layer assignment algorithm for rlc crosstalk minimization
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23/4/22
Layer Assignment Algorithm for RLC Crosstalk Minimization
Bin Liu, Yici Cai, Qiang Zhou, Xianlong Hong
Tsinghua University
23/4/22
Outline
Motivation and ObjectiveMotivation and Objective Crosstalk Cost Estimation Algorithm Description Experimental Results Conclusion
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Motivations
Crosstalk is becoming a vital factor in high performance designs.
Crosstalk consideration in early design stages is necessary.
Appropriate method to evaluate the crosstalk cost is needed for early crosstalk avoidance and mitigation.
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Motivations
Layer assignment is an ideal step to address crosstalk. Global routing: flexible but inaccurate Detailed routing: accurate but lacks
flexibility As an intermediate stage, layer
assignment combines accuracy and flexibility
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Objective
Minimize RLC crosstalk risk in layer assignment.
Minimize congestion in each layer. Cooperate with further crosstalk
mitigation techniques like shield insertion.
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Outline
Motivation and Objective Crosstalk Cost EstimationCrosstalk Cost Estimation Algorithm Description Experimental Results Conclusion
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Noise Budgeting
Assumption : Noises on each segment of a wire are independent on each other.
Given the noise constraint at a sink, we can distribute the noise onto each segment along the net according to segment length.
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Cost Function
Basically, on one GRC Edge
So the cost of crosstalk on one GRC is
1,
_ ,t
l l k
real noise N k l
_ _cost real noise noise budget
_noise budget L
1,
,t
k k l
Lcost N k l
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Further Explanation
From another perspective: The same noise voltage on longer
nets is more dangerous, the severity is proportional to the length
Severity is inversely proportional to the constraint
So, we use as a weight factor of noise
L
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Noise Calculation
Various noise models with different considerations can be used.
Capacitive and Inductive Statistically, the noise voltage is
approximately proportional to the congestion of sensitive nets
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Outline
Motivation and Objective Crosstalk Cost Estimation Algorithm DescriptionAlgorithm Description Experimental Results Conclusion
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Problem Formulation Given:
Number of layers The GRC’s that a net passes Noise constraint at each sink Sensitive matrix Routing resources on each layer
Output: Layer pair for each net
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Problem Formulation
Optimize Crosstalk cost Congestion
Congestion should be even at each layer Including the shields to be inserted
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Algorithm Flow
2-phase Algorithm Initial solution
Assign the nets according to the weight Among all possible assignment, the least-
cost solution is adopted Optimize using simulated annealing
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Algorithm FlowRepeatRepeat Ei is the unprocessed GRG-Edge with max W(e) RepeatRepeat
Nj is the unassigned net on Ei with max W(Nj)ForFor all layer pairs Assuming Nj is assigned to current layer pair Calculate global costEnd forEnd forAssign Nj to the layer pair with min global costUpdate the congestion of the layer pair
UntilUntil all nets that pass Ei has been assignedUntilUntil all GRG-Edges has been processed
Optimize the solution with simulated annealingOutput final solution
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Strategies
Areas with larger congestion should take priority
Nets with higher sensitive rate should take priority
Nets with lower noise tolerance should take priority
Longer nets should take priority
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Outline
Motivation and Objective Crosstalk Cost Estimation Algorithm Description Experimental ResultsExperimental Results Conclusion
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Experimental Results
Industry instance, IBM benchmark Compared to a previous layer
assignment work in ASICON’03 Max coupling length of a net Congestion
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Experimental Results
max congestion ratio, ours/pre
SensRate
20% 30% 40% 50%
Gdc0.875/
0.80.875/
0.80.8/0.8
0.8/0.8
mibm01
0.55/0.45
0.5/0.45
0.45/0.45
0.5/0.45
mibm02
0.78/0.67
0.78/0.67
0.89/0.67
0.78/0.67
mibm07
0.64/0.56
0.62/0.56
0.60/0.56
0.62/0.56
mibm08
0.71/0.57
0.71/0.57
0.71/0.57
0.71/0.57
mibm10
0.57/0.57
0.63/0.57
0.6/0.57
0.6/0.57
max coupling length, ours/pre
SensRate
20% 30% 40% 50%
Gdc52/99
73/116
96/166
145/182
mibm01
74/123
74/137
83/224
125/240
mibm02
26/48
29/67
50/95
74/118
mibm07
32/54
62/67
78/91
129/213
mibm08
55/79
73/151
146/251
176/233
mibm10
91/135
130/171
155/233
216/340
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Outline
Motivation and Objective Crosstalk Cost Estimation Algorithm Description Experimental Results ConclusionConclusion
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Conclusion Crosstalk control in layer assignment is
necessary and effective. With consideration of net length, we
could prevent longer nets suffer too much noise.
Successive work could combine layer assignment with track assignment, wire sizing/spacing to achieve better results. This work is on going.
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Thanks.
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