Overview

Objective: Complex large designs are normally synchronous, with sequential circuits generally accounting for a large portion of the system. To construct them, registers are needed. For that reason, the discussion on sequential circuits, which spans Chapters 13 15, starts with the study of registers. Such units can be separated into two kinds, called latches and flip-flops. The former can be further divided into SR and D latches, while the latter can be subdivided into SR, D, T, and JK flip-flops. All six are studied in this chapter, but special attention is given to the D latch and to the D flip-flop, because they are responsible for almost the totality of register-based applications.

Chapter Contents

13.1 Sequential versus Combinational Logic

As described in Section 11.1, a combinational logic circuit is one in which the outputs depend solely on the current inputs. Thus the system is memory less and has no feedback loops, as in the model of Figure 13.1(a). In contrast, a sequential logic circuit is one in which the outputs do depend on previous system states, so storage elements are necessary, as well as a clock signal that is responsible...