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

9.3 FINITE-STATE MACHINE DESIGN CONCEPTS

The general structure of a sequential circuit consists of two parts: flip-flops and combinational circuits. The flip-flops hold the state memory of the sequential circuit. Depending on how the output of the finite-state machine is related to the state of the flip-flops, two structure models can be constructed:

1. The Moore model. In the Moore model the output of the finite-state machine depends only on the present state of the flip-flops. In other words, the output does not depend on the present inputs but, rather, on the previous inputs. The output does not explicitly include the inputs in its logic expression. The output is not valid until the flip-flops are updated during the current clock cycle. Figure 9.3 illustrates the structure of the Moore model. There is no direct connection between the inputs and the outputs of a finite-state machine. The output of a finite-state machine is a function of its present state.
2. The Mealy model. The Mealy model is the other type of sequential circuit. The Mealy model is a variation of the Moore model in which the output of the finite- state machine depends on the present states of the flip-flops and the present inputs of the finite-state machine. The logic expression of the output depends explicitly on the inputs of the finite-state machine. The output is valid before the flip-flops are updated during the current clock cycle. Figure 9.4 illustrates the structure of the Mealy model.

In either model, the ...