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Transcript
```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.

3
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
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

 Power grid modeling


Inductances [Abdul Ghani et al ICCAD’06]
Capacitive couplings between VDD and GND networks
 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.
5
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
8
The System Equation
 Time domain

G: conductance
 v(t): nodal voltage noises
^
 I(t): current excitations

 Discretization with time step t
where
9
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.
11
Our Problem Formulation
 For each node j

The formulation actually computes the worst noise at
node j for all time slots kt
 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
13
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
16
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.
17
Overestimation without Transient
Constraints for a Random Node
18
Average Runtime per Node
19
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