Enabling LOS delay test with slow scan enable

Delay defects can be detected using Launch-off-capture (LOC) and Launch-off-shift (LOS) based delay test techniques. In terms of delay test coverage and test set size, LOS is more effective compared to LOC. However, to exercise LOS test a high speed scan enable signal is required. The cost of implementing a high speed global scan enable signal is prohibitively high. In practice, most of the commercial designs employing full scan design support only LOC based delay test. In this paper, we propose a new scan flip-flop design that is capable of exercising both LOS and LOC based delay test with a slow scan enable signal. The proposed design can achieve much higher delay fault coverage by exercising both LOS and LOC test. Furthermore, in a mixed mode scan test environment the proposed scan flip-flop can be used both as a serial scan cell as well as a random access scan (RAS) cell.