1. Minimisation of crosstalk
in VLSI
1
Presented By: Subhradeep Mitra
Ankita Dutta
Paramita Sau
Debanjana Biswas
(Mca Students of Rajabazar sc. College)

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Outlines
• VLSI DESIGN CYCLE
• PHYSICAL DESIGN CYCLE
• ROUTING
• PHASES OF ROUTING
• CROSSTALK OVERVIEW
•EFFECT OF CROSSTALK
•HOW TO MINIMIZE CROSSTALK
•HOW TO PREVENT CROSSTALK
• CONCLUSION
•REFERENCES

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VLSI Design Cycle
• The VLSI design cycle starts with a formal specification of a VLSI chip
follows a series of steps, and eventually produces a packaged chip
The steps included into the VLSI Design Cycle are as follows :
• System Specification
• Architectural Design
• Behavioral or Functional Design
• Logic Design
• Circuit Design
• Physical Design
• Fabrication
• Packaging, Testing and Debugging

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Physical Design Cycle
The input of the physical design cycle is a circuit diagram and the output is
the layout of the circuit. This is accomplished in several stages. The stages are
following:-
1) Partitioning
2) Floorplanning and Placement
3) Routing
4) Compaction
5) Extraction and verification

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Routing
The Routing is to locate a set of wires in the routing space that connect all the nets in
the net list.The capacity of channels width of wires, and wire crossing often need to
be taken into consideration.
 Problem :
 Input :
• Netlist
• Timing budget for typically critical nets
• Placement information including location of blocks, location of pins in
the block boundary, location of the I/O pins on the chip boundary etc.
 Output :
• Geometric layouts of all nets

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 Objectives of Routing:
• Minimize the total wire length
• Minimize the no of layers ( fewer layer is less expensive)
•Minimize the no of vias,bends i.e. completing all connection without
increasing the chip area
• Minimize the crosstalk

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PHASES OF ROUTING:-
Routing
Global Routing Detailed Routing
Line Routing Maze Routing
Channel
Routing
Switch Box
Routing

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Global Routing:-.
• It‟s the first phase of routing and generates a „loose‟ route for each net.
• Minimize the total wire length
• Minimize running time
•The global routing considers the entire layout.
• This routing consists of three distinct phases-
i. Region Definition
ii. Region Assignment
iii. Pin Assignment
Global Routing

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Detailed Routing:-
• It‟s the second phase, of routing to finds the actual geometric layout of
each net within the assigned routing regions.
• Three types of detailed routing methods:-
i) Channel Routing
ii) 2-D Switchbox Routing
iii) 3-D Switchbox Routing
• Channel Routing 2-D Switchbox 3-D switchbox
• If the switchbox or channels are unroutable without
a large expansion, global routing to be done again
Detailed Routing

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Crosstalk
1. Mutual inductance (magnetic field)
2. Mutual capacitance (electric field)
 Definition :
The electromagnetic coupling of a signal from one conductor to
another is called as crosstalk
 Crosstalk can be classified into the following two types:

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1. Mutual Inductance :
• Mutual inductance Lm induces current from a driver line onto a
quiet line by means of the magnetic field.
• The mutual inductance Lm will inject a voltage noise onto the
victim proportional to the rate of change of the current on the
driver line. The magnitude of this noise is calculated as

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• Due to the recent trends of higher aspect ratios and lower spacing
between signal lines, the coupling capacitance is becoming
significant. Also the vertical height has not been scaled down .
2. Mutual Capacitance:-

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• If one line is switching and the other line is steady the energy transfer
through the coupling capacitance results in a glitch on the quiet line
• The inductively induced voltages were generally negligible compared to the
effects of the parasitic capacitances of the interconnect lines.
• The interconnect capacitance is modeled as Mutual Capacitance.
d(Vvictim – Vdriver)
dt

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Crosstalk induced noise:
Crosstalk is the result of mutual capacitance Cm in
conjunction with mutual inductance Lm between adjacent
conductors.
For simplicity and for measuring the magnitude of noise
induced on to the adjacent transmission lines we have to
consider the two terms :
• Near-end-crosstalk
• Far-end-crosstalk

