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 is\ 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 ouputs need not simply be the outputs from the flip-flops (as with the synchronous counters) but could be functions of these (present state) signals.

General (Moore and Mealy) circuits
The next state of a general synchronous sequential circuit is dependent not 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.

Moore model
The Moore model describes a general synchronous sequential circuit where the external outputs are only functions of the circuit's present states (i.e. the flip-flops' 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).

Mealy model
The Mealy model is the most general since not only is the next state dependent upon the present state 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.

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