The speed of diversity: Exploring complex FPGA routing topologies for the global metal layer

The rapid growth of wire RC delay with technology scaling has put increasing pressure on FPGA architects to make more efficient use of the different layers available in the metal stack. While commercial FPGA architectures have implemented the majority of inter-logic-block wiring on the lower metal layers and a small fraction of wires on the least-resistive upper metal layers, published explorations have largely ignored the question of how to exploit the different layers of the metal stack, focusing instead on very simple interconnect topologies and physical models. We generate VPR architectures and detailed area and delay models at the 22nm node and present enhancements to VPR that enable us to describe and evaluate complex interconnect topologies. We use our new architectures and tool enhancements to explore complex interconnect patterns suitable for modern unidirectional architectures and suggest topologies to connect wires on the semi-global and global metal layers. The proposed topologies improve the critical path routing delay by 17% compared to architectures with no global layer wires, and by 5-13% compared to architectures with global layer wires using the default VPR switch pattern.