Modeling Sequential Functionality
In Chap. 3 techniques were presented to describe the behavior of concurrent systems. The modeling techniques presented were appropriate for combinational logic because these types of circuits have outputs dependent only on the current values of their inputs. This means a model that continuously performs signal assignments provides an accurate model of this circuit behavior. When we start looking at sequential circuits (i.e., D-flip-flops, registers, finite state machine, and counters), these devices only update their outputs based upon an event, most often the edge of a clock signal. The modeling techniques presented in Chap. 3 are unable to accurately describe this type of behavior. In this chapter we describe the VHDL constructs to model signal assignments that are triggered by an event to accurately model sequential logic. We can then use these techniques to describe more complex sequential logic circuits such as finite state machines and register transfer level systems.