Optimizing effective interconnect capacitance for FPGA power reduction

We propose a technique to reduce the effective parasitic capacitance of interconnect routing conductors in a bid to simultaneously reduce power consumption and improve delay. The parasitic capacitance reduction is achieved by ensuring routing conductors adjacent to those used by timing critical or high activity nets are left floating - disconnected from either VDD or GND. In doing so, the effective coupling capacitance between the conductors is reduced, because the original coupling capacitance between the conductors is placed in series with other capacitances in the circuit (series combinations of capacitors correspond to lower effective capacitance). To ensure unused conductors can be allowed to float requires the use of tri-state routing buffers, and to that end, we also propose low-cost tri-state buffer circuitry. We also introduce CAD techniques to maximize the likelihood that unused routing conductors are made to be adjacent to those used by nets with high activity or low slack, improving both power and speed. Results show that interconnect dynamic power reductions of up to ~15.5% are expected to be achieved with a critical path degradation of ~1%, and a total area overhead of ~2.1%.