Home » Difference Between Combinational Logic Circuit and Sequential Logic Circuit

Combinational Logic Circuit vs Sequential Logic Circuit

Digital Circuits are the circuits which use discrete voltage levels for its operation, and the Boolean logic for mathematical interpretation of these operations. Digital circuits use abstract circuit elements called gates, and each gate is a device whose output is a function of inputs alone. Digital circuits are used to overcome the signal attenuation, noise distortion present in Analog circuits. Based on the relations between the inputs and the outputs, Digital circuits are divided into two categories; Combinational Logic Circuits and Sequential Logic Circuits.

More about Combinational Logic Circuits

Digital circuits whose outputs are a function of present inputs are known as Combinational Logic circuits. Therefore, combinational logic circuits have no capability of storing a state inside them. In computers, arithmetic operations on stored data are performed by combinational logic circuits. Half adders, full adders, multiplexers (MUX), demultiplexers (DeMUX), encoders, and decoders are elementary level implementation of combinational logic circuits. Most components of Arithmetic and Logic Unit (ALU) are also comprised of combinational logic circuits.

Combinational logic circuits are mainly implemented using Sum of Products (SOP) and Products of Sum (POS) rules. Independent working states of the circuit are represented with Boolean algebra. Then simplified and implemented with NOR, NAND and NOT Gates.

More about Sequential Logic Circuits

Digital circuits whose output is a function of both present inputs and the past inputs (in other words, present state of the circuit) are known as sequential logic circuits. Sequential circuits have the ability to retain the previous state of the system based on the present inputs and the previous state; therefore, sequential logic circuit is said to have memory and used to store data in a digital circuit. The simplest element in sequential logic is known as a latch, where it can retain the previous state (latches the memory / state). Latches are also known as flip-flops (f-f’s) and, in true structural form, it is a combinational circuit with one or more outputs fed back as inputs. JK, SR (Set-Reset), T (Toggle), and D are commonly used flip flops.

Sequential logic circuits are used in almost every type of memory elements and finite state machines. Finite State Machine is a digital circuit model in which possible states if the system is finite. Almost all the sequential logic circuits use a clock, and it triggers the operation of the flip flops. When all the flip-flops in the logic circuit are triggered simultaneously, the circuit is known as a synchronous sequential circuit, while the circuits that are not triggered simultaneously are known as asynchronous circuits.

In practice, most of the digital devices are based on a mixture of combinational and sequential logic circuits.


What is the difference between Combinational and Sequential Logic Circuits?

• Sequential logic circuits have its output based on the inputs and the present states of the system, while combinational logic circuit’s output is based only on the present inputs.

• Sequential Logic Circuits have a memory, while combinational logic circuits do not have the ability to retain data (state)

• Combinational Logic Circuits are used mainly for arithmetic and Boolean operations, while sequential logic circuits are used for storage of data.

• Combinational logic circuits are built with logic gates as the elementary device while, in most cases, sequential logic circuits have (f-f’s) as the elementary building unit.

• Most sequential circuits are clocked (triggered for operation with electronic pulses), while combinational logic does not have clocks.


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