Sequential Test Generation with Advanced Illegal State

TPG for synchronous sequential circuits has received wide attention over the last two decades, yet unlike for (full-scan) combinational circuits, for many sequential benchmark circuits 100% fault efficiency still cannot be reached. This illustrates the complexity of sequential circuit ATPG. The huge search space, which exists during sequential circuit TPG, is the main reason for this complexity. Powerful techniques and heuristics are required to cope with this search space. One way to reduce the search space is the detection of illegal states. These states cannot be justified with an initialization sequence. In this paper, we propose new techniques to find illegal states and to remove the over-specification of these states by searching common fractions in the list of illegal states. Experimental results demonstrate the importance of an as complete as possible illegal state list: Higher fault efficiencies are reached for the sequential ISCAS '89 circuits [1] and industrial circuits, together with a large reduction of CPU time.