Lecture_17

# Lecture_17 - 11/15/2011 Overview: Partial-Scan &amp; Scan...

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11/15/2011 1 ECE 553: TESTING AND TESTABLE DESIGN OF DIGITAL SYSTEMS Design for Testability (DFT) - 2 11/15/2011 2 Overview: Partial-Scan & Scan Variations • Definition Partial-scan architecture • Scan flip-flop selection methods • Cyclic and acyclic structures • Partial-scan by cycle-breaking • Scan variations Scan-hold flip-flop (SHFF) • Summary 11/15/2011 3 Partial-Scan Definition • A subset of flip-flops is scanned. • Objectives: – Minimize area overhead and scan sequence length, yet achieve required fault coverage – Exclude selected flip-flops from scan: • Improve performance • Allow limited scan design rule violations – Allow automation: • In scan flip-flop selection • In test generation – Shorter scan sequences 11/15/2011 4 Partial-Scan Architecture FF FF SFF SFF Combinational circuit PI PO CK1 CK2 SCANOUT SCANIN TC 11/15/2011 5 Scan Flip-Flop Selection Methods • Testability measure based – Use of SCOAP: limited success. • Structure based: – Cycle breaking – Balanced structure • Sometimes requires high scan percentage • ATPG based: – Use of combinational and sequential TG 11/15/2011 6 Cycle Breaking • Difficulties in ATPG • S-graph and MFVS problem • Test generation and test statistics • Partial vs. full scan • Partial-scan flip-flop

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11/15/2011 2 11/15/2011 7 Difficulties in Seq. ATPG • Poor initializability. • Poor controllability/observability of state variables. • Gate count, number of flip-flops, and sequential depth do not explain the problem. • Cycles are mainly responsible for complexity. • An ATPG experiment: Circuit Number of Number of Sequential ATPG Fault gates flip-flops depth CPU s coverage TLC 355 21 14* 1,247 89.01% Chip A 1,112 39 14 269 98.80% * Maximum number of flip-flops on a PI to PO path 11/15/2011 8 Benchmark Circuits Circuit PI PO FF
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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_17 - 11/15/2011 Overview: Partial-Scan &amp; Scan...

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