3. Index
• ROUTING
• CROSSTALK OVERVIEW
• EFFECTS OF CROSSTALK
• APPROACHES TO AVOID CROSSTALK
• METHODS TO MINIMISE CROSSTALK
• CONCLUSION
• REFERENCES
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4. Routing
• Problem
Given a placement, and a fixed number of metal
layers, find a valid pattern of horizontal and vertical
wires that connect the terminals of the nets
Levels of abstraction:
Global routing
Detailed routing
• Objectives
Cost components:
1. Area (channel width) – min congestion
2. Wire delays – timing minimization in previous levels
3. Number of layers (fewer layers less expensive)
4. Additional cost components: number of bends, vias
5. Minimisation of crosstalk
4
6. Global vs. Detailed Routing
• Global routing
Input: detailed placement, with exact
terminal locations
Determine “channel” (routing region)
for each net
Objective: minimize area (congestion),
and timing (approximate)
• Detailed routing
Input: channels and approximate
routing from the global routing phase
Determine the exact route and layers
for each net
Objective: valid routing, minimize area
(congestion), meet timing constraints
Additional objectives: min via, power
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8. Multiple Terminal nets:Steiner Tree
Steiner tree(aka Rectilinear Steiner tree- RST):
A tree connecting multiple terminals.
Original points:”Demand points”- set D.
Added points:”Steiner points”- set S.
Edges horizontal or vertical only.
Steiner Minimum Tree (SMT)
Similar to minimum spanning tree (MST)
– But finding SMT is NP-complete
Many good heuristics introduced to find SMT
Algorithm
1 . Find MST
2 . Pass horizontal and vertical lines from each terminal to
get the Hannan grid (optimal solution is on this grid)
3. Convert each edge of the MST to an L-shaped route
on Hannan grid (add a Steiner point at the corner of L)
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11. VLSI trends:
– Device size is decreasing.
– Increase the no of transistors, interconnection wires
– Size of the channel is decreased.
Effects:
Increasing coupling effect (inductive & capacitive) between
interconnection wires
Result:
crosstalk
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13. Mutual Inductance and Capacitance
Crosstalk is the coupling of energy from one line to another via :
Mutual inductance(magnetic field)
Mutual capacitance(electric field)
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14. Mutual Inductance and Capacitance
Mechanism of coupling
the circuit element representing this transfer of energy are the familiar
equations:
Δ IB= -Cm d(VB - VA) and Δ VB= -Lm dIA
dt dt
Mutual inductance will induce current on the victim line opposite of the driving
current(Lenz’s Law).
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15. Crosstalk induced noise
The near and far end victim line currents sum to produce the near
and far end crosstalk noise.
Coupled currents:
I near=Icm + I lm I far=Icm - I lm
• Current induced by capacitive coupling goes to both directions
• Current induced by inductive coupling goes opposite to the drive current
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16. Crosstalk induced noise
“Voltage profile of coupled noise”
• Near end crosstalk is always positive
• currents from Lm and Cm always add and flow into the node.
• For PCB’s far end crosstalk is “usually” negative
• current due to Lm larger than current due to Cm.
• Note that far end crosstalk can be positive.
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17. Noise: A Key Stopper in Mixed Signal Systems
Skin effect.
Dielectric absorption
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20. Effects of crosstalk (contd…)
3.Crosstalk can lead to :
- logic faults(especially in dynamic circuits).
- Voltage overshoot(stress,forward biased PN junctions)
4. 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.
5.Timing noise
skew(DC component of timing noise).
jitter(AC component of timing noise).
6. EMI and violation of EMC requirements.
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22. Segregation / Spacing / Ground Shielding (2)
• Segregation : Dividing many
(noisy) and less(quiet) signal
transition wire and merging
group by group.(use with
shielding)
• Spacing : the method that
signal wire to shun each other,
when signal net is close to
each other (routing channel is
not wide)
• Shielding : blocking signal line
with ground line to minimize
signal interference to the other
wire.(ground bounce occurs
and must broaden the ground
line)
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23. Net Ordering
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.
• Left : Unordered
track permutation
• Right : Ordered
track permutation
for minimizing
crosstalk
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24. Layer Assignment
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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25. Various Techniques To Reduce
Crosstalk
The following PCB design techniques can significantly
reduce crosstalk in micro-strip or strip-line layouts:
SOME TECHNIQUES ARE A RULE OF THUMB.
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26. 1. Widen spacing S between the signallines as much as routing restrictions will allow.
2. Design the transmission line so that the conductor is as close to the ground plane as
possible. This couples the transmission line tightly to the ground plane and helps
decouple it from adjacent signals.
3. Use differential routing techniques where possible, especially for critical nets.
4. Route signals on different layers orthogonal to each other, if there is significant coupling.
5. Minimize parallel run lengths between signals, routing with short parallel sections
and minimize long coupled sections between nets. 26
27. CROSSTALK ESTIMATION
Crosstalk-estimation:
Bounded partitioning:
partitions the X-talk bound of each net into the regions its go
through
Net ordering:
orders the net in each regions to require as few spare track as
possible
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28. Crosstalk Constraints Global Routing
(CCGR)
• NP-hard problem
• Two stage heuristic approach
– New Steiner tree formulation to minimize the total X-talk.
– X-talk on each net is estimated.
– nets having X-talk violation is then ripped up and re-routed
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29. 6.Minimum X-talk Steiner tree
Routing graph G={V,E}
Minimized X-talk Extended Global Routing solution for nets 1…. M-1
is given
Place Mth net such that routing topologies for 1 ………. M-1 are kept and
total X-talk is minimized
Rip-Up & Rerouting
If solution violates crosstalk constraints
Rip-up them
Re-route them one net at a time in the order of
decreasing violation
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30. EXAMPLE
Nets 1,2,3,4, are routed and net 5 is Shortest path root (not
{p1 & p2} c1=2,c2=3,c3=30, feasible): r1=12,r2=20,r3=4,
c4=36,c5=35 r4=0,r5=32
30
33. Crosstalk reduction techniques(contd…)
8. A structure for reducing crosstalk in VLSI circuits :
the empty areas(3) are filled with ground connections(7 & 11) fig2 below
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34. Conclusion
NP-hard problem.
Many others approaches are available.
Some GA based approaches are very successfully implemented
Optimal Solution are taken because some other constraints are there like
wire length and congestion .
Also add routing of virtex 5 diagonal from whitepaper
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