What is a Latch#
wiki:
Latch (English: latch), also known as bolt, is an electronic circuit used to store information in asynchronous sequential logic circuits systems in digital circuits. A latch can store one bit of information and is usually found in groups, some of which have special names, such as "4-bit latch" (can store four bits) or "8-bit latch" (can store eight bits), etc.
A latch is a digital circuit that can store a single bit of information and maintain its value until a new input signal updates it. They are used as temporary storage elements in digital systems to store binary information. Latches can be implemented using various digital logic gates, such as AND, OR, NOT, NAND, and NOR gates.
Latches are basic storage elements that operate using signal levels (rather than signal transitions). Latches controlled by clock transitions are called flip-flops. Latches are level-sensitive devices. In the design of asynchronous sequential circuits, latches are very useful. They are sensitive to input voltage and do not rely on clock pulses. Flip-flops that do not use clock pulses are referred to as latches.
Why Can Latches Store Information?#
The feedback mechanism is the core of a latch's ability to store information. It is the way a latch "remembers" its current state.
In the diagram, note how the output of the NAND gate (Q and Q̅, pronounced "Q bar") is fed back to another gate as input. This creates a closed loop (feedback loop). As shown in the diagram:
When S' = 0, R' = 1, the SR latch is set, Q = 1, Q' = 0. At this point, even if S' is switched to S' = 1, due to the feedback input from the bottom NAND, the output of the top NAND remains 1, meaning Q remains 1 and Q' remains 0, thus achieving state retention under specific conditions (when both S and R are 0).
This loop can maintain the "set" state. Even if the "S" input changes, as long as "R" remains 0, the latch will maintain its state. The same principle applies when the latch is in the "reset" state (Q = 0, Q̅ = 1). The feedback loop ensures that the output remains stable and reflects the last valid input.
Thus, the key mechanism for latches to store signals is feedback:
- The output of the latch is connected back to the input through a feedback path, allowing the output state to be maintained even when the input signal is disconnected.
Types of Latches#
In digital electronics, there are different types of latches:
- SR Latches
- Gated SR Latches
- D Latches
- Gated D Latches
- JK Latches
- T Latches
SR Latch#
wiki:
The simplest latch is the "SR latch" (also known as "RS latch"), where "S" stands for "Set" and "R" stands for "Reset." This latch is composed of a pair of interleaved NAND logic gates. The stored bit is represented by Q at the output.
The S-R latch (i.e., Set-Reset latch) is the simplest form of latch, implemented with two inputs: S (Set) and R (Reset). When both S and R inputs are 1, the latch is in an undefined state. They are also referred to as preset and clear states. The SR latch is a basic component of all other types of flip-flops.
SR Latch Truth Table:
SR latch implemented using NAND:
D Latch#
The D latch is also known as a transparent latch, implemented with two inputs: D (Data) and a clock signal. As long as the clock signal is high, the output of the latch will follow the input at the D terminal. When the clock signal is low, the output of the latch is stored and held until the next clock rising edge.
Advantages and Disadvantages of Latches#
Advantages#
Some of the advantages of latches are listed below.
- Easy to Implement: Latches are simple digital circuits that can be easily implemented using basic digital logic gates.
- Low Power Consumption: Latches consume less power compared to other sequential circuits such as flip-flops.
- High Speed: Latches can operate at high speeds, making them suitable for use in high-speed digital systems.
- Low Cost: Latches are inexpensive to manufacture and can be used in low-cost digital systems.
- Versatility: Latches can be used for various applications, such as data storage, control circuits, and flip-flop circuits.
Disadvantages#
Some of the disadvantages of latches are listed below.
- No Clock: Latches do not have a clock signal to synchronize their operations, making their behavior unpredictable.
- Unstable State: Latches can sometimes enter into an unstable state when both inputs are at 1. This can result in unexpected behavior in the digital system.
- Complex Timing: The timing of latches can be complex and difficult to specify, making them less suitable for real-time control applications.