Clock signal
Encyclopedia
In electronics
and especially synchronous digital circuit
s, 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
. Although the word signal has a number of other meanings, the term here is used for "transmitted energy that can carry information".
A clock signal is produced by a clock generator
. Although more complex arrangements are used, the most common clock signal is in the form of a square wave
with a 50% duty cycle
, usually with a fixed, constant frequency. Circuits using the clock signal for synchronization may become active at either the rising edge, falling edge, or, in the case of double data rate
, both in the rising and in the falling edges of the clock cycle.
s (ICs) of sufficient complexity use a clock signal in order to synchronize different parts of the circuit, cycling at a rate less than the worst-case internal propagation delay
s. In some cases, more than one clock cycle is required to perform a predictable action. As ICs become more complex, the problem of supplying accurate and synchronized clocks to all the circuits becomes increasingly difficult. The preeminent example of such complex chips is the microprocessor
, the central component of modern computers, which relies on a clock from a crystal oscillator
.
The only exceptions are asynchronous circuit
s such as asynchronous CPUs.
A clock signal might also be gated, that is, combined with a controlling signal that enables or disables the clock signal for a certain part of a circuit. This technique is often used to save power by effectively shutting down portions of a digital circuit when they are not in use, but comes at a cost of increased complexity in timing analysis.
s use only a "single phase clock" -- in other words, they transmit all clock signals on (effectively) 1 wire.
s, a "two-phase clock" refers to clock signals distributed on 2 wires, each with non-overlapping pulses. Traditionally one wire is called "phase 1" or "phi1", the other wire carries the "phase 2" or "phi2" signal.
http://www.princeton.edu/~wolf/modern-vlsi/Overheads/CHAP5-2/sld010.htm
http://tams-www.informatik.uni-hamburg.de/applets/hades/webdemos/12-gatedelay/40-tpcg/two-phase-clock-gen.html
http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/twophase.html
http://www.hpc.msstate.edu/mpl/distributions/scmos/scmos_doc/cells/cgf104.html
MOS ICs typically used dual clock signals (a two-phase clock) in the 1970s. These were generated externally for both the 6800 and the 8080. The next generation of microprocessors incorporated the clock generation on chip. The 8080 had a 2 MHz clock but the processing throughput was similar to the 1 MHz 6800. The 8080 require more clock cycles to execute a processor instruction. The 6800 had a minimum clock rate of 100 kHz while the 8080 could be halted. Higher speed versions of both microprocessors were released by 1976.
The 6501 required an external 2-phase clock generator.
The MOS Technology 6502
used the same 2-phase logic internally, but also included a two-phase clock generator on-chip, so it only needed a single phase clock input, simplifying system design.
In some early microprocessors such as the National Semiconductor
IMP-16 family, a multi-phase
clock was used. In the case of the IMP-16
, the clock had four phases, each 90 degrees apart, in order to synchronize the operations of the processor core and its peripherals.
Some ICs use four-phase logic
.
Most modern microprocessors and microcontroller
s use a single-phase clock, however.
s use a "clock multiplier" which multiplies a lower frequency external clock to the appropriate clock rate
of the microprocessor. This allows the CPU to operate at a much higher frequency than the rest of the computer, which affords performance gains in situations where the CPU does not need to wait on an external factor (like memory or input/output
).
As long as the minimum and maximum clock times are respected, the time between clock edges can vary widely from one edge to the next.
Such digital devices work just as well with a clock generator that dynamically changes its frequency, such as spread-spectrum clock generation, PowerNow!
, Cool'n'Quiet
, SpeedStep
, etc.
Devices that use static logic do not even have a maximum clock time; such devices can be slowed down and paused indefinitely, then resumed at full clock speed at any later time.
, such as precision analog-to-digital converter
s, use sine wave
s rather than square waves as their clock signals, because square waves contain high-frequency harmonic
s that can interfere with the analog circuitry and cause noise. Such sine wave clocks are often differential signals
, because this type of signal has twice the slew rate
, and therefore half the timing uncertainty, of a single-ended signal
with the same voltage range. Differential signals radiate less strongly than a single line. Alternatively, a single line shielded by power and ground lines can be used.
