Finding unreachable states of sequential circuits
Hiroyuki Yotsuyanagi, K. Kinoshita · Technology Reports of the Osaka University · 1999
Some states in a sequential circuit can not be reached by any input sequence. A state is said to be unreachable if it has no transition from any other state. A test generator wastes a time if tries to bring a state of a circuit to an unreachable state. Therefore the information about unreachable states in the circuit is useful for reducing test generation time. We have reported that obtaining strongly unreachable states, which are part of unreachable states, is useful to simplify the sequential circuit. In this paper, we present a procedure to identify strongly unreachable states for the circuit which is described by the gate-level and whose state transition table is not given. The input functions of flip-flops are used to identify the invalid combination of state values. Since the procedure to identify unreachable states requires large memory for the circuits with many flip-flops, we also present a procedure to partition the set of flip-flops in order to restrict the number of flip-flops. Experimental results for benchmark sequential circuits show the effectiveness of the proposed procedure.