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Sequential logic

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	Topics Sequential logic Topic Home In digital circuitDigital circuit A digital circuit is based on a number of discrete voltage levels, as distinct from an analog circuit that uses continuous v... theory, sequential logic is a type of logic circuit whose output depends not only on the present input but also on the history of the input. This is in contrast to combinational logicCombinational logic ----In digital circuit theory, combinational logic is a type of logic circuit whose output is a function of the present inp... , whose output is a function of, and only of, the present input. In other words, sequential logic has storageComputer storage Computer storage, computer memory, and often casually memory refer to computer components, devices and recording... (memory) while combinational logic does not. Sequential logic is therefore used to construct some types of computerFacts About Computer A computer is a machine for manipulating data according to a list of instructions known as a program.... memoryComputer storage Computer storage, computer memory, and often casually memory refer to computer components, devices and recording... , other types of delay and storage elements, and finite state machineFinite state machine A finite state machine or finite automaton is a model of behavior composed of states, transitions and actions.... s. Most practical computer circuits are a mixture of combinational and sequential logic. There are two types of finite state machine that can be built from sequential logic circuits: Moore machineMoore machine In the theory of computation, a Moore machine is a finite state automaton where the outputs are determined by the current st... : the output depends only on the internal state. (Since the internal state only changes on a clock edge, the output only changes on a clock edge too). Mealy machineMealy machine In the theory of computation, a Mealy machine is a type of transducer, a finite state machine that generates an output for e... : the output depends not only on the internal state, but also on the inputs. Synchronous sequential logic Nearly all sequential logic today is 'clocked' or 'synchronous logic' logic: there is a 'clock' signal, and all internal memory (the 'internal state') changes only on a clock edge. The basic storage element in sequential logic is the flip-flopFlip-flop (electronics) Overview In electronics and digital circuits, the flip-flop or bistable multivibrator is a pulsed digital circuit capable of se... . The main advantage of synchronous logic is its simplicity. Every operation in the circuit must be completed inside a fixed interval of time between two clock pulses, called a 'clock cycle'. As long as this condition is met (ignoring certain other details), the circuit is guaranteed to be reliable. Synchronous logic also has two main disadvantages, as follows. 1. The clock signal must be distributed to every flip-flop in the circuit. As the clock is usually a high-frequency signal, this distribution consumes power and dissipates heat. Even the flip-flops that are doing nothing consume a small amount of power, thereby generating waste heat in the chip. 2. The maximum possible clock rate is determined by the slowest logic path in the circuit, otherwise known as the critical path. This means that every logical calculation, from the simplest to the most complex, must complete in one clock cycle. One way around this limitation is to split complex operations into several simple operations, a technique known as 'pipelining'. This technique is prominent within microprocessor design, and helps to improve the clock rate of modern processors. Clocked sequential system In digital electronics, a clocked sequential system is a system whose output depends only on the current stateState (computer science) In information processing, a state is the complete set of properties transmitted by an object to an observer via one or more... , whose state changes only when a global clock signal changes, and whose next-state depends only on the current state and the inputs. Nearly all digital electronic devices are designed as clocked sequential systems. Notable exceptions include digital asynchronous logic systems. In particular, nearly all computers are designed as clocked sequential systems. Notable exceptions include analog computerAnalog computer An analog computer is a form of computer that uses electrical or mechanical phenomena to model the problem being solved by u... s and clockless CPUsAsynchronous circuit Summary An asynchronous circuit is a circuit in which the parts are largely autonomous.... . Typically each bit of the "state" is contained in its own flip-flopFlip-flop (electronics) In electronics and digital circuits, the flip-flop or bistable multivibrator is a pulsed digital circuit capable of se... . Combinational logicCombinational logic ----In digital circuit theory, combinational logic is a type of logic circuit whose output is a function of the present inp... decodes the state into the output signals. More combinational logic encodes the current state and the inputs into the next-state signals. The next-state signals are latched into the flipflops under the control of the global clock signal (a wire connected to every flip-flop). A clocked sequential system is a kind of Moore machineMoore machine In the theory of computation, a Moore machine is a finite state automaton where the outputs are determined by the current st... . See also synchronous circuitSynchronous circuit A synchronous circuit is a digital circuit in which the parts are synchronized by a clock signal....