An economical scan design for sequential logic test generation

Abstract

A method of partial scan design in which the selection of scan flip-flops is aimed at breaking up the cyclic structure of the circuit is presented. Experimental data are given to show that the test generation complexity may grow exponentially with the length of the cycles in the circuit. This complexity grows only linearly with sequential depth. Graph-theoretic algorithms are presented to select a minimal set of flip-flops for eliminating cycles to reduce sequential depth. Tests for the resulting circuit can be efficiently generated by a sequential logic test generator. An independent control of the scan clock allows the insertion of scan sequences within the vector sequence produced by the test generator. Experimental results on a 5000 gate circuit show that a test coverage above 98% could be obtained by scanning just 5% of the flip-flops. In addition, the authors give the design of a scan flip-flop to reduce the input pin and signal routing overheads in a single-clock design.<>