chip in both the horizontal and vertical directions. This paper shows why this result, which is contrary to the intuition of many FPGA architects, is true. While a uniform routing architecture is the most area-efficient, several nonuniform and directionally biased architectures are fairly area-efficient provided that appropriate choices are made for the pin positions on the logic blocks and the logic block array aspect ratio.