SELF-CHECKING COMBINATIONAL-CIRCUITS WITH UNIDIRECTIONALLY INDEPENDENT OUTPUTS

In this paper we propose a structure dependent method for the systemat ic design of a self-checking circuit which is well adapted to the faul t model of single gate faults and which can be used in test mode. Acco rding to the fault model considered, maximal groups of independent and unidirectionally independent outputs of an arbitrarily given combinat ional circuit are determined. A parity bit is added to every group of independent outputs. A few additional outputs are added to every group of unidirectionally independent outputs. In the error free case, thes e groups of unidirectional independent outputs together with their cor responding additional outputs are elements of a unidirectional error d etecting code; for example, a Berger code or an r-out-of-s code. It is demonstrated how the pairs of (unidirectionally) independent outputs of a given circuit can be determined. A simple heuristic solution for this problem based on a modified circuit graph is also given. The maxi mal classes of (unidirectionally) independent outputs can be computed as cliques of a dependency graph where the nodes of the graph are the outputs of the circuit. The applicability of the proposed method is de monstrated for the MCNC benchmarks circuits.