A 900 Mb/s bidirectional signaling scheme

Traditional approaches to the interchip communication problem have consisted of unidirectional signal flow on either a point-to-point interconnect or a shared bus. For systems in which the electrical length of the interconnect is significant in relation to the edge rate or fundamental frequency of the signals traversing them and operation at the highest possible speeds is desired, the interconnect medium consists of controlled impedance traces terminated in their characteristic impedance. The goal of these interconnect designs is to obtain the optimum combination of bandwidth, number of wires, power, and cost. The traditional schemes require a wire for each component output along with the required termination resistors in order to achieve the highest possible bandwidth. The bidirectional method described here allows data transmission simultaneously in two directions over one wire. This doubles the effective bandwidth per pin over a point-to-point unidirectional scheme operating at the same frequency. The line termination is provided by the driver, eliminating discrete terminations from the board. When the drivers at both ends of the line are in the same state, no power is consumed in the I/O's. This can result in significant power savings. Bidirectional schemes have been proposed previously but required current mode signaling and explicit terminations.