Post on 11-Dec-2015
Feb 13, 2008 E0-286@SERC 1
VLSI Testing Testability Analysis
VLSI Testing Testability Analysis
Virendra SinghIndian Institute of Science (IISc)
Bangalorevirendra@computer.org
E0-286: Testing and Verification of SoC Design
Lecture - 11
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PurposePurposeNeed approximate measure of:
Difficulty of setting internal circuit lines to 0 or 1 by setting primary circuit inputsDifficulty of observing internal circuit lines by observing primary outputs
Uses:Analysis of difficulty of testing internal circuit parts – redesign or add special test hardwareGuidance for algorithms computing test patterns – avoid using hard-to-control linesEstimation of fault coverageEstimation of test vector length
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OriginsOriginsControl theoryRutman 1972 -- First definition of controllabilityGoldstein 1979 -- SCOAP First definition of observability First elegant formulation First efficient algorithm to compute
controllability and observabilityParker & McCluskey 1975 Definition of Probabilistic Controllability
Brglez 1984 -- COP 1st probabilistic measures
Seth, Pan & Agrawal 1985 – PREDICT 1st exact probabilistic measures
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Testability AnalysisTestability Analysis
Involves Circuit Topological analysis, but no test vectors and no search algorithm
Static analysis
Linear computational complexityOtherwise, is pointless – might as well use automatic test-pattern generation and calculate:
Exact fault coverageExact test vectors
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Types of MeasuresTypes of Measures
SCOAP – Sandia Controllability and ObservabilityAnalysis ProgramCombinational measures:
CC0 – Difficulty of setting circuit line to logic 0CC1 – Difficulty of setting circuit line to logic 1CO – Difficulty of observing a circuit line
Sequential measures – analogous:SC0SC1SO
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Range of SCOAP MeasuresRange of SCOAP Measures
Controllabilities – 1 (easiest) to infinity (hardest)Observabilities – 0 (easiest) to infinity (hardest)Combinational measures: Roughly proportional to # circuit lines that
must be set to control or observe given lineSequential measures: Roughly proportional to # times a flip-flop
must be clocked to control or observe given line
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Goldstein’s SCOAP MeasuresGoldstein’s SCOAP Measures
AND gate O/P 0 controllability:output_controllability = min (input_controllabilities)
+ 1AND gate O/P 1 controllability:output_controllability = Σ (input_controllabilities)
+ 1XOR gate O/P controllabilityoutput_controllability = min (controllabilities of
each input set) + 1Fanout Stem observability:Σ
or min (some or all fanout branch observabilities)
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Controllability ExamplesControllability Examples
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Controllability ExamplesControllability Examples
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Observability ExamplesObservability ExamplesTo observe a gate input:Observe output and make other input values non-controlling
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Observability ExamplesObservability ExamplesTo observe a fanout stem:Observe it through branch with best observability
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Levelization Algorithm Levelization Algorithm Label each gate with max # of logic levels from primary inputs or with max # of logic levels from primary outputAssign level # 0 to all primary inputs (PIs)For each PI fanout:
Label that line with the PI level number, &Queue logic gate driven by that fanout
While queue is not empty:Dequeue next logic gateIf all gate inputs have level #’s, label the gate with the maximum of them + 1;Else, requeue the gate
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Controllability - Level 0Controllability - Level 0
Circled numbers give level number. (CC0, CC1)
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Controllability - Level 2Controllability - Level 2
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Combinational ControllabilityCombinational Controllability
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Observability for Level 1Observability for Level 1
Number in square box is level from primary outputs (POs).(CC0, CC1) CO
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Observabilities - Level 2Observabilities - Level 2
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Thank YouThank You