Synchronous circuit

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A synchronous circuit is a digital circuit in which the changes in the state of memory elements are synchronized by a clock signal. In a sequential digital logic circuit, data is stored in memory devices called flip-flops or latches. The output of a flip-flop is constant until a pulse is applied to its "clock" input, upon which the input of the flip-flop is latched into its output. In a synchronous logic circuit, an electronic oscillator called the clock generates a string of pulses, the "clock signal". This clock signal is applied to every storage element, so in an ideal synchronous circuit, every change in the logical levels of its storage components is simultaneous. Ideally, the input to each storage element has reached its final value before the next clock occurs, so the behaviour of the whole circuit can be predicted exactly. Practically, some delay is required for each logical operation, resulting in a maximum speed at which each synchronous system can run.

In information technology and computer science, a program is described as stateful if it is designed to remember preceding events or user interactions; the remembered information is called the state of the system.

In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits.

In digital circuit 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. This is in contrast to combinational logic, whose output is a function of only the present input. That is, sequential logic has state (memory) while combinational logic does not.

To make these circuits work correctly, a great deal of care is needed in the design of the clock distribution networks. Static timing analysis is often used to determine the maximum safe operating speed.

Static timing analysis (STA) is a simulation method of computing the expected timing of a digital circuit without requiring a simulation of the full circuit.

Nearly all digital circuits, and in particular nearly all CPUs, are fully synchronous circuits with a global clock. Exceptions are often compared to fully synchronous circuits. Exceptions include self-synchronous circuits, [1] [2] [3] [4] globally asynchronous locally synchronous circuits, and fully asynchronous circuits.

Globally asynchronous locally synchronous (GALS) is an architecture for designing electronic circuits which addresses the problem of safe and reliable data transfer between independent clock domains. GALS is a Model of Computation (MoC) that emerged in the 1980s. It allows to design computer systems consisting of several synchronous islands interacting with other islands using asynchronous communication, e.g. with FIFOs.

An asynchronous circuit, or self-timed circuit, is a sequential digital logic circuit which is not governed by a clock circuit or global clock signal. Instead it often uses signals that indicate completion of instructions and operations, specified by simple data transfer protocols. This type of circuit is contrasted with synchronous circuits, in which changes to the signal values in the circuit are triggered by repetitive pulses called a clock signal. Most digital devices today use synchronous circuits. However asynchronous circuits have the potential to be faster, and may also have advantages in lower power consumption, lower electromagnetic interference, and better modularity in large systems. Asynchronous circuits are an active area of research in digital logic design.

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In the theory of computation, a Moore machine is a finite-state machine whose output values are determined only by its current state. This is in contrast to a Mealy machine, whose output values are determined both by its current state and by the values of its inputs. The Moore machine is named after Edward F. Moore, who presented the concept in a 1956 paper, “Gedanken-experiments on Sequential Machines.”

In the theory of computation, a Mealy machine is a finite-state machine whose output values are determined both by its current state and the current inputs. This is in contrast to a Moore machine, whose output values are determined solely by its current state. A Mealy machine is a deterministic finite-state transducer: for each state and input, at most one transition is possible.

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In digital logic and computing, a counter is a device which stores the number of times a particular event or process has occurred, often in relationship to a clock signal. The most common type is a sequential digital logic circuit with an input line called the clock and multiple output lines. The values on the output lines represent a number in the binary or BCD number system. Each pulse applied to the clock input increments or decrements the number in the counter.

Digital electronics Electronic circuits that utilize digital signals

Digital electronics or digital (electronic) circuits are electronics that operate on digital signals. In contrast, analog circuits manipulate analog signals whose performance is more subject to manufacturing tolerance, signal attenuation and noise. Digital techniques are helpful because it is a lot easier to get an electronic device to switch into one of a number of known states than to accurately reproduce a continuous range of values.

In digital circuits, a shift register is a cascade of flip flops, sharing the same clock, in which the output of each flip-flop is connected to the 'data' input of the next flip-flop in the chain, resulting in a circuit that shifts by one position the 'bit array' stored in it, 'shifting in' the data present at its input and 'shifting out' the last bit in the array, at each transition of the clock input.

Static random-access memory Semiconductor memory

Static random-access memory is a type of semiconductor memory that uses bistable latching circuitry (flip-flop) to store each bit. SRAM exhibits data remanence, but it is still volatile in the conventional sense that data is eventually lost when the memory is not powered.

In digital circuit design, register-transfer level (RTL) is a design abstraction which models a synchronous digital circuit in terms of the flow of digital signals (data) between hardware registers, and the logical operations performed on those signals.

