Detection of transistor stuck-open faults in asynchronous inputs of scan cells

Abstract

Typically faults in gates internal to scan cells are assumed to be detected using what are known as flush tests. Recently we have shown that several scan cell internal faults are not detected either by the existing flush tests or by ATPG tests. A new set of tests to detect such faults were proposed. These works considered faults in fully-synchronous flip-flops. In many designs asynchronous inputs are used to set and/or reset flip-flops. Considering a scan cell implementation used in an industrial design we show that stuck-open faults in some transistors driven by asynchronous inputs require two new flush tests. Such faults, if left undetected, cause functional failures. The two new tests increase the overall stuck-open fault coverage of each scan cell by approximately 5%. This will significantly improve the overall test quality due to the large number of scan cells contained in large industrial designs. © 2008 IEEE.