Digital logic circuits can be classified into “combinational” and “sequential.” A combinational logic circuit is one whose output solely depends on its current inputs. Sequential circuits, on the other hand, are built using combinational circuit and memory elements called “flip-flops”. These circuits generate output which depends on the current and previous states.
A combinational circuit is one where the output at any time depends only on the present combination of inputs at that point of time with total disregard to the past state of the inputs. The logic gate is the most basic building block of combinational logic. The logical function performed by a combinational circuit is fully defined by a set of Boolean expressions.
A combinational circuit consists of n input variables and m output variables or simply outputs. Since the number of input variables is n, there are 2n possible combinations of bits at the input. If m is the number of output variable then there are m boolean functions corresponding to each output.
A block diagram of combinational circuit is shown in figure below.
Sequential circuit is a combination of a combinational circuit and a memory elements connected in feedback path. The memory elements are devices capable of storing binary information within them.The binary information stored in the memory elements at any given time defines the state of the sequential circuit. The sequential circuit receives binary information from external inputs. Thus, a sequential circuit is specified by a time sequence by a time sequence of inputs, outputs and internal states.
A block diagram of sequential circuit is shown in figure below.
Differences between combinational and sequential circuits
|S.No.||Combinational circuit||Sequential Circuit|
|1||In combinational circuits no memory element is present.||In sequential circuits memory element is present.|
|2||The present values of its outputs are determined solely by the present values of its inputs.||The present values of its outputs are determined by the present values of its inputs and its present state.|
|3||The behavior of combinational circuit is described by the set of output functions.||The behavior of sequential circuit is described by the set of next-state functions and the set of output functions.|
|4||More hardware is required for the realization of combinational circuits.||Less hardware is required for the realization of sequential circuits.|
|5||Its operation can be described by truth table.||Its operation can be described by truth table and timing diagram.|
|6||Combinational circuit are more expensive.||Sequential circuits are cheaper.|
|7||Combinational circuit are faster in speed.||Due to the memory elements the speed of sequential circuit is slow.|
|8||The designing of combinational circuit is easier.||The designing of sequential circuit is complicated.|
|9||There is no requirement of the feedback.||Feedback is required in this circuit.|
|10||Examples of combinational circuits are half adder, full adder, magnitude compactor, multiplexer, demultiplexer, etc.||Examples of sequential circuits are flip flop, ragisters, counters, etc.|