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What is crosstalk ?

How it occurs ?



An unwanted coupling from a

neighboring signal wire to a network

node introduces Interference. Theinjected noise depends upon the

transient value of the other signals

routed in the neighborhood



The digital integrated circuit designing is rapidly approaching apoint where conventional device signal models are no longer

effective in predicting the behavior of ICs. The maintenance of

signal integrity possesses a challenge in designs implemented in

deep sub-micron technology that operates at speeds approaching

10GHz and above. Smaller devices are more condensed in

geometry and smaller signal swings are used to achieve high-

speed performance. This high-speed approach results in higher

noise-coupling rate and lower noise margin as the side effects.

Consequently, signal integrity and interconnectmodeling becomes more critical and a limiting factor for avoiding

crosstalk



Crosstalk may occur over

many paths like



Inductive Crosstalk and Capacitive

crosstalk :This type of Crosstalk comes into

existance when the interconnects

are routed close to each

other,signals on the line crosstalk to

each other via near fieldelectromagnetic coupling



Substrate Crosstalk:

The common substrate serves as a channelfor signal coupling when Interconnects are

placed far a apart,such a noise source is

called Substrate Crosstalk.They play asignificant negative role in mixed signal ICs

where low resistive silicon substrate can be

modeled as resistive and capacitive networkand noise can spread globally through the

network.



Power/Ground CrossTalk:

Signals may effect one

another via a shared power

supply and ground leading toPower Supply and Ground

Crosstalk



Return Signal Crosstalk:

When a pair of signals share a returnpath having a finite impedance, a

transition on one signal induces a

voltage across the shared returnimpedance that appears as a noise

on the other signal thereby giving

scope to return signal crosstalk



Crosstalk effects:

Crosstalk primarily causes noise on non-

switching wires as

A capacitor does not like to change its

voltage instantaneously.

A wire has high capacitance to its

neighbor and when the neighbor

switches from 1-> 0 or 0->1, the wiretends to switch too called capacitive

coupling or crosstalk



Types of Crosstalk



Crosstalk can be a difficult phenomenon tograsp, particularly since there are two types of

crosstalk, forward and backward, which behave

quite differently. Though, the magnitude of

forward crosstalk increases as the length of the

coupled region increases, its pulse width

remains nearly constant and independent of the

length of the coupled region.



However in case of backward crosstalk,

there is nearly constant magnitude

independent of the length of the

coupled region (as long as the coupled

region is "long enough"). But its pulse

width is twice as long as the coupled

region. Crosstalk can be a serious

problem in some designs as it hassome very subtle effects that are hard

to recognize. As a result:



Parasitic coupling capacitanceand inductance between

interconnect introduces crosstalk

between signals



Level of crosstalk depends on

signal frequency, layout geometryand material characteristics



Crosstalk sensitivity varies - some

nodes (pre-charge control

function) may be very sensitive,others (static gate nodes)



Minimize crosstalk by re-designing layout to minimize

coupling C & L (e.g. by increasingspacing between critical signals)



Backward Crosstalk:

In case of backward crosstalk, the backward crosstalk

pulse is (almost) constant amplitude in magnitude buttwice as wide as the propagation time represented by

the coupled region. This backward crosstalk pulse

width is trouble understanding. The width is not a

function of coupling strength. Width is purely afunction of the length of the coupled region. Hence

the backward crosstalk pulse width is purely a

function of the length of the coupled region. Backward

crosstalk grows fairly quickly to a constant magnitudepulse whose pulse width is twice as wide as the

propagation time down the coupled region.



Forward Crosstalk:

As the aggressor signal moves forward one more increment,

the first two victim elements get kicked further along and

bump into the third element. This continues as the aggressor

keeps kicking the forward crosstalk elements along. By the

time the aggressor signal reaches the end of its trace, the

forward crosstalk elements are all bunched together at the farend of the victim trace. The forward crosstalk signal never

travels in the reverse direction, it doesn't care whether there is

a reflecting barrier or not. The quantity of the forward

elements represents the magnitude of the forward crosstalk

signal. The forward crosstalk signal will continue to increase inmagnitude the longer is the coupled region. Although there is a

theoretical limit to how high the forward crosstalk signal can

grow, it is never likely to reach that limit on circuit boards (the

coupled region can't be long enough).



For simplicity the forward crosstalk signal can be

assumed to grow larger and larger as the

coupled region increases. All the elements

representing the signal are clustered on top of

each other. This represents the width of the

forward crosstalk pulse. In theory, the width of the forward crosstalk pulse is no wider than the

rise time of the aggressor signal that creates it.

The pulse starts to build as the aggressor signal

starts to rise, and it finishes building when the

aggressor signal has reached its maximum value.



Forward versus Backward Crosstalk



RULES TO AVOID CROSSTALK

1. Greater separation between traces

is better.

2. Lower frequency harmonics and

slower rise times are better.3. Keep the sensitive traces in

stripline environments.



WHAT IS INTERCONNECTS ?



In past, on-chip interconnect wires were not

considered to be a major issue and had only beenconsidered in special cases or when performing high-

precision analysis. However, with the introduction of

deep-submicron semiconductor technologies, there

have been rapid changes. While the gate delay used todominate the net delay, the on-chip interconnects

delay now account for up to 60% of the total delay in a

deep submicron design. The on-chip interconnect

delay needs to be accurately quantified; as any error inthe on-chip interconnect delay can translate into a

large portion of error in the total delay



Another phenomenon that occurs in a deep submicron

design is that in order to maintain proper resistance

(and therefore voltage drop) in each conducting wire,the conductors height is not reduced, if at all, as fast as

the width. Due to the different aspect ratio, the

coupling capacitance between adjacent wires, which

was ignored in past is now significant. The frequency-dependent resistance and inductance (i.e., the skin

effect) of each conductor can be readily obtained by

subdividing the cross section of each conductor into

many segments, replacing each segment by a resistorand inductor in series, and then

reducing the aggregate resistors and inductors through

a circuit formulation



Classification of VLSI Interconnects ?



At the lowest level are the polysilicon

lines characterized as RC lines.



M etal interconnects on chip



Chip to chip interconnects

on a module.



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30 September 2010 33

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