In CMOS circuits, gate capacitances are charged and uncharged continually. A capacitor does not dissipate energy, but energy is wasted in the driving transistors. In reversible computing
, inductor
s can be used to store this energy and reduce the energy loss, but they tend to be quite large. Alternatively, using a sine wave clock, CMOS transmission gates and energy-saving techniques, the power requirements can be reduced.
In a large microprocessor, the power used to drive the clock signal can be over 30% of the total power used by the entire chip.
The clock signal must be propagated with a clock distribution network. This is often done with a recursive H tree
. The whole structure with the gates at the ends and all amplifiers in between have to be loaded and unloaded every cycle. To save energy, unused parts of the tree may be temporarily cut off (clock gating).
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
and especially synchronous digital circuit
Digital circuit
Digital electronics represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state...
s, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome
Metronome
A metronome is any device that produces regular, metrical ticks — settable in beats per minute. These ticks represent a fixed, regular aural pulse; some metronomes also include synchronized visual motion...
to coordinate actions of circuits
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
. Although the word signal has a number of other meanings, the term here is used for "transmitted energy that can carry information".
A clock signal is produced by a clock generator
Clock generator
A clock generator is a circuit that produces a timing signal for use in synchronizing a circuit's operation. The signal can range from a simple symmetrical square wave to more complex arrangements...
. Although more complex arrangements are used, the most common clock signal is in the form of a square wave
Square wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
with a 50% duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
, usually with a fixed, constant frequency. Circuits using the clock signal for synchronization may become active at either the rising edge, falling edge, or, in the case of double data rate
Double data rate
In computing, a computer bus operating with double data rate transfers data on both the rising and falling edges of the clock signal. This is also known as double pumped, dual-pumped, and double transition....
, both in the rising and in the falling edges of the clock cycle.
Digital circuits
Most integrated circuitIntegrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s (ICs) of sufficient complexity use a clock signal in order to synchronize different parts of the circuit, cycling at a rate less than the worst-case internal propagation delay
Propagation delay
Propagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics or physics...
s. In some cases, more than one clock cycle is required to perform a predictable action. As ICs become more complex, the problem of supplying accurate and synchronized clocks to all the circuits becomes increasingly difficult. The preeminent example of such complex chips is the microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
, the central component of modern computers, which relies on a clock from a crystal oscillator
Crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
.
The only exceptions are asynchronous circuit
Asynchronous circuit
An asynchronous circuit is a circuit in which the parts are largely autonomous. They are not governed by a clock circuit or global clock signal, but instead need only wait for the signals that indicate completion of instructions and operations. These signals are specified by simple data transfer...
s such as asynchronous CPUs.
A clock signal might also be gated, that is, combined with a controlling signal that enables or disables the clock signal for a certain part of a circuit. This technique is often used to save power by effectively shutting down portions of a digital circuit when they are not in use, but comes at a cost of increased complexity in timing analysis.
Single-phase clock
Most modern synchronous circuitSynchronous 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...
s use only a "single phase clock" -- in other words, they transmit all clock signals on (effectively) 1 wire.
Two-phase clock
In synchronous circuitSynchronous 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...
s, a "two-phase clock" refers to clock signals distributed on 2 wires, each with non-overlapping pulses. Traditionally one wire is called "phase 1" or "phi1", the other wire carries the "phase 2" or "phi2" signal.
http://www.princeton.edu/~wolf/modern-vlsi/Overheads/CHAP5-2/sld010.htm
http://tams-www.informatik.uni-hamburg.de/applets/hades/webdemos/12-gatedelay/40-tpcg/two-phase-clock-gen.html
http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/twophase.html
http://www.hpc.msstate.edu/mpl/distributions/scmos/scmos_doc/cells/cgf104.html
MOS ICs typically used dual clock signals (a two-phase clock) in the 1970s. These were generated externally for both the 6800 and the 8080. The next generation of microprocessors incorporated the clock generation on chip. The 8080 had a 2 MHz clock but the processing throughput was similar to the 1 MHz 6800. The 8080 require more clock cycles to execute a processor instruction. The 6800 had a minimum clock rate of 100 kHz while the 8080 could be halted. Higher speed versions of both microprocessors were released by 1976.
