A sequential logic circuit implements a function using the notion of circuit state along with the usual logic components such as OR gates, NOT gates, etc. This circuit state is a function of past inputs and is implemented by some form of memory. So we can say that in a sequential logic circuit, outputs are a function of current inputs and past inputs. This is in contrast with combinational logic where the circuit has no notion of past inputs — its outputs are a function solely of current inputs.
The SR flip-flop is an example of a sequential logic circuit.
Kleitz, W. Digital Microprocessor Fundamentals. 3rd Edition. Prentice Hall, 2000.
Mano, M. Morris, and Kime, Charles R. Logic and Computer Design Fundamentals. 2nd Edition. Prentice Hall, 2000.
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