ABSTRACT
The values of future steady-state outputs of a purely combinatorial logic circuit depend only on its current inputs. In previous chapters, we have seen numerous examples of such logic circuits. The time between the introduction of input signals and the generation of the corresponding steady-state output signals is characterized as the inherent propagation delay of the circuit. In comparison, in sequential circuits the future values of the outputs are dependent on both the present and past values of the inputs. There are many digital applicationswhere the signals arefirst interpretedby the systemand thennecessaryoutputs are generated in accordancewith the sequence inwhich the input signals are received. Such systems require logic circuits that respond to the past history of the inputs. In general, sequential circuits have built-in feedback paths in them. Accordingly, one or
more of the outputs just generated may contribute to the future values of the same or a different set of outputs. Sequential circuits typically store information and are used widely in digital systems. Counters and registers are typical examples of the sequential circuits. Properly designed counters, for example, can be used to count the number of days. Every 24 h, the circuit needs to conclude that a day has just passed and accordingly increment the latest day count by 1. Thus, this circuit must be able to store information, rememberwhat was the value of theday count 24h ago, and then shouldbe able toupdate the value of theday count. An integral part of a sequential circuit is the memory unit. The memory elements are
also referred to as bistable electronic circuits; that is, they exist indefinitely in one of two binary states. Binary data are stored in amemory element by transitioning it into the 1 state to store a 1 and the 0 state to store a 0. The one or more inputs feeding the memory circuits are known as excitation inputs since they excite the circuit to reach a newer steady state. The two commonly used sequential memory elements are latches and flip-flops. Typically, the flip-flop circuit output indicates the latest state of thememory element. There are a number of different flip-flops available, each differing from one another in the number of inputs they have and in the manner in which its binary state is affected by the inputs. The changes in the values of the outputs of flip-flops often are a direct consequence of the frequency with which the circuit inputs change their values. However, there exists a special class of sequential memory device, known as a monostable multivibrator, whose output is often independent of the changes or rate of changes in the inputs. In this chapter, we introduce the characteristics of the various flip-flops.
