Fault effects in asynchronous sequential logic circuits

The paper demonstrates the effects of single stuck-at faults in Huffman-model asynchronous sequential logic circuits (ASLCs). The fault effects include equivalent-state redundant faults, invalid-state redundant faults and state oscillations. Equivalent-state redundant faults in ASLCs may be generated by violation of the fundamental mode constraint noncritical races or delays. On the other hand, invalid-state redundant faults are caused either by the existence of invalid states, or by improperly assigning the "don\´t-care" terms. State oscillations are generally caused by the presence of critical races. Based on the fault effects, this paper presents a set of rules for synthesising oscillation-free ASLCs in the presence of faults. As far as synthesising testable ASLCs is concerned, the race-free UDC state assignment is much better than STT state assignment.