Isomorph-redundancy in sequential circuits

Design of irredundant and fully testable nonscan sequential circuits is a m ajor concern of logic synthesis. as the presence of undetectable faults may render an ATPG intractable. This paper outlines some intriguing properties of isomorph faults, which are sequentially undetectable as well as redunda nt. An isomorph fault in a sequential circuit makes the state diagram of th e faulty machine identical to that of the fault-free machine under certain renaming of states. Examples of reduced sequential machines whose circuit r ealization is combinationally irredundant, but isomorph-redundant, are hard to construct and very little is known about them. In this paper, many curi ous examples of such sequential circuits are presented wherein a single stu ck-at fault causes isomorphic faulty machines. An infinite family of such c ircuits may, in fact, be constructed. It is shown that even two-level irred undant circuits obtained by synthesis tools may admit isomorph-redundancy u nder multiple stuck-at faults. Various classifications and related properti es of isomorph faults are also reported. These results reveal new insight a nd understanding of redundancy in sequential circuits.