Difference between Combinational and Sequential Circuit

Nov 30 -0001

Compare, distinguish and explain what is the difference between Combinational and Sequential Circuit. Comparison and Differences.

In digital circuit theory, sequential logic is a type of logic circuit whose output depends not only on the present value of its input signals but on the sequence of past inputs, the input history as well. This is in contrast to combinational logic, whose output is a function of only the present input.

Sequential circuits

Sequential circuits are those which are dependent on clock cycles and depends on present as well as past inputs to generate any output. The sequential logic has memory. Sequential logic circuits is a form of binary circuit; its design employs one or more inputs and one or more outputs.

A Sequential logic circuits is a form of binary circuit; its design employs one or more inputs and one or more outputs, whose states are related to some definite rules that depends on previous states. ... Examples of such circuits include clocks, flip-flops, bi-stables, counters, memories, and registers.

There are two types of sequential circuit, synchronous and asynchronous. Synchronous types use pulsed or level inputs and a clock input to drive the circuit(with restrictions on pulse width and circuit propagation). Asynchronous sequential circuits do not use a clock signal as synchronous circuits do.

Flip flop is a sequential circuit which generally samples its inputs and changes its outputs only at particular instants of time and not continuously. Flip flop is said to be edge sensitive or edge triggered rather than being level triggered like latches.

The figure below shows both a state table and a timing diagram for this simple memory. The state table shows the state which the device enters after an input (the next state), for all possible states and inputs. For this device, the output is the value stored in the memory.

Combinational Circuit

Combination Logic Circuits are made up from basic gates (AND, OR, NOT) or universal gates (NAND, NOR) gates that are "combined" or connected together to produce more complicated switching circuits. These logic gates are the building blocks of combinational logic circuits. An example of a combinational circuit is a decoder, which converts the binary code data present at its input into a number of different output lines, one at a time producing an equivalent decimal code at its output.

In this output depends only upon present input. Combinational logic does not have memory. They employ a feedback loop to give output back to input.

A combinational logic circuit performs an operation assigned logically by a Boolean expression or truth table. Examples of common combinational logic circuits include: half adders, full adders, multiplexers, demultiplexers, encoders and decoders.

Combinational vs Sequential Circuit

S.No.	Combinational circuits	Sequential circuits
1	Output of any instance of time depends only upon the input variables.	Output is generated dependent upon the present input variables and also on the basis of past history of these inputs.
2	Memory unit is not required. i.e. it doesn’t allocate any memory to the elements.	Memory unit is required. . i.e. it allocates any memory to the elements
3	Faster	Slower
4	Easy to design	Difficult to design
5	Examples: Half and full adder, Half and full subtractor, MUX , DEMUX	Examples: Flip flops, Shift registers, Binary counters