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Pseudorandom Testability – Study of the Effect of the Generator Type

Pseudorandom Testability – Study of the Effect of the Generator Type. Petr Fišer, Hana Kubátová Czech Technical University in Prague Dept. of Computer Science & Engineering Karlovo nám. 13, CZ-121 35, Prague 2, Czech Rep. E-mail: fiserp@fel.cvut.cz, kubatova@fel.cvut.cz. Outline.

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Pseudorandom Testability – Study of the Effect of the Generator Type

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  1. Pseudorandom Testability – Study of the Effect of the Generator Type Petr Fišer, Hana Kubátová Czech Technical University in Prague Dept. of Computer Science & Engineering Karlovo nám. 13, CZ-121 35, Prague 2, Czech Rep. E-mail: fiserp@fel.cvut.cz, kubatova@fel.cvut.cz

  2. Outline • Introduction • LFSR Structure • Fault Coverage Statistics • Influence of LFSR • Mixed-Mode BIST • Column-Matching • BIST Design Results • Conclusions

  3. Introduction • Most of BIST methods are based on LFSR • Fault coverage strictly depends on the LFSR type

  4. LFSR • Parameters: • Generating polynomial • Seed

  5. Fault Coverage Statistics • Pseudo-Random Testability • Depends on the number of hard-to-detect faults • The number of pseudo-random test patterns strictly depends on the LFSR polynomial & seed • This number notably varies

  6. Fault Coverage Statistics – complete fault coverage

  7. Fault Coverage Statistics – non-complete fault coverage

  8. Influence of the LFSR type

  9. Influence of the LFSR type • Important observation – the fault coverage capabilities are steadily distributed • Primitive polynomials are not needed (if the period is satisfied) • Best poly: one-tap

  10. Mixed-Mode BIST • The test is divided into two phases • Pseudo-Random – try to detect easy-to-detect faults • Deterministic – generate deteterministic patterms

  11. Column-Matching • LFSR produces code words • These have to be transformed into deterministic patterns (APTG) => Output Decoder

  12. Column-Matching • Mixed-Mode BIST • Simulate first n LFSR patterns • Determine undetected faults • Compute a test for them (APTG) • Make a decoder producing test from LFSR patterns > n

  13. Influence of the test length

  14. Influence of LFSR

  15. Conclusions • The effect of the LFSR type on the fault coverage was studied • FC strictly depends on LFSR poly & seed • But cannot be computed • Primitive polynomial is not necessary • The best poly is one-tap • Shown in praxis – Column-Matching BIST

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