Digital Logic Testing and Simulation, 2nd Edition

CHAPTER 5

Sequential Logic Test

5.1 INTRODUCTION

The previous chapter examined methods for creating sensitized paths in combinational logic extending from stuck-at faults on logic gates to observable outputs. We now attempt to create tests for sequential circuits where the outputs are a function not just of present inputs but of past inputs as well. The objective will be the same: to create a sensitized path from the point where a fault occurs to an observable output. However, there are new factors that must be taken into consideration. A sensitized path must now be propagated not only through logic operators, but also through an entirely new dimension—time. The time dimension may be discrete, as in synchronous logic, or it may be continuous, as in asynchronous logic.

The time dimension was ignored when creating tests for faults in combinational logic. It was implicitly assumed that the output response would stabilize before being measured with test equipment, and it was generally assumed that each test pattern was independent of its predecessors. As will be seen, the effects of time cannot be ignored, because this added dimension greatly influences the results of test pattern generation and can complicate, by orders of magnitude, the problem of creating tests. Assumptions about circuit behavior must be carefully analyzed to determine the circumstances under which they prevail. ...