In this introductory text we will define two broad classes of sequential circuits, namely; asynchronous and synchronous.
• The timing of the operation of asynchronous circuits, as the name implies, is controlled by any external timing mechanism. Rather, as soon as changes are made at the inputs of such a circuit they take effect at the outputs. The simplest form of memory in such circuits is just a wire forming the feedback connection.
Synchronous circuits are those which possess a clock of some sort which regulates the feedback process. Hence the tuning of changes in the outputs, in response to changes at the inputs (which may have occurred some times before), are controlled by the kicking* of a clock. Consequently, the timing of the operation of sequential circuits can be, and usually is, synchronised to other parts of a larger circuit. The memory in such circuits is itself made up of specialised logic circuits (called flip-flops) that essentially act as digital storage elements.
Fig. 5.3 shows the block diagrams for these two types of sequential circuits. For obvious reasons, synchronous sequential circuits are also referred to as clocked circuits, whilst asynchronous ones are known as undocked or free running. Although asynchronous circuits are more difficult to design than synchronous ones they do have certain benefits. These include the fact that because they are free running their speed of operation is limited solely by the characteristics of the components from which they are built and not by the speed at which they are clocked. Consequently, asynchronous circuits have the potential to work at higher speeds than synchronous ones. Also: some systems may require a circuit to respond immediately to changing inputs (i.e. the inputs cannot be synchronised to the rest of the circuit); in very large circuits the unavoidable delays as a signal traverses the whole circuit may mean exact synchronisation is not possible，and finally flip-flops which are essential digital circuit components are themselves asynchronous circuits.
In this brief introduction the general properties and structure of sequential circuits have been introduced, together with the idea of a broad classification of such circuits as either asynchronous or synchronous.
The remainder of the chapter is split into three sections. The first is an introduction to asynchronous sequential logic circuits; the second looks at how asynchronous circuits operate via the analysis of a number of such circuits; whilst the third considers the design, and associated problems, of these circuits.
The following three chapters are also concerned with sequential logic. Chapter 6 covers flip-flops which are themselves asynchronous circuits that act as digital storage elements and are used as the memory in synchronous sequential circuits.
Chapters 7 and 8 cover synchronous sequential circuits, beginning with counters before moving on to more general examples.