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A latch is an asynchronous sequential circuit whose output changes immediately with the change in the applied input. A latch is used to store 1 bit information in a digital system, so it is considered as the most elementary memory element.
In this chapter, we will explain in detail about latches in digital electronics along with their types and applications.
What is a Latch?
In digital electronics, a latch is an asynchronous sequential circuit that can store 1-bit information. It is used as the fundamental memory element in digital circuits.
A latch can have two stable states namely, set and reset. The set state is denoted by the logic 1 and the reset state is represented by the logic 0. Due to these two stable states, a latch is also known as a bistable-multivibrator. The state of a latch toggles according to the applied input.
The most important thing to be noted about latches is that they do not have a clock signal for synchronization. That is why they are called asynchronous sequential circuits.
The logic gates are the fundamental building blocks of latches. Since there is no synchronization and clock signal used. Hence, the latches operate immediately on the application of input signals.
Characteristics of Latches
Some key characteristics of latches are explained below −
- Latches can store 1-bit of digital information that can be represented using either logic 0 or logic 1. Thus, the latches are mainly used as memory elements in digital circuits.
- Latches have a feedback mechanism that allows them to maintain their current state as it is until the next input is applied.
- The operation of latches is completely controlled by applied inputs that means the output of the latches updates based on the change in the input signals.
Types of Latches
The following are the main types of latches that used in digital circuits and systems −
- SR Latch
- JK Latch
- D Latch
- T Latch
Let us now discuss about each type of latch in detail.
SR Latch
The SR latch is a type of latch which has two input lines designated as S and R. Where, S represents the Set input and R represents the Reset input. Thus, it is also known as Set-Reset Latch.
The SR latch has two stable states namely Set state (S) and Reset state (R). The block diagram of the SR latch is shown in the following figure.
In the case of SR latch, the S input sets the output Q to 1 and Q” to 0. On the other hand, the R input sets the output Q to 0 and Q” to 1. In case, when both S and R inputs are high, the latch is said to be in forbidden state.
The complete operation of the SR latch for different input combinations is described in the following truth table −
| Inputs | Outputs | Comment | ||
|---|---|---|---|---|
| S | R | Q | Q” | |
| 0 | 0 | Q | Q” | No change |
| 0 | 1 | 0 | 1 | Reset state |
| 1 | 0 | 1 | 0 | Set state |
| 1 | 1 | X | X | Forbidden state |
The SR latch can be implemented by connecting two NOR gates in a cross-coupled manner as shown in the following figure.
JK Latch
The JK latch is another type of latch which has two inputs namely, J and K. Here, the input J is similar to S input and the input K is similar to R input in an SR latch.
The operation of the JK latch is similar to that of the SR latch but it does not have the forbidden state. Instead, it has a toggle state in which the outputs Q and Q” swap their states when both inputs J and K are 1.
Therefor, the JK latch is mainly designed to overcome the problem of forbidden state in the SR latch.
The block diagram of the JK latch is shown in the following figure −
The truth table given below describes the operation of the JK latch for different input combinations −
| Inputs | Outputs | Comment | ||
|---|---|---|---|---|
| J | K | Q | Q” | |
| 0 | 0 | Q | Q” | No change |
| 0 | 1 | 0 | 1 | Reset state |
| 1 | 0 | 1 | 0 | Set state |
| 1 | 1 | Q” | Q | Toggle state |
From this truth table, it is clear that the problem of forbidden state is addressed by implementing the toggle state.
The logic circuit of the JK latch consists of a combination of two NOR gates and two AND gates as shown in the following figure.
D Latch
The D Latch, also known as Data latch or transparent latch, is a type of bistable multivibrator which has two input signals namely, D (Data) input and E (Enable) input.
The output Q of the D latch is same as the input applied at the D input line as long as the E input is high. When the E input goes low, the output of the D latch is held as it is until the new input is applied to the D input.
The block diagram of the D latch is shown in the following figure.
The truth table given below explains the operation of the D latch −
| Inputs | Outputs | Comment | ||
|---|---|---|---|---|
| D | E | Q | Q” | |
| 0 | 0 | Q | Q” | No change |
| 0 | 1 | 0 | 1 | Reset state |
| 1 | 0 | Q | Q” | No change |
| 1 | 1 | 1 | 0 | Set state |
The logic circuit diagram of the D latch is depicted in the following figure −
T Latch
T latch is a type of latch that toggles its output state (Q) when a logic 1 is applied to its input line. Hence, it is also known as toggle latch.
The T latch is implemented by connecting the J and K inputs of the JK latch together as shown in the following block diagram.
The truth table describing the operation of the T latch is shown below −
| Input | Present State | Next State | ||
|---|---|---|---|---|
| T | Q | Q” | Q | Q” |
| 0 | 0 | 1 | 0 | 1 |
| 0 | 1 | 0 | 1 | 0 |
| 1 | 0 | 1 | 1 | 0 |
| 1 | 1 | 0 | 0 | 1 |
The logic circuit diagram of the T latch is shown in the following figure −
Applications of Latches
The latches find several applications in the field of digital electronics. They are most elementary storage components used to store one bit of information in digital systems.
Some of the common applications of latches are listed here −
- Latches are used as 1-bit memory element in digital systems.
- Latches are used to design digital registers which are employed for storage and manipulation of data in microprocessors and microcontrollers.
- Latches are used to design flip-flops which are basically the synchronized latches.
- Latches are also used in communication systems for temporary data storage or buffering purposes.
Conclusion
In this chapter, we explained different types of latches used in digital systems along with some examples of applications of latches.
In conclusion, a latch is a 1-bit storage device made up of logic gates. It is a type of asynchronous sequential logic circuit which do not have a clocked signal for synchronization.
In digital systems, latches are used to serve some key functions like temporary data storage, data flow control, etc.
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