12 Difference Between Combinational And Sequential Logic Circuit

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What Are Combinational Circuit (CC)?

Combinational circuits are circuits made up of inputs, outputs and two or more logic gates. A logic gate is a basic building block of any electronic circuit. The circuits have no memory, timing or feedback loops, there operation is instantaneous.  A combination logic circuit performs an operation assigned logically by a Boolean expression or truth table.

The result is that combinational logic circuits have no feedback and any changes to the signals being applied to their inputs will immediately have an effect at the output. In other words, in a combinational logic circuit, the logic gates are combined in such a way that the output is dependant at all times on the combination of its input.

Examples of common combination logic circuits include:

  • Half adder
  • Full adder
  • Multiplexer
  • De-multiplexer
  • Comparators
  • Subtractors
  • Encoder
  • Decoder

What You Need To Know About Combinational Circuit

  1. Combinational circuit is the type of circuit in which output is independent of time and only relies on the input present at that particular instant.
  2. A combinational circuit consists of inputs, logic gates and outputs.
  3. There is no further classification of combinational circuits.
  4. The primary building blocks of combinational circuits are logic gates.
  5. In combination circuit, output does not depend on the time instant; no feedback is required for its next output generation.
  6. Combinational circuits are not capable of storing data (state). They do not contain any memory element.
  7. Combination circuits are independent of clock signal.
  8. Half adder, full adder, multiplexer, de-multiplexer, comparators, subtractors, encoder and decoder are some examples of combination circuits.
  9. The combination circuit does not require triggering for its operation.
  10. The behavior of combination circuit can be defined by either Boolean expression or truth table.
  11. Combination circuit is faster and better in performance.
  12. There is no feedback in combinational circuits.

What Is Sequential Circuit?

Sequential logic circuits are circuits whose input depends not only on the present value of its input signals but on the sequence of past inputs, the output history as well. The word Sequential means that things happen in a sequence, one after another. In sequential logic circuits, the actual clock signal determines when things will happen next.

In other words, the output state of a sequential logic circuit is a function of the following three states, the preset input, the past input and the past output. Sequential logic circuits remember these conditions and stay fixed in their current state until the next clock signal changes one of the states, giving sequential logic circuits memory. Sequential logic circuits can be constructed to produce either simple edge-triggered flip-flops or more complex sequential circuits such as storage registers, shift registers, memory devices or counters.

Sequential logic circuits can be dived into three main categories, which are:

  • Event Driven e.g asynchronous circuits which change state immediately when enabled.
  • Clock Driven e.g synchronous circuits that are synchronized to a specific clock signal.
  • Pulse Driven –which is a combination of the two that responds to triggering pulses

Examples Of sequential Logic Circuits Include:

  • Flip flops
  • Memory devices
  • Counters
  • Shift registers

What You Need To Know About Combinational Circuit

  1. Sequential circuit is the type of circuit where output not only relies on the current input but also depends on the previous output.
  2. A sequential circuit can be considered as a combinational circuit with storage elements.
  3. Sequential circuits are classified into two types based on the timing of feedback signals. They are synchronous and asynchronous sequential circuits.
  4. The elementary building units of sequential logic circuit are flip flops (binary storage device).
  5. In sequential circuit, output relies on its previous feedback therefore output of previous input is being transferred as feedback used with input for next output generation.
  6. Sequential circuits have the memory space required for storing the current states sent as control inputs (enable) for the next set of operations. They contain memory elements that store data in digital circuits.
  7. Synchronous sequential circuits use clock pulses whereas the asynchronous sequential circuits do not use clock pulses.
  8. Flip flops, memory devices, counters and shift registers are some examples of sequential circuits.
  9. Synchronous sequential circuits need to be triggered for its operation.
  10. The behavior of the sequential circuit can be defined using State equation (also referred to as Transition equation) or State table.
  11. Sequential circuits are comparatively slower and have low performance.
  12. There is need for storage of previous outputs.

Difference Between Combinational And Sequential Logic Circuit In Tabular Form

BASIS OF COMPARISON COMBINATIONAL LOGIC CIRCUIT SEQUENTIAL LOGIC CIRCUIT
Description Combinational circuit is the type of circuit in which output is independent of time and only relies on the input present at that particular instant.   Sequential circuit is the type of circuit where output not only relies on the current input but also depends on the previous output.  
Components They consist of inputs, logic gates and outputs.   They can be considered as a combinational circuit with storage elements.  
Classification There is no further classification of combinational circuits.   They are classified into two types based on the timing of feedback signals. They are synchronous and asynchronous sequential circuits.  
Building Unit The primary building blocks of combinational circuits are logic gates.   The elementary building units of sequential logic circuit are flip flops (binary storage device).  
Output The output does not depend on the time instant; no feedback is required for its next output generation.   Output relies on its previous feedback therefore output of previous input is being transferred as feedback used with input for next output generation.  
Memory They do not contain any memory element.   They contain memory elements that store data in digital circuits.  
Clock Signal Combination circuits are independent of clock signal.   Synchronous sequential circuits use clock pulses whereas the asynchronous sequential circuits do not use clock pulses.  
Examples Half adder, full adder, multiplexer, de-multiplexer, comparators, subtractors, encoder and decoder Flip flops, memory devices, counters and shift registers  
Triggering They do not require triggering for its operation.   Synchronous sequential circuits need to be triggered for its operation.  
Behavior The behavior of combination circuit can be defined by either Boolean expression or truth table.   The behavior of the sequential circuit can be defined using State equation (also referred to as Transition equation) or State table.  
Performance They faster and better in performance.   They are comparatively slower and have low performance.  
Feedback There is no feedback in combinational circuits.   There is need for storage of previous outputs.