Sequential Logic circuits

Sequential logic circuits in digital electronics are circuits whose output depends not only on the present input but also on the past history of inputs. This is possible because they use memory elements (such as flip-flops or latches) to store data.

Unlike combinational circuits, where the output changes instantly with the input, sequential circuits remember previous states, making them suitable for tasks that require storage, timing, and control.

Clock Signal:
A Clock signal in a sequential circuit is a timing signal used to synchronize the operations of the circuit. It is usually a square wave that continuously alternates between a high level (1) and a low level (0).

In sequential circuits, the clock signal controls when the memory elements (like flip-flops) can change their state. This ensures that all parts of the circuit work in a coordinated manner.

Comparison between Combinational and Sequential Circuits

Latch in Digital Electronics

A latch is a basic memory device in sequential circuits that can store one bit of information. It is a bistable circuit, meaning it has two stable states (0 or 1). Latches are the simplest form of sequential circuits.

Types of Latches
The following are the main types of latches that are used in digital circuits and systems

1. SR Latch
2. JK Latch
3. D Latch
4. T Latch

SR Latch:

The SR Latch is a type of latch that has two input lines, labeled S and R, where S stands for the Set input and R stands for the Reset input. Therefore, it is also called a Set-Reset Latch. The SR Latch has two stable states, known as the Set state (S) and the Reset state (R). The block diagram of the SR Latch is shown in the following figure. The SR latch can be implemented by connecting two NOR gates or NAND gates in a cross-coupled manner, as shown in the following figure.

In the case of an SR Latch, the S (Set) input sets the output Q to 1 and Q’ to 0. Conversely, the R (Reset) input sets Q to 0 and Q’ to 1. When both S and R inputs are high, the latch enters a forbidden state.

The complete operation of the SR Latch for different input combinations is shown in the following truth table.

JK Latch 

The JK Latch is another type of latch that has two inputs, labeled J and K. Here, the J input is similar to the S input of an SR Latch, and the K input is similar to the R input.

The operation of the JK Latch is similar to that of the SR Latch, but it does not have a forbidden state. Instead, it has a toggle state, in which the outputs Q and Q’ swap their states when both J and K inputs are 1. Therefore, the JK Latch is mainly designed to overcome the problem of the forbidden state in the SR Latch.

The block diagram of the JK Latch is shown in the following figure

The JK Latch eliminates the forbidden state problem of the SR Latch.

D latch:

A D Latch is a type of digital storage device with two inputs: D (Data) and E (Enable). When the Enable (E) input is high, the output (Q) matches the Data (D) input. When the Enable input goes low, the output holds the last value until the Data input changes again. The block diagram of the D latch is shown 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 a 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

Applications of Latches

Latches are fundamental building blocks in digital electronics. They serve as basic memory elements and are widely used in a variety of systems. Here are the most common applications of latches:

• Used to store a single bit of data (1 or 0).
• Used in CPU registers to hold data temporarily.
• Temporarily holds data between operations or devices.
• Removes noise and false triggers from push buttons or switches.
• Stores the current state in finite state machines (FSMs).
• Holds control signals until they are needed by another part of the circuit.
• Helps synchronize signals in asynchronous systems.
• Used in building binary counters and digital timers.
• Controls when data is allowed to pass through a circuit.
• Holds data to drive displays until new data is available.

Flip-Flops

A flip-flop is a sequential digital electronic circuit having two stable states that can be used to store one bit of binary data. Flip-flops are the fundamental building blocks of all memory devices.

Types of Flip-Flops

There are mainly four types of flip-flops, and these are 

• S-R Flip-Flop
• J-K Flip-Flop
• D Flip-Flop
• T Flip-Flop

S-R Flip-Flop

An S-R flip-flop (Set-Reset flip-flop) is a basic digital memory circuit used to store one bit of information, either a 0 or a 1. It has two inputs: S (Set) and R (Reset), and two outputs: Q and Q’ (the complement of Q).S-R Flip Flop can be constructed by using  NOR Gate or  NAND Gate

When in this circuit when S is set as active, the output Q would be high and the Q’ will be low. If R is set to active then the output Q is low and the Q’ is high. Once the outputs are established, the results of the circuit are maintained until S or R get changed, or the power is turned off.

Truth Table of S-R Flip-Flop

Characteristics Equation of S-R Flip-Flop

When the clock is triggered:

• If J = 0 and K = 0, the output Q remains unchanged (no change).
• If J = 0 and K = 1, the output Q is reset to 0.
• If J = 1 and K = 0, the output Q is set to 1.
• If J = 1 and K = 1, the output Q toggles — it switches from 0 to 1 or from 1 to 0.

Truth Table of JK Flip-Flop

D Flip-Flop

A D flip-flop, also known as a Data or Delay flip-flop, is one of the simplest and most commonly used flip-flops in digital electronics. It is used to store one bit of data and works with a clock signal. The D flip-flop has two main inputs: D (Data input), and Clock (CLK), and two outputs: Q (current state) Q’ (complement of Q)

T Flip-Flop

A T flip-flop, or Toggle flip-flop, is a type of flip-flop that changes (toggles) its output on every clock pulse when its input T is HIGH (1). It is mainly used in counters and frequency dividers. The T flip-flop has One input: T (Toggle), one clock input, and Two outputs: Q and Q’ (complement of Q)

Applications of Flip-Flops

Applications of flip-flops are:

• Used to store a single bit of binary data (1 or 0) in memory elements like registers and RAM.
• Flip-flops are combined to form registers that store multi-bit data in microprocessors and digital systems.
• T and JK flip-flops are commonly used in building binary counters (up-counters, down-counters, ring counters).
• Flip-flops divide the input clock frequency by 2, 4, 8, etc., useful in digital clocks and timers.
• Flip-flops are connected in series to shift data bit by bit — used in serial-to-parallel or parallel-to-serial data conversion.
• Flip-flops hold the current state in finite state machines used in control units and automation.
• Used to clean noisy signals from mechanical switches and prevent multiple triggering.
• Used to generate precise delays, clock pulses, and control signals in microcontrollers and processors.
• Helps in synchronizing signals between asynchronous systems or different clock domains.
• Flip-flops can detect and store the occurrence of specific events (like a button press or signal transition).

Compiled by Er. Basant Kumar Yadav