The general form of a synchronous sequential circuit is shown in Fig. 8.1. To recap, this has: external inputs, A, and outputs, Z; a combinational block can be considered in two parts; and 'memory’ in the form of flip-flops. The two parts of the combinational block serve to provide the internal outputs to the flip-flops, Y, and the external outputs, Z.
Obviously a circuit could have a simpler form and still be a synchronous sequential circuit. For instance it may have no external inputs or the external outputs may be functions of only the flip-flop's outputs (the present state variables). Consideration of such simplified circuits leads to a useful way of classifying sequential synchronous circuits.
8.2.1 Autonomous circuits
Autonomous circuits are those with no external inputs (except for the clock line) and which therefore perform independently (autonomously) of other circuits around them. Such circuits move through a set cycle of states as the circuit ^ clocked. The synchronous counters in the last chapter come into this category- However, the states of a general autonomous circuit obviously need not follow a binary sequence and furthermore the external outputs need not simply be the outputs from the flip-flops (as with the synchronous counters) but could be functions of these (present state) signals. An example of an autonomous circuit is presented in Section 8.3.1.
8.2.2 General (Moore and Mealy) circuits
The next state of a general synchronous sequential circuit is dependent nor only on the present state, as in an autonomous circuit, but also on the external inputs. Such general circuits can be further subdivided into two classes which are commonly referred to as Moore and Mealy models.
The Moore model describes a general synchronous sequential circuit where the external outputs arc only functions of the circuit^ present states (i.e. the flip-flops5 outputs). Because of this in the state diagram of such a circuit the external outputs can be linked explicitly to the nodes (i.e. states). An example of such a circuit is given in Section 8.3.2.
The Mealy model is the most general since not only is the next state dependent upon the present stale and the external inputs, but the external outputs are also functions of both of these sets of variables. Since the external outputs also depend upon the external inputs then in the state diagram of Mealy circuits the external outputs cannot simply be associated with a node but rather must be linked to the arrows (connecting the nodes) which are labelled with the output conditions as appropriate.