FLIP-FLOPS AND OTHER RELATED DEVICES
The digital circuits discussed so far have been combinational i.6., the outputs at any instant of time are entirely dependent upon the inputs present at that time. Most of the digital systems used in practice do not have only combinational circuits but also include memory elements. Thus the sequential circuits are made up of combinational circuits and memory elements.
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A block diagram of a sequential circuit is shown in Fig. 6.1. It consists of a combinational Circuit to which memory elements are connected to form a feedback path. The memory elements are devices capable of storing information. The information stored in the memory element at any given time defines the state of the sequential circuit. The sequential circuit receives external inputs and from. outputs of memory elements and generates signals for external outputs.
The above block diagram shows that the external outputs in the sequential circuits are a function of external inputs as well as the present state of the memory element.
There are two types of sequential circuits as.
(1) Synchronous Sequential Circuits.
(2) Asynchronous Sequential Circuits.
A sequential circuit whose inputs and outputs are considered at the discrete time
intervals defined by external means such as clock 18 called a synchronous sequential ’ circuit.
The circuits whose behavior depends upon the order in which its, input signals change and can be -, affected at any instant of time, are known as asynchronous sequential circuits.
An asynchronous circuit can be thought of as a. combinational circuit with feedback. the next state logic F as a function of the current state and Input. The output logic G determines the output as a function of the current state and input. Both F and G are strictly combinational logic circuits. For the Mealy machine, we can write.
Next state = F (current state, input)
Output = G (current state, input)
In some sequential machines the output depends on the state alum:
Output = G (current state)
Such a circuit is called Moore
The only difference between the two state machine models is in how outputs are generated.
The logic circuits studied so far have been combinational circuits whose output levels at any instant of time are dependent on the levels present at the inputs at that time. Any prior input level condition has no effect on the present Outputs because combination and logic circuit has no memory. Most of the digital systems are made up of combinational as well as memory elements known as sequential circuits.
The most important memory element is Flip-flop which is basically a bistable multivibrator. Bistable elements (latches and flip-flops) have two stable states. They are capable of residing in either of these two states indefinitely. These two states are set and reset. Because at their ability to retain a given state, these elements are useful as storage elements. The flip-flop is a basic building block for sequential circuits such as counters, registers, memories, control logic etc.