cross talk-impact of interconnect parasitics
TRANSCRIPT
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Impact of Interconnect
Parasitics• Reduce Robustness
• Affect Performance • Increase delay
• Increase power dissipation
Classes of Parasitics
• Capacitive
• Resistive
• Inductive
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INTERCONNECT
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Capacitive Cross Talk
X
Y V X
C XY
C Y
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Capacitive Cross Talk
Dynamic Node
3 x 1 mm overlap: 0.19 V disturbance
C Y
C XY
V DD
PDN
CLK
CLK
In 1 In 2 In 3
Y
X 2.5 V
0 V
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Capacitive Cross Talk
Driven Node
tXY = R Y(CXY+C Y)
Keep time-constant smaller than rise time
V (Volt)
0
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0 1 0.8 0.6
t (nsec)
0.4 0.2
X
Y V X
R Y
C XY
C Y
tr ↑
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Dealing with Capacitive Cross
Talk
• Avoid floating nodes
• Protect sensitive nodes
• Make rise and fall times as large as possible• Differential signaling
• Do not run wires together for a long distance
• Use shielding wires
• Use shielding layers
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Shielding
GND
GND
Shielding wire
Substrate ( GND )
Shielding layer
V DD
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Impact of Cross Talk on Delay
r is ratio between capacitance to GND and to neighbor
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Structured Predictable
Interconnect
S
S SV V S
G
S
S V
G
V
S
S SV V S
G
S
S V
G
V
Example: Dense Wire Fabric ([Sunil Kathri])Trade-off:• Cross-coupling capacitance 40x lower, 2% delay variation
• Increase in area and overall capacitance
Also: FPGAs, VPGAs
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Interconnect Projections
Low-k dielectrics• Both delay and power are reduced by dropping interconnect
capacitance
• Types of low-k materials include: inorganic (SiO2), organic
(Polyimides) and aerogels (ultra low-k)
• The numbers below are on the
conservative side of the NRTS roadmap
Generation 0.25mm
0.18mm
0.13mm
0.1mm
0.07mm
0.05mm
Dielectric
Constant
3.3 2.7 2.3 2.0 1.8 1.5
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Encoding Data Avoids Worst-Case
Conditions
Encoder
Decoder
Bus
In
Out
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Driving Large Capacitances
V in V out
C L
V DD
• Transistor Sizing• Cascaded Buffers
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Using Cascaded Buffers
C L = 20 pF
In Out
1 2 N
0.25 m process
Cin = 2.5 fFtp0 = 30 ps
F = CL/Cin = 8000
fopt = 3.6 N = 7tp = 0.76 ns
(See Chapter 5)
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Output Driver Design
Trade off Performance for Area and Energy
Given t pmax find N and f
• Area
• Energy
m inm inm in
12
1
1
1
1...1 A
f
F A
f
f A f f f A
N N
driver
2221211
1...1 DD
L DDi DDi
N
driver V f
C V C
f
F V C f f f E