Combinational Networks " KUANG - WEI CHIANG AND ZVONKO

This correspondence points out that the scope of application of the matrix model for MOS complex gates proposed by El.ziq and Su' does not cover general structures of this type. Also, an example is given to show that a complete detection test set for single stuck-at faults may not be able to detect all multiple faults in a fanout-free and irredundant MOS complex gate. Index Terms -Fault detection, MOS logic circuits, test generation. In a recent paper,1 El-ziq and Su considered the test generation problem for detecting and locating stuck-at faults in MOS complex gates. They proposed a matrix model for representation of MOS complex gates. Based on this model, they developed test generation procedures which were presented and illustrated in the paper. However, the applicability of their matrix model is limited to MOS complex gates with series-parallel connection structures in the driver part of the circuit. It is well known that a MOS transistor is a bilateral device [1]. This bilateral property enables one to use MOS transistors as bidirectional switches or transmission gates. Thus, it is possible to construct MOS complex gates with more general nonseries-parallel driver structures. A simple example is shown in Fig. 1. Here, we note that the transistor driven by C is used as a bidirectional switch. Currents flow in different directions through this transistor in the paths A -C-E and B-C-D from the gate output terminal to ground. This complex gate cannot be represented by the matrix model. A more natural model for representation of MOS complex gates without such a limitation is the connection graph [2]. El-ziq and Su also showed that for an irredundant fanout-free MOS complex gate with a series-parallel driver structure, a comManuscript received September 6, 1983. K. -W. Chiang is with the Department of Electrical Engineering, University of Ottawa, Ottawa, Ont., Canada. Z. G. Vranesic is with the Department of Electrical Engineering, University of Toronto, Toronto, Ont., Canada. 'Y.M. El-ziq and S.Y.H. Su, IEEE Trans. Comput., vol. C-31, pp. 129-139, Feb. 1982. V00 f w AD +ACE +BCD +BE