This paper explores the relationship between the routability of a Field-Programmable Gate Array (FPGA) and the flexibility of its interconnection structures. The flexibility of an FPGA is determined by the number and distribution of switches used in the interconnection. While good routability can be obtained with a high flexibility, a large number of switches will result in poor performance and logic density because each switch has significant delay and area.
The minimum number of switches required to achieve good routability is determined by implementing several industrial circuits in a variety of interconnection architectures. These experiments indicate that high flexibility is essential for the connection block that joins the logic blocks to the routing channel, but a relatively low flexibility is sufficient for switch blocks at the junction of horizontal and vertical channels. Furthermore, it is necessary to use only a few more routing tracks than the absolute minimum possible with structures of surprisingly low flexibility.
Jonathan Rose and Stephen Brown, "Flexibility of Interconnection Structures in Field-Programmable Gate Arrays," IEEE Journal of Solid State Circuits, Vol. 26, No. 3, pp. 277-282, March 1991.