# The Introduction of Synchronous sequential circuits

Synchronous sequential circuits were introduced in Section 5.1 where firstly sequential circuits as a whole (being circuits with ‘memory’) and then the differences between asynchronous and synchronous sequential circuits were discussed. You should be familiar with these ideas, and in particular the general form of a synchronous sequential circuit (see Figs 8.1 and 5.3) before continuing with this chapter.

As with asynchronous sequential circuits, the operation of synchronous sequential systems is based around the circuit moving from state to state. However, with synchronous circuits the state is determined solely by the binary pattern stored by the flip-flops within the circuit. (In Chapter 5 this was referred to as the internal state of the circuit.) Since each flip-flop can store a 0 or 1 then a circuit with n flip-flops has T possible states. Note that all states are stable since the present and next state variables are not connected directly but isolated due to the (not-transparent) flip-flops. The analysis and design of these circuits is based upon determining the next state of the circuit (and the external outputs) given the present state and the external inputs. This is therefore one application of the flip- flops* next state equations introduced in Chapter 6.

Following the introduction to sequential circuits in Section 5.1, Chapter 5 then dealt exclusively with asynchronous sequential circuits, concluding with an in- depth analysis of an SR flip-flop. Chapter 6 continued this theme of flip-flops which then meant that we could begin to look at synchronous sequential circuits since these use flip-flops as their ^memory5.

Chapter 7 looked at counters, which themselves are often considered as basic digital building blocks, and are therefore important digital circuits. The synchronous counters designed in Chapter 7 are in fact (simple types of) synchronous sequential circuits. In this chapter following a description of the way that synchronous sequential circuits can be classified，we will look at further examples of such circuits.