Covering Test Holes of Functional Broadside Tests

Abstract

Functional broadside tests were developed to avoid overtesting of delay faults. The tests achieve this goal by creating functional operation conditions during their functional capture cycles. To increase the achievable fault coverage, close-to-functional scan-based tests are allowed to deviate from functional operation conditions. This article suggests that a more comprehensive functional broadside test set can be obtained by replacing target faults that cannot be detected with faults that have similar (but not identical) detection conditions. A more comprehensive functional broadside test set has the advantage that it still maintains functional operation conditions. It covers the test holes created when target faults cannot be detected by detecting similar faults. The article considers the case where the target faults are transition faults. When a standard transition fault, with an extra delay of a single clock cycle, cannot be detected, an unspecified transition fault is used instead. An unspecified transition fault captures the behaviors of transition faults with different extra delays. When this fault cannot be detected, a stuck-at fault is used instead. A stuck-at fault has some of the detection conditions of a transition fault. Multicycle functional broadside tests are used to allow unspecified transition faults to be detected. As a by-product, test compaction also occurs. The structure of the test generation procedure accommodates the complexity of producing functional broadside tests by considering the target as well as replacement faults together. Experimental results for benchmark circuits demonstrate the fault coverage improvements achieved, and the effect on the number of tests.