In this tutorial, you will explore the fundamental concepts of J-K Flip-Flops, including their definition, characteristics, and the differences between J-K and SR Flip-Flops. You will also learn about designing J-K Flip-Flops using NAND gates, understand their advantages and disadvantages, and discover their applications in various digital circuits.
Contents:
- What is a J-K Flip-Flop?
- SR Flip-flop in Sequential Circuits
- JK Flip-flop in Sequential Circuits
- Differences Between SR and JK Flip-Flops
- Designing JK Flip-Flop Using NAND Gates
- Advantages and Disadvantages of a JK Flip-flop
- Applications of JK Flip-flops
What is a J-K Flip-Flop?
A J-K Flip-Flop is a type of bistable multivibrator, which means it has two stable states (0 and 1) and can store one bit of information. It is an improvement over the basic S-R (Set-Reset) Flip-Flop, solving the undefined state issue that arises when both inputs are high in the S-R configuration. The J-K Flip-Flop is one of the most widely used flip-flops in digital circuits because of its versatility.
SR Flip-flop in Sequential Circuits
The following diagram explains the block diagram of an S-R flip-flop.
- SR flip-flop is a 1-bit memory-bistable device that consists of two inputs S and R where S represents SET and R represents RESET.
- An edge triggered clock is needed for the proper working of the flip-flop to produce the next state outputs.
- The next state output expression of a D flip-flop is, Q+ = S + R’Q. This is also called the characteristic equation of the SR flip-flop.
- There are two limitations of SR flip-flops using NOR and NAND gates. One is the invalid state and another is when SET ‘S’ and RESET ‘R’ both inputs are set to ‘0’, then the condition is avoided.
JK Flip-flop in Sequential Circuits
The following diagram explains the block diagram of a JK flip-flop.
- The diagram represents a JK flip-flop. It consists of two inputs J and K along with an edge triggered clock.
- It produces two outputs Q and Q’. They represent the current state outputs of the flip-flop.
- A JK flip-flop is called as universal flip-flop because all other flip-flops such as SR flip-flop, D flip-flop, T flip-flop can be derived from it.
- The characteristic equation of a JK flip-flop is, Q+ = JQ’ + K’Q. Q+ represents the next state output of the flip-flop.
Differences Between SR and JK Flip-Flops
The differences between an SR and a JK flip-flop are explained in the points below.
| Parameters | SR Flip-Flop | JK Flip-Flop |
|---|---|---|
| Inputs | Inputs are represented by S and R. | Inputs are represented by J and K. |
| Characteristic Equation | The characteristic equation of SR flip-flop is Q+ = S + R’Q. | The characteristic equation of JK flip-flop is Q+ = JQ’ + K’Q. |
| Universal Flip-Flop | It is not a universal flip-flop. | It is a universal flip-flop. |
| Toggle Mode | It doesn’t operate on toggle mode. | It can operate on toggle mode. |
| Logic Gates | The circuit of an SR flip-flop requires fewer logic gates compared to JK flip-flop. | The circuit of a JK flip-flop requires more logic gates compared to SR flip-flop. |
| Problem | It mainly suffers from invalid input problem. | It mainly suffers from the race-around problem. |
Designing JK Flip-Flop Using NAND Gates
The following diagram explains the design of a JK flip-flop using NAND gates.
- Two 2-input NAND and two 3-input NAND gates are needed to design a JK Flip-Flop.
- The inputs for the NAND3 gate are J, CLK, and the previous state output Q’. The inputs for the NAND4 gate are K, CLK, and the previous state output Q.
- The clock used here is a positive edge triggered clock. The circuit will produce its next state output (Q+) only when it receives a positive clock edge.
- The function table of a JK flip-flop is shown below.
CLK J K Q+ – X X No change ↑ 0 0 No change ↑ 0 1 0 ↑ 1 0 1 ↑ 1 1 Toggle
Advantages and Disadvantages of a JK Flip-flop
The advantages and disadvantages of a JK flip-flop are explained below:
Advantages:
- JK flip-flop removed the invalid input problem that occurs in SR flip-flop, rather it produces a toggle output state when both inputs J and K are HIGH.
- It solved the switching problem of SR flip-flop i.e. when SET ‘S’ and RESET ‘R’ inputs of an SR flip-flop are set to 0, the condition is avoided.
Disadvantages:
- JK flip-flop suffers from the “race around” problem that occurs due to continuous oscillation of outputs in toggle mode when level triggered clock is used.
Applications of JK Flip-flops
The applications of JK flip-flops are explained below:
- JK flip-flops are used in event detector circuits. These detector circuits change their state when an event occurs and retain its state till the event gets cleared.
- JK flip-flops are used in counter circuits for counting pulses. They are also used as data synchronizers in combinational circuits.
- JK flip-flops are used in master-slave configuration to remove the race around problem that occurs in toggle output mode.
Key Points to Remember
Here is the list of key points we need to remember about “JK Flip Flop”.
- A J-K Flip-Flop is a bistable multivibrator with two inputs (J and K) and two outputs (Q and Q’), improving upon the SR Flip-Flop by eliminating the undefined state when both inputs are high.
- The next state output of a J-K Flip-Flop is defined by the characteristic equation
Q+ = JQ′ + K′Q, where Q+ represents the next state based on current inputs and previous state. - The J-K Flip-Flop is classified as a universal flip-flop, meaning it can be configured to function as any other type of flip-flop, such as SR, D, or T Flip-Flops.
- The primary advantages include resolving the invalid input problem of SR Flip-Flops and providing toggle functionality, while the main disadvantage is its susceptibility to the race-around problem in toggle mode with level-triggered clocks.
- J-K Flip-Flops are widely used in event detection circuits, counters, data synchronization in combinational circuits, and in master-slave configurations to mitigate race conditions.
