Design-for-testability for multi-cycle broadside tests by holding of state variables

Abstract

This article describes a design-for-testability approach for increasing the transition fault coverage of multi-cycle broadside tests. Earlier methods addressed two-cycle tests. The importance of multi-cycle tests results from the ability to produce more compact test sets than possible with two-cycle tests, from the fact that when multi-cycle tests are applied at-speed, they can detect defects that are not detected by two-cycle tests and from their ability to avoid overtesting of delay faults. The approach described in this article is based on holding the values of selected state variables constant during the functional clock cycles of a multi-cycle broadside test. This allows new tests to be produced, which are different from broadside tests, without relying on nonfunctional toggling of state variables as in earlier methods for two-cycle tests. Experimental results show significant improvements in transition fault coverage using a fixed set of hold configurations for two types of multi-cycle broadside test sets: (1) test sets that are stored and applied from an external tester, and (2) functional broadside test sets that are generated using on-chip hardware.