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Combinational and Sequential Logic Circuits

1950

Digital circuits are categorized into two main types: combinational and sequential logic. In combinational logic, the output is a pure function of the present input values only (e.g., an adder). In sequential logic, the output depends not only on the current inputs but also on the past sequence of inputs, as these circuits have memory elements (e.g., a flip-flop).

Combinational logic circuits are memory-less. Their output at any given time is determined solely by the combination of input values at that same instant. They are implemented using basic logic gates like AND, OR, NOT, NAND, and NOR. Examples include adders, subtractors, comparators, multiplexers (which select one of several input signals), and demultiplexers (which send an input signal to one of many outputs). The relationship between inputs and outputs can be described by a truth table or a set of Boolean functions.

Sequential logic circuits, in contrast, incorporate memory. Their state, stored in memory elements like flip-flops or latches, is a summary of the history of past inputs. The output is a function of both the current inputs and the current state. This memory capability allows them to perform tasks that combinational circuits cannot, such as counting or storing data. Sequential circuits are further divided into synchronous and asynchronous types. Synchronous circuits use a clock signal to update their state at discrete time intervals, ensuring orderly operation. Asynchronous circuits update their state in response to changes in the input signals, which can be faster but are more complex to design and analyze correctly.

UNESCO Nomenclature: 2205

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