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Vectorless Verification of RLC
Power Grids with Transient
Current Constraints
Xuanxing Xiong and Jia Wang
Electrical and Computer Engineering
Illinois Institute of Technology
Chicago, Illinois, United States
November, 2011
Agenda
Power Grid Verification
Proposed Approach
Experimental Results
2
Power Grid Verification
Verify that the power supply noises are within certain
acceptable range
Noises depend on the patterns of currents drawn
General idea for power grid verification
First, specify currents
Second, compute noises
Simulation-based verification
DC & Transient analysis
Need to simulate a large number of current vectors to cover
usual use scenarios
No guarantee the worst noise (but not overpessimistic) can be
found.
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Vectorless Power Grid Verification
Apply optimization to find a current vector that leads
to the worst power supply noise
[Kouroussis et al DAC’03] [Qian et al ISPD’04]
Objective: maximizing power supply noise
Constraints: feasible current set all possible current vectors
No need to explicitly enumerate all possible current vectors
Trade-off: accuracy of feasible current set and solution
efficiency
Linear current constraints: linear programming
Steady-state vectorless verification
For worst-case DC scenarios and provide bounds for RC
powergrid.
Early works are limited to small problem sizes. But recent
advances [Abdul Ghani et al DAC’09] [Xiong et al DAC’10,
ICCAD’10] have improved solution efficiency drastically.
4
Transient Vectorless Verification
Transient behaviors are more realistic
Steady-state verification could be overpessimistic.
Power grid modeling
Inductances [Abdul Ghani et al ICCAD’06]
Capacitive couplings between VDD and GND networks
[Avci et al ICCAD’10]
Current modeling
Max delta constraints [Ferzli et al TCAD’10]
Current slope constraints [Du et al ISQED’10]
Current conservation constraints [Avci et al ICCAD’10]
Power constraints [Cheng et al ISPD’11]
However, there is no constraint to restrict the
transient behavior of individual current sources.
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Our Contribution
A framework for transient vectorless verification of
RLC power grids
With both VDD & GND networks
Propose transient constraints for current sources
To capture the fact that a gate/block will only draw current
when it is switching
Prove the transient vectorless verification problem
can be decomposed into a transient power grid
anlysis problem and an optimization problem
Be able to leverage research works on fast power grid
simulation
6
Agenda
Power Grid Verification
Proposed Approach
Experimental Results
7
Integrated RLC Power Grid
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The System Equation
Time domain
G: conductance
M/C: represent self-inductance/capactiance links
v(t): nodal voltage noises
^
I(t): current excitations
Discretization with time step t
where
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Current Constraints
[Kouroussis et al DAC’03] and [Avci et al ICCAD’10]
Local Constraints
Global Constraints
Current Conservation Constraints
10
Our Transient Current Constraints
Nts: number of time steps
IT: nx1 upper bound vector
Transient constraints may be extracted from the
circuit by switching activity analysis, e.g.
[Morgado et al ICSD’09] and [Morgado et al TODAES’09]
11
Our Problem Formulation
For each node j
The formulation actually computes the worst noise at
node j for all time slots kt
If the cumulative effects of voltage noises are of
interests, e.g. similar to [Evmorfopoulos et al
ICCAD’10], the objective function can be
12
Property of System Equation
There exists a unique series of nxn matrices S1, S2, ...
Sk, Sk+1, ..., such that
jth column of Sk can be computed as
Sk is symmetric. So
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Our Problem Decompostion
For each node j:
Sub-problem I: transient analysis with current
excitation ej to compute cj,k
Sub-problem II: linear programming (LP) to compute
worst-case voltage noises
14
Agenda
Power Grid Verification
Proposed Approach
Experimental Results
15
Experimental Setup
Implement the RLCVN in C++
Use PCG with a random-walk based preconditioner for
transient analysis
Adopt MOSEK to solve the LP problems
Randomly generate 6 RLC power grids with 4 metal
layers, 1.2V VDD, and various constraints
Time step = 10ps, number of time steps Nts = 100
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A Simple Case Study
Left: no transient constraint, max voltage drop is 118.4mV.
Right: IT = 200mA, max voltage drop at node j is 86.5mV.
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Overestimation without Transient
Constraints for a Random Node
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Average Runtime per Node
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Conclusion & Future Work
The proposed transient constraints make the voltage
noise predicitons more realistic.
The proposed decomposition results in an effective
method for transient vectorless verification.
To handle even larger power grid verification
problems, it is necessary to research more efficient
algorithms to solve the LP problems for worst-case
voltage noises.
20
Thanks!
21
Our RLCVN Algorithm
Can be extended to verify the integral of voltage
noise without any computational overhead
22