Improved$n$-Detection Test Sequences Under Transparent Scan

Abstract

The quality of test sequences for scan circuits under a test-application scheme called transparent scan as n-detection test sequences is studied. A transparent-scan sequence T is obtained from a compact single-detection combinational test set C. It is shown that for the same number of clock cycles required to apply C, the transparent-scan sequence T detects faults more times than C. It is also noted that a transparent-scan sequence based on a combinational test set contains unspecified values. The effects of specifying the unspecified values of the transparent-scan sequence on the quality of the sequence are studied by considering a random specification of these values. A procedure for modifying the scan-select subsequence of a (fully specified) transparent-scan sequence so as to improve its quality as an n-detection test sequence is also described. Finally, the extension of a transparent-scan test sequence into an n-detection test sequence that detects every target fault at least n times is considered. The results show a slower increase in test-application time with n than when combinational test sets are considered