In other words, the output state of a “sequential logic circuit” is a function of the following three states, the “present input”, the “past input” and/or the “past output”. This means that sequential logic circuits are able to take into account their previous input state as well as those actually present, a sort of “before” and “after” effect is involved with sequential circuits. The outputs are affected only with the application of a clock pulse.Unlike Combinational Logic circuits that change state depending upon the actual signals being applied to their inputs at that time, Sequential Logic circuits have some form of inherent “Memory” built in. The synchronization is achieved by a timing device known as a system clock, which generates a periodic train of lock pulses. Synchronous sequential circuit:Ī sequential circuit whose behavior can be defined from the knowledge of its signal at discrete instants of time is referred to as a synchronous sequential circuit. In these systems, the memory elements are affected only at discrete instants of time. The memory elements used in sequential circuits are flip-flops which are capable of storing binary information. Since the designs of asynchronous circuits are more tedious and difficult, their uses are rather limited. And in such a case, the system becomes unstable. However, in an asynchronous circuit, events are allowed to occur without any synchronization. There is no need to wait for a clock pulse. Therefore, in general, asynchronous circuits are faster than synchronous sequential circuits. The commonly used memory elements in these circuits are time-delay devices. The output will be affected whenever the input changes. Asynchronous sequential circuit:Ī sequential circuit whose behavior depends upon the sequence in which the input signals change is referred to as an asynchronous sequential circuit. There are two types of sequential circuits, and their classification is a function of the timing of their signals. Thus, a sequential circuit is specified by a time sequence of inputs, outputs, and internal states. The next state of the storage elements is also a function of external inputs and the present state. The block diagram demonstrates that the outputs in a sequential circuit are a function not only of the inputs, but also of the present state of the storage elements. These external inputs also determine the condition for changing the state in the storage elements. The sequential circuit receives binary information from external inputs that, together with the present state of the storage elements, determine the binary value of the outputs. The binary information stored in these elements at any given time defines the state of the sequential circuit at that time. The storage elements are devices capable of storing binary information. It consists of a combinational circuit to which storage elements are connected to form a feedback path. A block diagram of a sequential circuit is shown in Figure below:. In sequential circuits, the output signals are fed back to the input side. On the other hand, the logic circuits whose outputs at any instant of time depend on the present inputs as well as on the past outputs are called sequential circuits. Moreover, in a combinational circuit, the output appears immediately for a change in input, except for the propagation delay through circuit gates. Till now we studied the logic circuits whose outputs at any instant of time depend only on the input signals present at that time are known as combinational circuits.
0 Comments
Leave a Reply. |