Lecture_10 - 10/6/2011 Overview Motivation ATPG Systems...

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10/6/2011 1 ECE 553: TESTING AND TESTABLE DESIGN OF DIGITAL SYSTES ATPG Systems and Testability Measures 10/6/2011 2 Overview • Motivation • ATPG Systems – Fault simulation effort – Test generation effort • Testability measures – Purpose and origins – SCOAP measures • Combinational circuit example • Sources of correlation error • Sequential circuit example – Test vector length prediction – High-Level testability measures • Summary 10/6/2011 3 Motivation • ATPG Systems – Increase fault coverage – Reduce over all effort (CPU time) – Fewer test vectors (test application time) • Testability measures – A powerful heuristic used during test generation (more on its uses in later slides) 10/6/2011 4 ATPG Systems • Reduce cost of fault simulation – Fault list reduction – Efficient and diverse fault simulation methods suited for specific applications and environment – Fault sampling method 10/6/2011 5 ATPG Systems • Reduce cost of test generation – Two phase approach to test generation • Phase 1: low cost methods initially – Many faults can be detected with little effort. For example use random vectors. – Initially the coverage rises rather fast. • Phase 2: use methods that target specific faults till desired fault coverage is reached 10/6/2011 6 ATPG Systems • Phase 1 issues: – When to stop phase 1 and switch to phase 2 • Continue as long as many new faults are detected • Do not keep a test that does not detect many new faults • When many consecutive vectors have been discarded • Etc. • .
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10/6/2011 2 10/6/2011 7 ATPG Systems • Phase 2 issues (during deterministic test generation): – Efficient heuristics for backtrace – What fault to pick next – Choice of backtrack limit – Switch heuristics – Interleave test generation and fault simulation – Fill in x’s (test compaction) – Identify untestable faults by other methods 10/6/2011 8 ATPG Systems • Efficient heuristics for backtrace – Easy/hard heuristic • If many choices to meet an objective, and satisfaction of any one of the choices will satisfy the objective – choose the easiest one first • If all conditions must be satisfied to meet the desired objective, choose the hardest one first – Easy hard can be determined • Distance from Pis and Pos • Testability measures 10/6/2011 9 ATPG Systems • Which fault to pick next – Target to generate tests for easy faults first • Hard faults may get detected with no extra effort – Target to generate tests for hard faults first • Easy faults will be detected any way, why waste time – Target faults near PIs – Target faults near POs – etc. 10/6/2011 10 ATPG Systems • Choice of backtrack limit (High backtrack more time) • It has been observed that as the number of backtrack increases the success rate goes down.Thus we may wish to keep low backtrack limit.
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This note was uploaded on 03/13/2012 for the course ECE 553 taught by Professor Ece553 during the Winter '08 term at Wisconsin.

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Lecture_10 - 10/6/2011 Overview Motivation ATPG Systems...

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