Jan. 9, 2007 VLSI Design Conference 2007 1

Spectral RTL Test Generation for Microprocessors

Nitin Yogi and Vishwani D. AgrawalAuburn UniversityDepartment of ECE

Auburn, AL 36849, USA

Jan. 9, 2007 VLSI Design Conference 2007 2

Outline

Microprocessor testing Issues Problem and Approach RTL faults Spectral analysis & test generation Test set compaction RTL DFT Experimental Results Conclusion

Jan. 9, 2007 VLSI Design Conference 2007 3

Microprocessor Testing Issues

Issues arising from Increased Design Complexity Increased Test Generation Complexity

Viable Test Method: RTL test generation Advantages:

Low testing complexity Early detection of testability issues

Increased Demands on Testing Viable Test Method: Functional at-speed tests Advantages:

Better defect coverage Detection of delay faults

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Problem and Approach

The problem is … Develop an RTL-based ATPG method to

generate functional at-speed tests. And our approach is …

Circuit characterization using RTL: RTL test generation Analysis of information content and noise in RTL

vectors. Test generation for gate-level implementation:

Generation of spectral vectors Fault simulation and vector compaction

Jan. 9, 2007 VLSI Design Conference 2007 5

Faults Modeled for an RTL Module

CombinationalLogic

FF

FF

Inputs Outputs

RTL stuck-at fault sites

A circuit is an interconnect of several RTL modules.

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Walsh Functions and Hadamard Spectrum

1 1 1 1 1 1 1 11 -1 1 -1 1 -1 1 -11 1 -1 -1 1 1 -1 -11 -1 -1 1 1 -1 -1 11 1 1 1 -1 -1 -1 -11 -1 1 -1 -1 1 -1 11 1 -1 -1 -1 -1 1 11 -1 -1 1 -1 1 1 -1

H8 =

w0

w1

w2

w3

w4

w5

w6

w7

Wal

sh f

unct

ions

(or

der

8)

• Walsh functions form an orthogonal and complete set of basis functions that can represent any arbitrary bit-stream.

• Walsh functions are the rows of the Hadamard matrix.

• Example of Hadamard matrix of order 8:

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Analyzing Bit-Streams

0 to -1

Bit-stream

Vector 1Vector 2

.

.

.

Inp

ut

1

Inp

ut

2

. . .

Bit-stream ofInput 2

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Spectral Characterization of a Bit-Stream

Bit stream to analyze

Correlating with Walsh functions by multiplying with Hadamard matrix.

Essential component (others regarded noise)

Hadamard Matrix

Bit stream

Spectral coeffs.

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Generation of New Bit-Streams

Perturbation

Generation of new bit-stream by multiplying with Hadamard matrix

Spectral components

Essential component

retained; noise components

randomly perturbed

New bit stream

Bits changed

Sign function

-1 to 0

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PARWAN Processor

Reference: Z. Navabi, Analysis and Modeling of Digital Systems. New York: McGraw-Hill, 1993.

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Power Spectrum for “Interrupt” Bit-Stream

Spectral Coefficients

Nor

mal

ized

Pow

er

Essential components

Some noise components

Randomlevel

(1/128)

Analysis of 128 test vectors.

Jan. 9, 2007 VLSI Design Conference 2007 12

Power Spectrum for “DataIn[5]” Signal

Theoretical random noise

level(1/128)

Nor

mal

ized

Pow

er

Spectral Coefficients

Some essential

components

Some noise components

Analysis of 128 test vectors.

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Power Spectrum for Random SignalN

orm

aliz

ed P

ower

Theoretical random noise

level(1/128)

Spectral Coefficients

Analysis of 128 random vectors.

Jan. 9, 2007 VLSI Design Conference 2007 14

Selecting Minimal Vector Sequences Using ILP

Fault simulation of new sequences A set of perturbation vector sequences {V1, V2, .. , VM} is

generated. Vector sequences are simulated and all gate-level faults

detected by each are obtained. Compaction problem

Find minimum set of vector sequences that cover all detected faults.

Minimize Count{V1, … ,VM} to obtain compressed seq. {V1,… ,VC}, where {V1, … ,VC} {V1, … , VM}, and Fault Coverage{V1, … ,VC} = Fault Coverage{V1, … ,VM}

Compaction problem is formulated as an Integer Linear Program (ILP) [1].

[1] P. Drineas and Y. Makris, “Independent Test Sequence Compaction through Integer Programming," Proc. ICCD’03, pp. 380-386.

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RTL DFT Goals of DFT:

Improve controllability and observability Most hard-to-detect faults were experimentally found to

have poor observability XOR tree as DFT

Low area overhead Low performance penalty Hard-to-detect RTL faults used for observation test points 10 observation test points selected

Hard-to-detect RTL faults

To test output

XOR tree

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Experimental Results

No of RTL Faults

No. of vectors

CPU (s) RTL coverage (%)

Gate-level fault

coverage(%)

737 134 640 96.30% 81.22%

RTL characterization

PARWAN processor

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Experimental Results

Gate-level Fault Coverage

Circuit

RTL Spectral ATPG*Gate-level ATPG*

(FlexTest)Random vecs.

Cov. (%)

No. of vecs.

CPU (secs)

Cov. (%)

No. of vecs.

CPU (secs)

Cov. (%)

No. of vecs.

Parwan 98.23% 2327 2442 93.40% 1403 26430 80.95% 2814

Parwan (with DFT)

98.77% 1966 2442 95.78% 1619 20408 87.09% 2948

*Sun Ultra 5, 256MB RAM

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Experimental Results

Parwan processor

0

20

40

60

80

100

1 10 100 1000 10000

No. of vectors

Te

st

co

ve

rag

e (

%)

RTL spectralATPG

Gate-levelATPG

Randomvectors

RTL faultvectors

Jan. 9, 2007 VLSI Design Conference 2007 19

Experimental Results

Parwan processor (with DFT)

0

20

40

60

80

100

1 10 100 1000 10000

No. of vectors

Te

st

co

ve

rag

e (

%)

RTL spectralATPG

Gate-levelATPG

Randomvectors

RTL faultvectors

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Conclusion

Spectral RTL ATPG technique applied to PARWAN processor.

Proposed ATPG method provides: Good quality “almost” functional at-speed tests Lower test generation complexity Enables testability appraisal at RTL

RTL based XOR tree as DFT was found to improve results.

An alternative approach: Use functional vectors instead of RTL vectors. Yogi and Agrawal, “Spectral Characterization of Functional

Vcetors for Gate-Level Fault Coverage Tests,” Proc. VDAT, August 2006

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Thank You !

Questions ?