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 , Prague 2, Czech Rep.

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

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

LFSR Parameters: Generating polynomial Seed

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

Fault Coverage Statistics – complete fault coverage

Fault Coverage Statistics – non-complete fault coverage

Influence of the LFSR type

Important observation – the fault coverage capabilities are steadily distributed Important observation – the fault coverage capabilities are steadily distributed Primitive polynomials are not needed (if the period is satisfied) Primitive polynomials are not needed (if the period is satisfied) Best poly: one-tap Best poly: one-tap

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

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

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

Influence of the test length benchrand / det.ud.vct.GEs c / / c / / / s / / / s / /

Influence of LFSR ud.vct.GEsud.vct.GEs

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