Crosstalk If both a wire and its neighbor are switching at the same time, the direction of the switching affects the amount of charge to be delivered and the delay of the switching. In the figure the coupling capacitor is Cadj and the charge delivered to this capacitor is Q=CadjV. V is the voltage change between A and B. If A switches and B does not V=VDD, i.e. the effective capacitance seen by A is just the capacitance to ground and to B. If both A and B switch in the same direction V=0, Cadj is effectively absent for delay purposes. If A and B switch in opposite directions V=2VDD. If wire A switches while wire B is supposed to remain constant, B partially switches. A B Cadj Cgnd Cgnd

Crosstalk Noise If B is floating the circuit can be modeled as a capacitive voltage divider If the floating node is now being actively driven but still constant the effect of the switching wire is reduced and the proper voltage can be restored by the driver once the switching activity is over. The designer has several ways to engineer wires for delay and coupling noise. The wire width, spacing and metal level (layer) are all parameters at the designer’s disposal. Widening a wire proportionally reduces resistance but increases the capacitance. Increasing spacing between wires reduces coupling capacitance and leaves the resistance unchanged. This slightly reduces the RC delay and significantly reduces coupling noise. Modern processes have six metal layers or more, with the lower layers being thin and optimized for tight routing pitch. The middle layers are often slightly thicker for lower resistance and better current handling capability.

Wire Engineering Upper layers are even thicker/wider to reduce global interconnect delays. Coupling between neighboring wires affects both the delay and signal integrity and can be avoided if adjacent wires do not switch. Critical signals are thus shielded with power or ground wires on one or both sides to eliminate coupling. This is costly in area, but maybe lest costly than increasing inter-wire spacing. Both resistance and capacitance increase with wire length and thus increase the RC delay. This RC delay maybe reduced by splitting the wire into N segments and inserting an inverter or buffer called a repeater to actively drive the wire. Using inverters as repeaters gives best performance, with each repeater adding some delay buffer. If the distance between the wires is long, the wire delay dominates. On the other hand if the distance is small the buffer delay dominates.