Mixed-Mode BIST Based on Column Matching

1. Mixed-Mode BIST Based on Column Matching Petr Fišer

2. Outline • Introduction to BIST • State-of-the-art Methods • Mixed-Mode BIST • Column-Matching Method • Experimental Results • A Summary of That Everything • Conclusions • What Could Be Done Yet

3. Introduction to BIST The BIST Structure • Generate test patterns • Apply the patterns to the circuit • Evaluate the response

4. Present BIST Method Mixed-Mode BIST • Combination of pseudo-random and deterministic BIST • The easy-to-detect faults are detected by pseudo-random patterns • Some patterns are generated deterministically, to detect the hard-to-detect faults

5. State-of-the-art Methods • Reseeding The pseudo-random test patterns are generated by LFSR, more LFSR seeds are applied • Weighted Pattern BIST Change the probability of occurrence of 1s and 0s in the PR sequence • Bit-Fixing, Bit-Flipping, Row-matching Modify the PR patterns by additional logic

6. Mixed-Mode BIST • Combination of pseudo-random and deterministic BIST • Two disjoint phases • Simple control logic • Simple design process

7. Column-Matching • LFSR produces code words • These have to be transformed into deterministic patterns(computed by ATPG) => Output Decoder

8. Column-Matching Basic Principle • Try to reorder test patterns, so that most of the Decoder outputs will be implemented as wires – A Column Match • This will be accomplished when the particular columns of the LFSR and test matrix will be equal • Direct match – even the Switch logic is eliminated

9. Column-Matching Example y0 = x4’ + x1 y1 = x3’ y2 = x2 x3’ + x2’ x4’ y3 = x0’ y4 = x4

10. Mixed-Mode Column-Matching • Simulate first n LFSR patterns • Determine undetected faults • Compute a test for them (ATPG) • Design the Decoder producing test from LFSR patterns > n

11. Mixed-Mode Column-Matching Example

12. Mixed-Mode Column-Matching Example

13. Experimental Results

14. Experimental Results

15. What Has Not Been Said Here Yet Well, there were more problems that had to be solved: • How to choose the lengths of the phases • How to improve the fault coverage in the pseudo-random phase • How to generate test vectors. Or better - what vectors to generate • How to choose the columns to be matched • How to find out if a particular column match is possible to be done • How to synthesize the Decoder logic

16. How to choose the lengths of the phases • Some trade-off has to be found • The PR-phase length influences fault coverage achieved  number of undetected faults  number of deterministic vectors to be generated • The Deterministic phase length influences the design time and BIST area overhead Fišer, P. - Kubátová, H.: Influence of the Test Lengths on Area Overhead in Mixed-Mode BIST, Proc. 9th Biennial Baltic Electronics Conference (BEC'04), Tallinn (Estonia), 3.-6.10.2004, pp. 201-204

17. How to improve the fault coverage in the pseudo-random phase • What LFSR polynomial and seed to choose? • Or a CA? • Or to modify the patterns somehow? Fišer, P. - Kubátová, H.: Pseudorandom Testability - Study of the Effect of the Generator Type, ECI'04, Herľany, SR, 22.-24.9.04, pp. 200-205 Fišer, P. - Kubátová, H.: Improvement of the Fault Coverage of the Pseudo-Random Phase in Column Matching BIST, Proc. 31th Euromicro Symposium on Digital Systems Design (DSD'05), Porto, (Portugal), 30.8. - 3.9.05, pp. 56-63 Fišer, P. - Kubátová, H.: Pseudorandom Testability - Study of the Effect of the Generator Type, Acta Polytechnica, Vol. 2, August 2005, CVUT, ISSN 1210-2709, pp. 47-54

18. How to generate test vectors.Or better - what vectors to generate • It’s better to generate deterministic vectors having many don’t cares • And moreover – we can improve the result by generating more vectors per one fault. It costs time, but the area overhead is smaller Well, it’s already done, there are positive results, but no publication yet.

19. How to choose the columns to be matched • NP-hard • Heuristic methods have to be used • Fast-search and Thorough-search methods proposed Fišer, P. - Kubátová, H.: Survey of the Algorithms in the Column-Matching BIST Method, Proc. 10th International On-Line Testing Symposium 2004 (IOLTS'04), Madeira, Portugal, 12.-14.7.2004, pp. 181

20. How to find out if a particular column match is possible to be done • NP-C, if don’t cares are present in the test • B-Matrix based approach Fišer, P. - Hlavička, J. - Kubátová, H.: Column-Matching BIST Exploiting Test Don't-Cares. Proc. 8th IEEE Europian Test Workshop (ETW'03), Maastricht (The Netherlands), 25.-28.5.2003, pp. 215-216

21. How to synthesize the Decoder logic • Two-level Boolean minimizer required • BOOM, FC-Min, BOOM-II developed Hlavička, J. - Fišer, P.: BOOM - a Heuristic Boolean Minimizer. Proc. International Conference on Computer-Aided Design ICCAD 2001, San Jose, California (USA), 4.-8.11.2001, pp. 439-442 Fišer, P. - Hlavička, J. - Kubátová, H.: FC-Min: A Fast Multi-Output Boolean Minimizer, Proc. 29th Euromicro Symposium on Digital Systems Design (DSD'03), Antalya (TR), 1.-6.9.2003, pp. 451-454 Fišer, P. - Hlavička, J.: BOOM - A Heuristic Boolean Minimizer, Computers and Informatics, Vol. 22, 2003, No. 1, pp. 19-51 Fišer, P. - Kubátová, H.: Two-Level Boolean Minimizer BOOM-II, Proc. 6th Int. Workshop on Boolean Problems (IWSBP'04), Freiberg, Germany, 23.-24.9.2004, pp. 221-228 … and more

22. Conclusions • Column-matching-based mixed-mode BIST method has been presented • Pseudo-random LFSR patterns are being transformed into deterministic vectors generated by ATPG by the Decoder • We try to match as many of the Decoder outputs as possible with its inputs, which yields no logic necessary to implement these outputs • Mixed-mode – two disjoint BIST phases introduced • Many more “minor” problems involved

23. What Could Be Done Yet • Adjust the width of a PRPG • More sophisticated methods should be found, to find a proper PRPG • Incorporate the ATPG into the design process – iterative test computation • Test-per-scan support • Partitioning of the CUT • Combine CM-BIST with other methods