Digital Logic Design, 4th Edition

9

Event driven circuits

9.1 Introduction

Some sequential circuits are driven by events rather than by a train of clock pulses. For example, a digital alarm will be activated by the event that raised the alarm. In this example it is the event that drives the logic, and since the events are frequently irregular occurrences, such a circuit is referred to as an asynchronous sequential circuit or, perhaps more meaningfully, as an event driven circuit.

Asynchronous circuits are also called fundamental mode circuits. The main characteristic of this type of circuit is that only one input is allowed to change at any given instant. Simultaneous changes are forbidden as, indeed, are changes that may take place before the circuit reaches a stable condition ...