In a synchronous digital system

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. These transitions follow the level change of a special signal called the clock...

,
the clock signal

Clock signal

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 utilized like a metronome to coordinate actions of circuits...

 is used to define a time
reference for the movement of data within that system. The clock distribution network (or clock tree, when this network forms a tree) distributes the clock signal(s) from a common point to all the elements that need it.
Since this function is
vital to the operation of a synchronous system, much attention has been given
to the characteristics of these clock signals and the electrical network

Electrical network

An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...

s used in their
distribution. Clock signals are often regarded as simple control signals;
however, these signals have some very special characteristics and attributes.

Considerations for clock signals

Clock signals are typically loaded with the greatest fanout

Fanout

In digital electronics, the fan-out of a logic gate output is the number of gate inputs to which it is connected.In most designs, logic gates are connected together to form more complex circuits. While no more than one logic gate output is connected to any single input, it is common for one output...

 and operate at the highest speeds of any signal, either
control or data, within the entire synchronous system. Since the data signals
are provided with a temporal reference by the clock signals, the clock
waveform

Waveform

Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...

s must be particularly clean and sharp. Furthermore, these clock
signals are particularly affected by technology scaling (see Moore's law

Moore's Law

Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....

), in that long global
interconnect lines become significantly more resistive as line dimensions are
decreased. This increased line resistance is one of the primary reasons for the
increasing significance of clock distribution on synchronous performance.
Finally, the control of any differences and uncertainty in the arrival times of
the clock signals can severely limit the maximum performance of the entire
system and create catastrophic race conditions in which an incorrect data
signal may latch within a register.

The clock distribution network often takes a significant fraction of the power consumed by a chip. Furthermore, significant power can be wasted in transitions within blocks, even when their output is not needed. These observations have led to a power saving technique called clock gating

Clock gating

Clock gating is a power-saving technique used in many synchronous circuits-Description:Clock gating is a popular technique used in many synchronous circuits for reducing dynamic power dissipation. Clock gating saves power by adding more logic to a circuit to prune the clock tree...

, which involves adding logic gates to the clock distribution tree, so portions of the tree can be turned off when not needed (when a clock can be safely gated may be determined either through automatic analysis of the circuit, or specified by the designer). The exact savings are very design dependent, but around 20-30% is often achievable.

Performance of clocked systems

Most synchronous digital

Digital

A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...

systems consist of cascaded banks of sequential
registers

Flip-flop (electronics)

In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information. 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...

 with combinational logic

Combinational logic

In digital circuit theory, combinational logic is a type of digital logic which is implemented by boolean circuits, where the 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...

 between each set of registers. The
functional requirements

Functional requirements

In software engineering, a functional requirement defines a function of a software system or its component. A function is described as a set of inputs, the behavior, and outputs ....

 of the digital system are satisfied by the logic
stages. The global performance and local timing requirements are satisfied by
the careful insertion of pipeline registers into equally spaced time windows to satisfy critical worst-case timing constraints. The proper design of the clock
distribution network ensures that these critical timing requirements are
satisfied and that no race conditions exist (see also clock skew

Clock skew

-In circuit design:In circuit designs, clock skew is a phenomenon in synchronous circuits in which the clock signal arrives at different components at different times...

).

The delay components that make up a general synchronous system are composed
of the following three individual subsystems: the memory storage elements, the
logic elements, and the clocking circuitry and distribution network.
Interrelationships among these three subsystems of a synchronous digital
system are critical to achieving maximum levels of performance and reliability.

Ongoing research

Novel structures are currently under development to ameliorate these issues and provide effective solutions. Important areas of research include resonant clocking techniques, on-chip optical interconnect, and local synchronization methodologies.

See also

• Electronic design automation
  Electronic design automation
  Electronic design automation is a category of software tools for designing electronic systems such as printed circuit boards and integrated circuits...
  
  
• Design flow (EDA)
  Design flow (EDA)
  Design flows are the explicit combination of electronic design automation tools to accomplish the design of an integrated circuit. Moore's law has driven the entire IC implementation RTL to GDSII design flows from one which uses primarily...
  
  
• Integrated circuit design
  Integrated circuit design
  Integrated circuit design, or IC design, is a subset of electrical engineering and computer engineering, encompassing the particular logic and circuit design techniques required to design integrated circuits, or ICs...
  
  
• Clock gating
  Clock gating
  Clock gating is a power-saving technique used in many synchronous circuits-Description:Clock gating is a popular technique used in many synchronous circuits for reducing dynamic power dissipation. Clock gating saves power by adding more logic to a circuit to prune the clock tree...
  
  
• H tree
  H tree
  The H tree is a family of fractal sets whose Hausdorff dimension is equal to 2...