A synchronous programming language is a computer programming language optimized for programming reactive systems. Computer systems can be sorted in three main classes: (1) transformational systems that take some inputs, process them, deliver their outputs, and terminate their execution; a typical example is a compiler; (2) interactive systems that interact continuously with their environment, at their own speed; a typical example is the web; and (3) reactive systems that interact continuously with their environment, at a speed imposed by the environment; a typical example is the automatic flight control system of modern airplanes. Reactive systems must therefore react to stimuli from the environment within strict time bounds. For this reason they are often also called real-time systems, and are found often in embedded systems.


The Muller C-element is a small digital block widely used in design of asynchronous circuits and systems. It has been specified formally in 1955 by David E. Muller and first used in ILLIAC II computer. In terms of the theory of lattices, the C-element is a semimodular distributive circuit, whose operation in time is described by a Hasse diagram. The C-element is closely related to the rendezvous and join elements, where an input is not allowed to change twice in succession. In some cases, when relations between delays are known, the C-element can be realized as a sum-of-product (SOP) circuit ,. Earlier techniques for implementing the C-element include Schmidt trigger, Eccles-Jordan flip-flop and last moving point flip-flop.

Metastability (electronics)

Metastability in electronics is the ability of a digital electronics system to persist for an unbounded time in an unstable equilibrium or metastable state. In digital logic circuits, a digital signal is required to be within certain voltage or current limits to represent a '0' or '1' logic level for correct circuit operation; if the signal is within a forbidden intermediate range it may cause faulty behavior in logic gates the signal is applied to. In metastable states, the circuit may be unable to settle into a stable '0' or '1' logic level within the time required for proper circuit operation. As a result, the circuit can act in unpredictable ways, and may lead to a system failure, sometimes referred to as a "glitch". Metastability is an instance of Buridan's paradox.

A ring counter is a type of counter composed of flip-flops connected into a shift register, with the output of the last flip-flop fed to the input of the first, making a "circular" or "ring" structure.

In integrated circuit design, dynamic logic is a design methodology in combinatory logic circuits, particularly those implemented in MOS technology. It is distinguished from the so-called static logic by exploiting temporary storage of information in stray and gate capacitances. It was popular in the 1970s and has seen a recent resurgence in the design of high speed digital electronics, particularly computer CPUs. Dynamic logic circuits are usually faster than static counterparts, and require less surface area, but are more difficult to design. Dynamic logic has a higher toggle rate than static logic but the capacitative loads being toggled are smaller so the overall power consumption of dynamic logic may be higher or lower depending on various tradeoffs. When referring to a particular logic family, the dynamic adjective usually suffices to distinguish the design methodology, e.g. dynamic CMOS or dynamic SOI design.

In digital electronic design a clock domain crossing (CDC), or simply clock crossing, is the traversal of a signal in a synchronous digital circuit from one clock domain into another. If a signal does not assert long enough and is not registered, it may appear asynchronous on the incoming clock boundary.

A frequency divider, also called a clock divider or scaler or prescaler, is a circuit that takes an input signal of a frequency, , and generates an output signal of a frequency:

Timing closure is the process by which a logic design consisting of primitive elements such as combinatorial logic gates and sequential logic gates is modified to meet its timing requirements. Unlike in a computer program where there is no explicit delay to perform a calculation, logic circuits have intrinsic and well defined delays to propagate inputs to outputs. In simple cases, the user can compute the path delay between elements manually. If the design is more than a dozen or so elements this is impractical. For example, the time delay along a path from the output of a D-Flip Flop, through combinatorial logic gates, then into the next D-Flip Flop input must satisfy the time period between synchronizing clock pulses to the two flip flops. When the delay through the elements is greater than the clock cycle time, the elements are said to be on the critical path. The circuit will not function when the path delay exceeds the clock cycle delay so modifying the circuit to remove the timing failure is an important part of the logic design engineer's task.

Flip-flop (electronics) circuit that has two stable states and can be used to store state information

In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information. A flip-flop is a bistable multivibrator. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in sequential logic. Flip-flops and latches are fundamental building blocks of digital electronics systems used in computers, communications, and many other types of systems.

The memory cell is the fundamental building block of computer memory. The memory cell is an electronic circuit that stores one bit of binary information and it must be set to store a logic 1 and reset to store a logic 0. Its value is maintained/stored until it is changed by the set/reset process. The value in the memory cell can be accessed by reading it.

Low Power flip-flops are flip-flops that are designed for low-power electronics, such as smartphones and notebooks. A flip-flop, or latch, is a circuit that has two stable states and can be used to store state information.

Vacuum tube computer computer using vacuum tubes instead of transistors (prevalent in 1950s)

A vacuum tube computer, now termed a first-generation computer, is a computer which uses vacuum tubes for logic circuitry. Although superseded by second generation, transistorized computers, vacuum tube computers continued to be built into the 1960s. These computers were mostly one-of-a-kind designs.