On the over-specification problem in sequential ATPG algorithms

The authors show that some ATPG (automatic test pattern generation) programs may err in identifying untestable faults. These test generators may not be able to find the test sequence for a testable fault, even allowed infinite run time, and may mistakenly claim it as untestable. The main problem of these programs is that the underlying combinational test generation algorithm may over-specify the requirements at the present state lines. A necessary condition that the underlying combinational test generation algorithm must satisfy is considered to ensure a correct sequential ATPG program. It is shown that the simple D-algorithm satisfies this condition while PODEM and the enhanced D-algorithm do not. The impact of over-specification on the length of the generated test sequence was studied. Over-specification caused a longer test sequence. Experimental results are presented