Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL

9.2 SYNCHRONOUS SEQUENTIAL CIRCUITS

In a combinational circuit, the outputs at any point in time are fully determined by the inputs present at that point in time. In a sequential logic circuit, the outputs depend not only on the present inputs but also on the past behavior of the circuit. Thus, a sequential logic circuit consists of a combinational circuit and memory elements that form a feedback system, as illustrated in Figure 9.1.

The sequential circuits described in Chapter 8 perform simple functions such as shifting and counting. Registers shift data inputs in response to a clock signal. Similarly, counters generate a predetermined sequence of states and have no inputs other than the initial conditions and the clock signal. The counters designed in Chapter 8 were constructed by cascading flip-flops to count in an orderly fashion. Changing the count order requires a sequence of data input. The design methods designed so far are not adequate to design a complex sequential circuit.

A sequential circuit which has additional inputs that may change its present state is also referred to as afinite-state machine (FSM). In general, the output sequence of an FSM depends on the input sequence and the present state of flip-flops of the FSM. There are two classes of sequential circuits, synchronous and asynchronous. If a clock signal is used to control the operation of a sequential circuit, the circuit is known as a synchronous sequential circuit. Asynchronous sequential circuits do not ...