The digital circuits discussed so far have been combinational, where the outputs are entirely dependent on the current inputs. Although every digital system is likely to have combinational circuits, most systems encountered in practice also include storage elements, which require that the system be described in terms of sequential logic. A block diagram of a sequential circuit is shown below.
It consists of a combinational circuit to which storage elements are connected to form a feedback path. The storage elements are devices capable of storing binary information. The binary information stored in these elements at any given time defines thestate of the sequential circuit at that time. The sequential circuit receives binary information from external inputs. These inputs, together with the present state of the storage elements, determine the binary value of the outputs. They also determine the condition for changing the state in the storage elements. From the block diagram we can see 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. The next state of the storage elements is also a function of external inputs and the present state. Thus, a sequential circuit is specified by a time sequence of inputs, outputs, and internal states. Combinational circuits are often faster than sequential circuits since the combinational circuits do not require memory elements where as the sequential circuits need memory devices to perform their operations in sequence. However, modern digital computers must have memories to function properly. Thus, sequential circuits are of prime importance in modern digital devices.
Types of Sequential circuits
Sequential circuits are classified in two main categories depending on timing of their signals.
1. Synchronous or clocked sequential circuits
2. Asynchronous or un-clocked sequential circuits.
Synchronous or clocked sequential circuits
A sequential circuit whose behavior can be defined from the knowledge of its signals at discrete instants of time is referred to as a synchronous sequential circuit. In such circuits, the memory elements are affected only at discrete instants of time. The synchronization is obtained by a timing device, called a system clock (or a master-clock generator) which generates a periodic train of clock pulses. The outputs are affected only with the application of clock pulse.
Asynchronous or un-clocked sequential circuits.
A sequential circuit whose behavior depends upon the sequence in which the input signals change is referred to as an asynchronous sequential circuit. The output is affected whenever there is a change in the inputs. The commonly used memory elements in such circuits are time delay devices. These circuits may be regarded as combinational circuits with feed back. These circuits are faster than synchronous sequential circuits. However, in an asynchronous circuit, events are allowed to occur without any synchronization. In such a case, the system becomes unstable which results in difficulties. To have a sequential circuit, a storage device is required to know what has happened in the past. The basic unit of storage is the flip – flop.
FLIP – FLOPS
Also called as LATCH. The simplest kind of sequential circuit is a memory cell that has only two states. It can be either a 0 or 1. Such two state sequential circuits are called flip-flops because they flip from one state to another and then flop back. A flip-flop is also known as bistable multivibrator, latch or toggle.
Types of Flip-Flops
Flip-Flops are of different types depending on how their inputs and clock pulses cause transition between two states. There are 4 basic types
1. S – R (Set-Reset)/R-S flip – flop
2. J-K flip-flop
3. D (delay)flip-flop
4. T (trigger or Toggle)flip-flop