Formal verification of partial good self test fencing structures
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Formal Verification of Partial Good Self-Test Fencing Structures. Rick Seigler, Gary Van Huben, Hari Mony. Outline. Overview of Partial LBIST Fencing Traditional Approach to Partial LBIST Fencing Verification Verification Model Overview Methodology Flow Verification Results

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Formal verification of partial good self test fencing structures l.jpg

Formal Verification of Partial Good Self-Test Fencing Structures

Rick Seigler, Gary Van Huben, Hari Mony


Outline l.jpg
Outline Structures

  • Overview of Partial LBIST Fencing

  • Traditional Approach to Partial LBIST Fencing Verification

  • Verification Model Overview

  • Methodology Flow

  • Verification Results

  • Tuning Considerations

  • Summary and Conclusions

Rick Seigler et al.


Overview of partial lbist fencing l.jpg

Core 1 Structures

Core 2

Sequential

Sequential

Sequential

Logic

Logic

Logic

Core 3

Core N

Overview of Partial LBIST Fencing

Multiple core chip with common logic

Common Logic

DesignUnder Test (DUT)

PartialGood Interface Core 1

PartialGood Fence Core 1

PartialGood Interface Core N

MISR

PartialGood Fence Core N

Red Latch Represents Non Partial Good Interface or Common Logic

Rick Seigler et al.


Traditional approach to partial lbist fencing verification l.jpg
Traditional Approach to Partial LBIST Fencing Verification Structures

  • Logic Simulation

    • Exercise LBIST procedure to obtain and verify LBIST signature

    • Major limitation is that simulation of LBIST procedure is inherently complex

      • Requires proper initialization

      • Requires complex driver sequencing

      • Even more complex with multiple clock domains

      • Time consuming to get running

      • Best case verification run times are typically measured in days and increases proportional to chain length

      • Not possible to prove correctness because can't cover all possible state transitions via simulation

Rick Seigler et al.


Verification model overview l.jpg

Sequential Structures

Sequential

Sequential

Sequential

Sequential

Sequential

Logic

Logic

Logic

Logic

Logic

Logic

Verification Model Overview

Formal Verification Model using SixthSense Sequential Equivalence Checking

DUT

Inactive state

Partial Good Interface Signal 1

Non-deterministic

Partial Good Fence Signal 1

Active state

Model 1

Driver

PartialGood Interface Signal N

MISR

Partial Good Fence Signal N

Equiv

Check

DUT

Partial Good Interface Signal 1

Partial Good Fence Signal 1

Model 2

Driver

X-State

Detect

PartialGood Interface Signal N

MISR

Partial Good Fence Signal N

Rick Seigler et al.


Methodology flow l.jpg
Methodology Flow Structures

STEP 1

IDENTIFY PG

INTERFACES

STEP 6

OVERRIDE SCAN INPUTS TO INVERTED LATCHES

STEP 4

CREATE X-STATE ASSERT

STEP 2

CREATE WRAPPER

STEP 5

CHECK PROPERTIES

STEP 7

REBUILD MODELS AND RE-CHECK PROPERTIES

STEP 3

CREATE

DRIVERS

Y

N

INVERSIONS ?

Y

N

N

Y

PROPERTY

VIOLATIONS ?

INVERSIONS ?

DESIGN

BUG(S)

DONE

Rick Seigler et al.


Verification results l.jpg
Verification Results Structures

Rick Seigler et al.


Tuning considerations l.jpg
Tuning Considerations Structures

  • Two primary challenges

    • Quickly find bugs

      • Used SAT-based Bounded Model Checking (BMC) on speculatively reduced model

    • Efficiently complete proofs

      • Imperative since model size and diameter limits the # of BMC cycles

  • Strategy: Sequential redundancy removal [MBPK 05] using assume-then-prove paradigm

    • Guess candidates using name comparison, semi-formal analysis, etc

    • Assume candidates to be redundant and create speculatively reduced model

    • Validate the correctness of candidates (proof step)

  • Bug Finding

    • BMC on original model ran out of resources due to model size and diameter

    • BMC on the spec-reduced model [MBPK 05] was successful and avoided resource crunch

  • Proof Completion

    • Inductive analysis insufficient; localization transformations very effective

    • Identified causal redundancy candidates that made proofs difficult; very useful

  • Rick Seigler et al.


    Summary and conclusions l.jpg
    Summary and Conclusions Structures

    • Case study on IBM z-Series multi-core chip demonstrated our partial lbist verification methodology is:

      • Scalable

        • More than a million latches and gates in DUT

      • Fast

        • Verification run times less than 30 minutes

      • Easy to implement

        • Knowledge of LBIST design details and sequences not required

        • Drivers easily auto-generated once partial good interfaces and fence signals identified

        • No complex assertions

      • Applicable to any partial good self-test structure

    • Six design bugs found and resolved prior to initial release

      • Very unlikely would have been discovered with simulation

    Rick Seigler et al.