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. This is in contrast to combinatorial logic, whose output is a function of, and only of, the present input. In other words, sequential logic has storage (memory) while combinatorial logic does not. Digital circuits are electric circuits based on a number of discrete voltage levels. ... This article is not about combinatory logic, a topic in mathematical logic. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

Sequential logic is therefore used to construct some types of computer memory, other types of delay and storage elements, and finite state machines. Most practical computer circuits are a mixture of combinatorial and sequential logic. A computer is a device or machine for processing information according to a program — a compiled list of instructions. ... This article needs to be cleaned up to conform to a higher standard of quality. ... Fig. ...

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. (Since the internal state only changes on a clock edge, the output only changes on a clock edge).
• Mealy machine: the output depends not only on the internal state, but also on the inputs.

Moore model: control of an elevator door In the theory of computation, a Moore machine is a finite state automaton where the outputs are determined by the current state alone (and not on the input). ... In the theory of computation, a Mealy machine is a finite state machine where the outputs are determined by the current state and the input. ...

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. This article is about the electronic component. ...

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 causes power dissipation - in other words, heat. Even the flip-flops that are doing nothing consume a small amount of power, thereby generating waste heat.
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.

Sine waves of various frequencies; the lower waves have higher frequencies than those above. ... // Mechanical power In physics, power (symbol: P) is the amount of work W done per unit of time t. ... A red-hot iron rod cooling after being worked by a blacksmith. ... Microprocessors, including an Intel 80486DX2 and an Intel 80386 A microprocessor (abbreviated as µP or uP) is a computer electronic component made from miniaturized transistors and other circuit elements on a single semiconductor integrated circuit (IC) (aka microchip or just chip). ... The clock rate is the fundamental rate in cycles per second, measured in hertz, at which a computer performs its most basic operations such as adding two numbers or transferring a value from one processor register to another. ...

Asynchronous sequential logic

Asynchronous sequential logic is the most general kind of sequential logic, but because of its flexibility it is also the most difficult kind to design. The basic storage element in asynchronous logic is the latch. Latches can change state at any time, depending on the transitions of other signals which may themselves be produced by other latches. The complexity of asynchronous circuits tends to rise very rapidly as the number of logic gates increases, so they tend to be used mostly in smaller applications. However, computer-aided design tools are appearing that can simplify the task, and permit more complex designs. The latch is a electronic circuit used to store information in asynchronous sequential logic systems. ... Computer-aided design (CAD) is the use of a wide range of computer-based tools that assist engineers, architects and other design professionals in their design activities. ...

It is possible to build mixed circuits containing synchronous flip-flops and asynchronous latches.