Download Crosstalk - WSU EECS

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.
A
Cgnd
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B
Cadj
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Cgnd
• In the figure the coupling
capacitor is Cadj and the charge
delivered to this capacitor is
Q=CadjV.
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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.
Crosstalk Noise
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If B is floating the circuit can be
modeled as a capacitive
Cadjvoltage
Vswitching
divider V floating 
Cgnd  Cadj
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.
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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
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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.
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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.