The main advantage of synchronous logic is its simplicity. At each cycle, the next state is determined by the current state and the value of the input signals when the clock pulse occurs. The state of the synchronous circuit only changes on clock pulses. The output of all the storage elements (flip-flops) in the circuit at any given time, the binary data they contain, is called the state of the circuit. The output of each flip-flop only changes when triggered by the clock pulse, so changes to the logic signals throughout the circuit all begin at the same time, at regular intervals, synchronized by the clock. The basic memory element in sequential logic is the flip-flop. In a synchronous circuit, an electronic oscillator called a clock (or clock generator) generates a sequence of repetitive pulses called the clock signal which is distributed to all the memory elements in the circuit. Nearly all sequential logic today is clocked or synchronous logic. In asynchronous circuits the state of the device can change at any time in response to changing inputs. In synchronous sequential circuits, the state of the device changes only at discrete times in response to a clock signal. When a "channel up" or "channel down" input is given to it, the sequential logic of the channel selection circuitry calculates the new channel from the input and the current channel.ĭigital sequential logic circuits are divided into synchronous and asynchronous types. The television stores the current channel as part of its state. In order for the channel selection to operate correctly, the television must be aware of which channel it is currently receiving, which was determined by past channel selections. However, if the television is on channel 8, pressing "up" switches it to channel "9". If the television is on channel 5, pressing "up" switches it to receive channel 6. Pressing the "up" button gives the television an input telling it to switch to the next channel above the one it is currently receiving. Virtually all circuits in practical digital devices are a mixture of combinational and sequential logic.Ī familiar example of a device with sequential logic is a television set with "channel up" and "channel down" buttons. Sequential logic is used to construct finite state machines, a basic building block in all digital circuitry. That is, sequential logic has state ( memory) while combinational logic does not. This is in contrast to combinational logic, whose output is a function of only the present input. In automata 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.
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