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 A TPG 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 A TPG. The results show nearly an order of magnitude or greater saving in the A TPG 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