Definition and generation of partially-functional broadside tests

Abstract

Functional and pseudo-functional broadside tests were defined to address the fact that testing a circuit under non-functional operation conditions, which are made possible by scanning in unreachable states, may result in overtesting or unnecessary yield loss. One of the reasons cited for overtesting is the high switching activity possible with unreachable states. Functional broadside tests allow only reachable states as scan-in states in order to address this issue. However, the exclusive use of functional (or pseudo-functional) broadside tests may result in a loss of fault coverage, which may be a concern for the long-term reliability of the chip. To address this issue, a new class of tests is defined, which is referred to as partially-functional broadside tests. The definition of a partially-functional broadside test allows the scan-in state to be an unreachable state. It measures the amount of deviation from functional operation by considering the minimum Hamming distance between the scan-in state and a reachable state. With a smaller minimum Hamming distance, the number of state variables that operate under functional operation conditions is likely to be higher. A procedure for computing what are called partially-reachable states, which are used for generating partially-functional broadside tests is described. We demonstrate that the switching activity under partially-functional broadside tests is close to that of functional broadside tests. Thus, partially-functional broadside tests operate the circuit under conditions that are close to functional operation conditions, helping to avoid overtesting because of high switching activity while improving fault coverage.