Combinational fault simulation in sequential circuits

We propose a very fast fault simulation method which is based on exact parallel critical path tracing developed for combinational circuits. To convert the sequential problem of fault simulation into the combinational one we introduce into the circuit a set of MISRs to improve the circuit´s observability. The role of these MISRs is to monitor signals on the global feedback loops, and on selected fan-out stems in the circuit. The given sequential circuit is partitioned into a set of sequential or combinational sub-circuits, with breakpoints at global feedback loops or at selected fan-out stems. The simulated test sequence is mapped into local sets of input patterns applied to the sub-circuits. For these local test patterns, each sub-circuit is fault simulated by exact parallel critical path tracing similarly as a combinational equivalent circuit. The feasibility and correctness of the method is shown, and the experimental results which demonstrate the speed-up achieved by the method are provided.