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• The current
generated on the
victim line due to
mutual capacitance
will split and flow
toward both ends of
the adjacent line.
• The current
generated on the
victim line due to
mutual inductance
will flow from the far
end toward the near
end of the victim line
• As a result, the crosstalk currents flowing toward the near and far ends can
be broken down into several components:
•Near end crosstalk is always positive
•Far end crosstalk is always negative

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Effects of Crosstalk:
• Signal Integrity Illustration
• Noise on Delay effect
• Destroying Local Information
• Timing Noise

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1. Signal Integrity Illustration:
The term Signal Integrity (SI) addresses two concerns in the electrical
design aspects – the timing and the quality of the signal. Due to crosstalk:
•the signal doesn’t reach its destination when it is supposed to
•when it gets there, it is not in good condition

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2. Noise on delay effect
Depending on the signal switching directions of the signals on the driver
and victim nets, the impact of interconnect cross-talk can be either,
•negative (increasing signal propagation time)
•positive (decreasing signal propagation time), which may cause setup-
or hold-time violations.

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3. Destroying local information:
When noise acts against a normally static signal, it can destroy the local
information carried by the static node in the circuit and ultimately result in
incorrect machine-state stored in a latch.
4. Timing noise
This includes the noise by,
• Skew – that is the DC components of the noise
• Jitter – that is the AC components of the noise

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Basic Approaches in Crosstalk Avoidance
 Segregation / Spacing / Ground Shielding
 Net Ordering
 Layer Assignment

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N oisy
R egion
Q uiet
R egion
Segregation Spacing
Extra
space
Segregation / Spacing/ Ground Shielding (1)

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Segregation / Spacing/ Ground Shielding (2)
 Segregation : Dividing many (noisy)
and less(quiet) signal transition wire
and merging group by group.(use with
shielding)
 Spacing : Spacing can be of two types:
 Increasing line spacing capacitive
coupling can be decreased.
 Widening metal wires decrease the
effect of capacitive coupling by
increasing the line to ground
capacitance to the total capacitance
ratio.
 Shielding : blocking signal line with
ground line to minimize signal
interference to the other wire.(ground
bounce occurs and must broaden the
ground line)
 Shielding type 1
 Shieling type 2

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Net Ordering
 Left : Unordered
track
permutation
 Right : Ordered
track
permutation for
minimizing
crosstalk
SS
SS
LL
LL
LL
SS
SS
LL
LL
LL
 Net ordering is used for minimize crosstalk-critical region
between each lines. When, long line and long line is close
together, crosstalk between them is more larger than long line
and short line. So, we must change the permutation of track for
minimizing crosstalk.

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Layer Assignment
O rdered net
Layer1
U nodered net
Layer2
Layer3
 When using more than 3 layer in channel routing, adjacent
signal wire in same layer results crosstalk. For example, left
figure makes more crosstalk than right.
 Layer assignment problem is solved by integer linear
programming or dynamic programming method.

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Minimize Crosstalk: Rule of Thumb
• Widen spacing S between the lines as much as routing restrictions will allow
• Minimize H while achieving the target impedance of the design
• Use differential routing techniques for critical nets

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•Routing adjacent layers orthogonal if there is significant inter layer
coupling
• If possible routing the signals on a strip layer or as an embedded micro-
strip to eliminate velocity variation
• Minimize parallel run lengths between signals, routing with short parallel
sections and minimize long coupled sections between nets
• If possible, use slow edge rates (with extreme caution on timing
uncertainty)

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Recently FinFET is used instead of CMOS in VLSI because, FinFET can
be significantly more power efficient than CMOS at the same gate
length.
Conclusion :
• Future scope :
• NP-hard problem
• Optimal solution are taken because some other
constraints are there like wire length and congestion

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References :
• “Algorithms for VLSI Physical design Automation by – Naveed
Sherwani
• “VLSI Design Automation” – Kia Bazragar
• J.J.Xiong & L.He.IEEE Transon CAD, 2008
• US PATIENT, PATINENT NO : US 6, 218, 631B1

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Thank You…