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In digital circuit
Digital circuit

Digital electronics are electronics systems that use digital signals. Digital electronics are representations of Boolean algebra and are used in computers, mobile phones, and other consumer products....
 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 logic
Combinational logic

In digital circuit theory, combinational logic is a type of logic circuit whose output is a pure function of the present input only. This is in contrast to sequential logic, in which the output depends not only on the present input but also on the history of the input....
, whose output is a function of, and only of, the present input. In other words, sequential logic has storage
Computer storage

Computer data storage, often called storage or memory, refers to computer components, devices, and recording medium that retain digital data used for computing for some interval of time....
 (memory) while combinational logic does not.

Sequential logic is therefore used to construct some types of computer
Computer

A computer is a machine that manipulates Data according to a list of Code .The first devices that resemble modern computers date to the mid-20th century , although the computer concept and various machines similar to computers existed earlier....
 memory
Computer storage

Computer data storage, often called storage or memory, refers to computer components, devices, and recording medium that retain digital data used for computing for some interval of time....
, other types of delay and storage elements, and finite state machine
Finite state machine

A finite state machine or finite state automaton or simply a state machine, is a model of behavior composed of a finite number of state s, transitions between those states, and actions....
s.






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In digital circuit
Digital circuit

Digital electronics are electronics systems that use digital signals. Digital electronics are representations of Boolean algebra and are used in computers, mobile phones, and other consumer products....
 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 logic
Combinational logic

In digital circuit theory, combinational logic is a type of logic circuit whose output is a pure function of the present input only. This is in contrast to sequential logic, in which the output depends not only on the present input but also on the history of the input....
, whose output is a function of, and only of, the present input. In other words, sequential logic has storage
Computer storage

Computer data storage, often called storage or memory, refers to computer components, devices, and recording medium that retain digital data used for computing for some interval of time....
 (memory) while combinational logic does not.

Sequential logic is therefore used to construct some types of computer
Computer

A computer is a machine that manipulates Data according to a list of Code .The first devices that resemble modern computers date to the mid-20th century , although the computer concept and various machines similar to computers existed earlier....
 memory
Computer storage

Computer data storage, often called storage or memory, refers to computer components, devices, and recording medium that retain digital data used for computing for some interval of time....
, other types of delay and storage elements, and finite state machine
Finite state machine

A finite state machine or finite state automaton or simply a state machine, is a model of behavior composed of a finite number of state s, transitions between those states, 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 machine
    Moore machine

    In the theory of computation, a Moore machine is a finite state transducer where the outputs are determined by the current state alone . The state diagram for a Moore machine will include an output signal for each state....
    : 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 machine
    Mealy machine

    In the theory of computation, a Mealy machine is a finite state transducer that generates an output based on its current state and an input....
    : 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-flop
Flip-flop (electronics)

In digital circuits, a flip-flop is a term referring to an electronic circuit that has two stable states and thereby is capable of serving as one bit of computer storage....
.

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 a relatively large amount of power and dissipates much 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 state
State (computer science)

In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as Lexical analysiss and parsers....
, 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 (microprocessors, digital clock
Digital clock

A digital clock is a type of clock that displays the time digitally, i.e. in cyphers, as opposed to an analog clock, where the time is displayed by hands....
s, mobile phone
Mobile phone

A mobile phone is a long-range, electronic device used for mobile voice or data communication over a network of specialized base stations known as cell sites....
s, cordless telephone
Cordless telephone

File:Phone.svgA cordless telephone or portable telephone is a telephone with a wireless handset that communicates via radio waves with a base station connected to a fixed telephone line, usually within a limited range of its base station ....
s, electronic calculator
Calculator

A calculator is a device for performing mathematical calculations, distinguished from a computer by having a limited problem solving ability and an interface optimized for interactive calculation rather than programming....
s, etc.) 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 computer
Analog computer

An analog computer is a form of computer that uses continuous physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved....
s and clockless CPUs
Asynchronous circuit

An asynchronous circuit is a electrical network in which the parts are largely autonomous. They are not governed by a clock circuit or global clock signal, but instead need only wait for the signals that indicate completion of instructions and operations....
.

Typically each bit of the "state" is contained in its own flip-flop
Flip-flop (electronics)

In digital circuits, a flip-flop is a term referring to an electronic circuit that has two stable states and thereby is capable of serving as one bit of computer storage....
. Combinational logic
Combinational logic

In digital circuit theory, combinational logic is a type of logic circuit whose output is a pure function of the present input only. This is in contrast to sequential logic, in which the output depends not only on the present input but also on the history of the input....
 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 machine
Moore machine

In the theory of computation, a Moore machine is a finite state transducer where the outputs are determined by the current state alone . The state diagram for a Moore machine will include an output signal for each state....
.

See also

  • Synchronous circuit
    Synchronous circuit

    A synchronous circuit is a digital circuit in which the parts are synchronized by a clock signal.In an ideal synchronous circuit, every change in the logical levels of its storage components is simultaneous....
  • Asynchronous circuit
    Asynchronous circuit

    An asynchronous circuit is a electrical network in which the parts are largely autonomous. They are not governed by a clock circuit or global clock signal, but instead need only wait for the signals that indicate completion of instructions and operations....