In a synchronous digital system, the clock signal is used to define a time reference for the movement of data within that system. The clock distribution network 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 networks used in their distribution. Clock signals are often regarded as simple control signals; however, these signals have some very special characteristics and attributes. A synchronous circuit is a circuit in which the parts are synchronized by means of a clock subcircuit. ... In electronics and especially synchronous digital circuits, a clock signal is a signal used to coordinate the actions of two or more circuits. ... This article does not cite any references or sources. ...

Considerations for clock signals

Clock signals are typically loaded with the greatest fanout, travel over the greatest distances, 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 waveforms must be particularly clean and sharp. Furthermore, these clock signals are particularly affected by technology scaling (see Moore's law), 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. Fanout is a measure of the ability of a logic gate output, implemented electronically, to drive a number of inputs of other logic gates of the same type. ... Waveform quite literally means the shape and form of a signal, such as a wave moving across the surface of water, or the vibration of a plucked string. ... Gordon Moores original graph from 1965 Growth of transistor counts for Intel processors (dots) and Moores Law (upper line=18 months; lower line=24 months) For the observation regarding information retrieval, see Mooers Law. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

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 lead to a power saving technique called clock gating, 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. Clock gating is one of the power-saving techniques used on the Pentium 4 processor. ...

Performance of clocked systems

Most synchronous digital systems consist of cascaded banks of sequential registers with combinational logic between each set of registers. The functional requirements 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.) For other uses, see Digital (disambiguation). ... In digital circuits, the flip-flop, latch, or bistable multivibrator is an electronic circuit which has two stable states and thereby is capable of serving as one bit of memory. ... This article is not about combinatory logic, a topic in mathematical logic. ... Functional requirements define the internal workings of the software: that is, the calculations, technical details, data manipulation and processing and other specific functionality that show how the use cases are to be satisfied. ... In computer architecture, a processor register is a small amount of very fast computer memory used to speed the execution of computer programs by providing quick access to frequently used values—typically, these values are involved in multiple expression evaluations occurring within a small region on the program. ... // In circuit design, clock skew (sometimes timing skew) is a phenomenon in synchronous circuits in which the clock signal (sent from the clock circuit) 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.

References

• E. G. Friedman (Ed.), Clock Distribution Networks in VLSI Circuits and Systems, ISBN 0-7803-1058-6, IEEE Press. 1995.
• E. G. Friedman, Clock Distribution Networks in Synchronous Digital Integrated Circuits , Proceedings of the IEEE, Vol. 89, No. 5, pp. 665-692, May 2001.
• V. G. Oklobdzija, V. M. Stojanovic, D. M. Markovic, and N. M. Nedovic, Digital System Clocking: High-Performance and Low-Power Aspects, ISBN 0-471-27447-X, IEEE Press/Wiley-Interscience, 2003.
• The power of RTL Clock-gating by Mitch Dale ,http://www.chipdesignmag.com/display.php?articleId=915

Adapted from Eby Friedman's column in the ACM SIGDA e-newsletter by Igor Markov
Original text is available at http://sigda.org/newsletter/2005/eNews_051201.html

See also

• Electronic design automation
• Design flow (EDA)
• Integrated circuit design
• Clock gating
• H tree

PCB Layout Program Electronic design automation (EDA) is the category of tools for designing and producing electronic systems ranging from printed circuit boards (PCBs) to integrated circuits. ... Design flows are the explicit combination of electronic design automation tools to accomplish the design of an integrated circuit. ... A simple CMOS Operational Amplifier Integrated circuit design, or IC design, is a subset of electrical engineering, encompassing the particular logic and circuit design techniques required to design integrated circuits, or ICs. ... Clock gating is one of the power-saving techniques used on the Pentium 4 processor. ...

Further reading/External links

One major source of additional detail on this subject (Clock distribution network) is the technical literature. The journals IEEE Transactions On Computer-Aided Design Of Integrated Circuits And Systems (Web page here) and ACM Transactions on Design Automation focus explicitly on Design Automation. In addition, many other journals carry EDA specific articles, and there are also many EDA-centric conferences. IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems, often abbreviated IEEE TCAD or IEEE Transactions on CAD, is a technical journal devoted to the design, analysis, and use of computer programs that aid in the design of integrated circuits and systems. ...

Most of these articles and proceedings are published by either the IEEE or the ACM. Desired articles can be found through the IEEE on-line library or the ACM digital library, which show abstracts, or [Google scholar], which shows snippets. The articles themselves are sometimes available for free on-line (both the IEEE and ACM allow authors to keep copies on their personal web sites.) If you are not so lucky, you can visit a technical library or download the article from the IEEE or ACM. Although almost all articles are available, on-line access to the full text is not free - it requires purchase, society membership, or a site license. Many schools and companies have site licenses already. As a last resort, a polite email to the author will sometimes yield a copy. Not to be confused with the Institution of Electrical Engineers (IEE). ... The Association for Computing Machinery, or ACM, was founded in 1947 as the worlds first scientific and educational computing society. ...