In digital circuit 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. Digital electronics are Electronics systems that use Digital signals Digital electronics are representations of Boolean algebra also see This is in contrast to combinational logic, whose output is a function of, and only of, the present input. In Digital circuit theory combinational logic (also called combinatorial logic) is a type of logic circuit whose output is a Pure function of the In other words, sequential logic has storage (memory) while combinational logic does not. Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital
Sequential logic is therefore used to construct some types of computer memory, other types of delay and storage elements, and finite state machines. A computer is a Machine that manipulates data according to a list of instructions. Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital 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: the output depends only on the internal state. In the Theory of computation, a Moore machine is a Finite state automaton where the outputs are determined by the current state (Since the internal state only changes on a clock edge, the output only changes on a clock edge too).
- Mealy machine: the output depends not only on the internal state, but also on the inputs. In the Theory of computation, a Mealy machine is a Finite state machine (and more accurately a Finite state transducer) that generates an output based
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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. In Digital circuits a flip-flop is a term referring to an Electronic circuit (a Bistable Multivibrator) that has two stable states and thereby
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 state, whose state changes only when a global clock signal changes, and whose next-state depends only on the current state and the inputs. Digital electronics are Electronics systems that use Digital signals Digital electronics are representations of Boolean algebra also see In Computer science and Automata theory, a state is a unique configuration of information in a program or machine
Nearly all digital electronic devices (microprocessors, digital clocks, mobile phones, cordless telephones, electronic calculators, etc. A microprocessor incorporates most or all of the functions of a Central processing unit (CPU on a single Integrated A digital clock is a type of Clock that displays the time Digitally i A cordless telephone or portable telephone is a Telephone with a Wireless handset that communicates via Radio waves with a Base station A calculator is device for performing mathematical calculations distinguished from a Computer by having a limited problem solving ability and an interface optimized for interactive ) are designed as clocked sequential systems. Notable exceptions include digital asynchronous logic systems. An asynchronous circuit is a circuit in which the parts are largely autonomous
In particular, nearly all computers are designed as clocked sequential systems. Notable exceptions include analog computers and clockless CPUs. An analog computer (spelt analogue in British English is a form of Computer that uses continuous physical phenomena such as electrical mechanical 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-flop. In Digital circuits a flip-flop is a term referring to an Electronic circuit (a Bistable Multivibrator) that has two stable states and thereby Combinational logic decodes the state into the output signals. In Digital circuit theory combinational logic (also called combinatorial logic) is a type of logic circuit whose output is a Pure function of the 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. In the Theory of computation, a Moore machine is a Finite state automaton where the outputs are determined by the current state
See also
References
- Katz, R, and Boriello, G. A synchronous circuit is a Digital circuit in which the parts are synchronized by a Clock signal. Contemporary Logic Design. Upper Saddle River, NJ: Prentice Hall. 2005. ISBN 0-201-30857-6.
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