1.
Introduction
2.
What is a JK flip flop?
2.1.
JK flip-flop Truth table
3.
Master-Slave JK Flip-flop
4.
FAQs
5.
Key takeaways
Last Updated: Mar 27, 2024

# JK flip flop

Aryan Raj
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## Introduction

A JK flip-flop is a sequential bi-state single-bit memory device named after its inventor Jack Kilby. The JK flip-flop is a gated SR flip flop with the addition of a clock input circuitry that prevents the invalid or illegal output condition that can occur when both inputs R and S are equal to logic level "1".

Flip flops are of the following types:

We have discussed other flip flops in our previous blogs. In this blog, we will dive deep into the JK flip flop.

## What is a JK flip flop?

A JK flip flop has very similar characteristics to an SR flip flop. The only difference between them is that the undefined condition for an SR flip-flop, i.e., Rn=Sn= 1 condition, is also included in this case.  Inputs J and K behave like inputs S and R to set and reset the flip flop.

When J = K = 1, the flip flop is in a toggle state, which means the output switches to its complementary state every time a clock passes. The data inputs are J and K, which are ANDed with Q' and Q respectively to obtain the inputs for S and R. A J-K flip flop thus obtained is shown in Figure below.

The only difference between SR and JK flip-flop is that when both inputs of SR flip flop is set to value 1, the circuit produces an output as the invalid states, but in the case of JK flip flop, there are no invalid states even if both 'J' and 'K' flip flops are set to value 1.

### JK flip-flop Truth table

Case 1: When the clock is applied and J = 0, whatever the value of Q'n (0 or 1), the output of NAND gate 1 is 1. Similarly, when K = 0, whatever the value of Qn (0 or 1), the output of gate 2 is also 1. Therefore, when J = 0 and K = 0, the inputs to the basic flip-flop are S = 1 and R = 1. This condition forces the flip-flop to remain in the same state.

Case 2: When the clock is applied and J = 0 and K = 1 & the previous state of the flip-flop is reset (i.e., Qn = 0 and Q'n = 1), then S = 1 and R = 1. Since S = 1 and R = 1, the basic flip-flop does not alter the state and remains in the reset state. But if the flip-flop is in set condition (i.e., Qn = 1 & Q'n = 0), then S = 1 and R = 0. Since S = 1 and R = 0, the basic flip-flop changes its state and resets.

Case 3: When the clock is applied and J = 1 and K = 0 and the previous state of the flip-flop is reset (i.e., Qn = 0 and Q'n = 1), then S = 0 and R = 1. Since S = 0 and R = 1, the basic flip-flop changes its state and goes to the set state. But if the flip-flop is already in set condition (i.e., Qn = 1 and Q'n = 0), then S = 1 and R = 1. Since S = 1 and R = 1, the basic flip-flop does not alter its state and remains in the set state.

Case 4: When the clock is applied and J = 1 and K = 1 and the previous state of the flip-flop is reset (i.e., Qn = 0 and Q'n = 1), then S = 0 and R = 1. Since S = 0 and R = 1, the basic flip-flop changes its state and goes to the set state. But if the flip-flop is already in set condition (i.e., Qn = 1 and Q'n = 0), then S = 1 and R = 0. Since S = 1 and R = 0, the basic flip-flop changes its state and goes to the reset state. So we find that for J = 1 and K = 1, the flip-flop toggles its state from set to reset and vice versa.

## Master-Slave JK Flip-flop

Basically, a master-slave flip-flop is a system of two flip flops—one being designated as the master and the other being the slave. The figure III below shows that a clock pulse is applied to the master, and the inverted form of the same clock pulse is applied to the slave. When    CLK = 1, the first flip-flop (i.e., the master) is enabled, and the outputs Qm and Q'm respond to the inputs J and K according to the table shown.

At this time, the second flip flop (i.e., the slave) is disabled because the CLK is LOW to the second flip flop. Similarly, when CLK becomes LOW, the master becomes disabled, and when the slave becomes active. Therefore, the outputs Q and Q' follow the outputs Qm and Q'm respectively.

Since the second flip-flop just follows the first one, it is referred to as a slave, and the first one is called the master. Hence, the configuration is referred to as a master-slave (M-S) flip-flop.

Fig I

Fig II

Fig III

The state of the master-slave flip-flop, shown in the figure given above, changes at the negative transition (trailing edge) of the clock pulse. Hence, it becomes negative triggering a master-slave flip-flop. This can be changed to a positive edge triggering flip-flop by adding two inverters to the system - one before the clock pulse is applied to the master and an additional one in between the master and the slave.

The logic symbol of a negative edge master-slave is shown in the figure given below. The system of master-slave flip-flops is not restricted to J-K master-slave only. There may be an S-R master-slave or a D master-slave, etc., in all of them the slave is an S-R flip-flop, whereas the master changes to J-K or S-R or D flip-flops.

Characteristic Table:

## FAQs

1. What is a clock in a flip flop?
A clock pulse is a time-varying voltage signal that controls a flip flop's operation (triggering). For exp, if a clock pulse is of frequency 1 Hz, the voltage it will supply will oscillate between X Volts and Y Volts(X and Y are any dc voltages), and this change occurs every half second.

2. What is the status of Q output when J= 1 and k= 0?
If inputs of J and K are both LOW (J = K = 0), there will be no change in Q, no matter how many times the clock pulse is applied. If K = 1 (HIGH) and J = 0 (LOW) the next clock edge resets Q output LOW (Q = 0). If K = 0 and J = 1, then the next clock edge sets Q output HIGH (Q = 1).

3. How is a J-K flip-flop made to toggle?
When j=k=1, the race condition occurs, which means both outputs want to be HIGH. Hence, there is a toggle condition where 0 becomes 1 and 1 become 0. That is device is either reset or set.

4. What is the toggle condition?
Condition of a flip-flop circuit in which the internal state of the flip-flop changes from value 0 to 1 or from 1 to 0.

5. What is J in JK flip-flop?
The JK flip flop was named after Jack Kilby, the Texas Instruments engineer that invented the integrated circuit in 1958. In his honour, the modified RS circuit that eliminated race conditions was named JK.

## Key takeaways

In this blog, we have discussed JK flip flop in complete detail with its truth table. We have also covered the Master-slave JK flip flop and how it works.

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