Before we get to start discussing flip flops in detail, let’s first quickly review the basics of the logic circuits in which flip flops are used.
Logic Circuits and Flip Flops
There are primarily two types of logic circuits: 1) combinational logic circuits, and 2) sequential logic circuits.
In combinational logic circuits, the output of a circuit at any time depends on the value of the inputs fed to the circuit at that particular time. On the other hand, in sequential logic circuits, the output of a circuit at any time not only depends on the present value of the inputs fed to the circuit, but also on the value of the past outputs acquired by the circuit.
So, we can say that sequential logic circuits have a memory element while combinational logic circuits don’t have a memory element.
It’s this requirement of a memory element in sequential logic circuits that flip flops are designed to serve.
Flip Flops –An Introduction
In the simplest words, flip flops are memory elements used in sequential circuits for storing the past outputs of sequential circuit systems.
A flip flop can store a past output indefinitely until prompted by an input signal to switch the states.
What type of input signal to be exact?
Depending on the type of flip flop, the “triggering” signal can either be:
- Level-based signal; or
- Edge-based signal
A typical flip flop circuit consists of:
- Two inputs—Set(S) and Reset(R)
- Clocking signal
- Two outputs—Normal and Complement
Flip flop circuits can mainly be constructed in two ways:
- Using combination of NOR gates
- Using combination of NAND gates
Let’s take a look at a flip flop circuit constructed from NOR gates and how it works.
NOR Gates Flip Flop Circuit
Here is a schematic diagram of a flip flop (NOR gates) circuit:
Flip flop (NOR gates) circuit
The flip flop circuit drawn above does not contain a clocking signal. This has been done so to keep the explanation of the working of the circuit simple and easy to understand.
We know that a NOR gate produces a “LOW” output if any of its inputs are high.
So, when S=1 and R=0:
The values of Q’ and Q will be“0” and “1” respectively.
If S now switches to “0” and R remains “1”; the values of Q’ and Q will be “0” and “1” respectively.
Likewise, when S=0 and R=1:
The values of Q and Q’ will be “0” and “1” respectively.
If R now returns to “0” and S remains “0”; the values of Q and Q’ will be “0” and “1” respectively.
What happens if both S and R are given a “HIGH” input?
In that case, both the outputs (Q and Q’) become “0”. Since that’s not possible, we must ensure that this state is avoided when operating a NOR gate flip flop.
Types of Flip Flops
Depending on the number of inputs they possess and the manner in which these inputs affect their binary state, flip flops can be divided into three types:
- RS Flip Flops
- D Flip Flops
- JK Flip Flops
RS flip flops are your normal NAND or NOR gate flip flops with a clock pulse generator. While D and JK flipflops are variations of RS flip flops having improved functionality.
Although flip flops can be designed using NOR or/and NAND gates, many system designers prefer using flip flop ICs for better results and convenient application.
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