Combinational test generation for various classes of acyclic sequential circuits

It is known that a class of acyclic sequential circuits called balanced circuits can be tested by combinational ATPG. The first contribution of this paper is a modified and efficient combinational single fault ATPG method for any general (not necessarily balanced) acyclic circuit. Without inserting real hardware, we create a “balanced” ATPG model of the circuit in which all reconverging paths have the same sequential depth. Some primary inputs are duplicated and each combinational ATPG vector for this model circuit is transformed into a test sequence. Although no time-frame expansion is used, a small set of faults still map onto multiple faults. Those are identified and dealt with again by the single fault combinational ATPG. The results show nearly an order of magnitude or greater saving in the ATPG CPU time over sequential ATPG. The second contribution consists of new partial-scan algorithms to obtain three subclasses of acyclic circuits, namely, internally balanced, balanced, and strongly balanced, which have been described in the literature. Results on ISCAS ’89 circuits show that such structures require extra scan overhead, sometimes almost approaching that of full-scan, and their advantages in ATPG are marginal considering the present contribution.