The 6501 required an external 2-phase clock generator.
The MOS Technology 6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...
used the same 2-phase logic internally, but also included a two-phase clock generator on-chip, so it only needed a single phase clock input, simplifying system design.
4-phase clock
A "4-phase clock" has clock signals distributed on 4 wires ( http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/fourphase.html, four phase logic ).In some early microprocessors such as the National Semiconductor
National Semiconductor
National Semiconductor was an American semiconductor manufacturer, that specialized in analog devices and subsystems,formerly headquartered in Santa Clara, California, USA. The products of National Semiconductor included power management circuits, display drivers, audio and operational amplifiers,...
IMP-16 family, a multi-phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
clock was used. In the case of the IMP-16
IMP-16
The IMP-16, by National Semiconductor, was the first multi-chip 16-bit microprocessor. It consisted of five PMOS integrated circuits: four four-bit RALU chips providing the data path, and one CROM providing control sequencing and microcode storage.The IMP-16 provided four 16-bit accumulators,...
, the clock had four phases, each 90 degrees apart, in order to synchronize the operations of the processor core and its peripherals.
Some ICs use four-phase logic
Four-phase logic
Four-phase logic is a type of, and design methodology for, dynamic logic; it enabled non-specialist engineers to design quite complex ICs, using either PMOS or NMOS processes.-History:R. K...
.
Most modern microprocessors and microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
s use a single-phase clock, however.
Clock multiplier
Many modern microcomputerMicrocomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
s use a "clock multiplier" which multiplies a lower frequency external clock to the appropriate clock rate
Clock rate
The clock rate typically refers to the frequency that a CPU is running at.For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry into a corresponding square wave...
of the microprocessor. This allows the CPU to operate at a much higher frequency than the rest of the computer, which affords performance gains in situations where the CPU does not need to wait on an external factor (like memory or input/output
Input/output
In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
).
Dynamic frequency change
The vast majority of digital devices do not require a clock at a fixed, constant frequency.As long as the minimum and maximum clock times are respected, the time between clock edges can vary widely from one edge to the next.
Such digital devices work just as well with a clock generator that dynamically changes its frequency, such as spread-spectrum clock generation, PowerNow!
PowerNow!
PowerNow! is speed throttling and power saving technology of AMD's processors used in laptops. The CPU's clock speed and VCore are automatically decreased when the computer is under low load or idle, to save battery power, reduce heat and noise...
, Cool'n'Quiet
Cool'n'Quiet
Cool'n'Quiet is a CPU speed throttling and power saving technology introduced by AMD with its Athlon 64 processor line. It works by reducing the processor's clock rate and voltage when the processor is idle. The aim of this technology is to reduce overall power consumption and lower heat...
, SpeedStep
SpeedStep
SpeedStep is a trademark for a series of dynamic frequency scaling technologies built into some Intel microprocessors that allow the clock speed of the processor to be dynamically changed by software...
, etc.
Devices that use static logic do not even have a maximum clock time; such devices can be slowed down and paused indefinitely, then resumed at full clock speed at any later time.
Other circuits
Some sensitive mixed-signal circuitsMixed-signal integrated circuit
A mixed-signal integrated circuit is any integrated circuit that has both analog circuits and digital circuits on a single semiconductor die.- Examples :...
, such as precision analog-to-digital converter
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
s, use sine wave
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
s rather than square waves as their clock signals, because square waves contain high-frequency harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
s that can interfere with the analog circuitry and cause noise. Such sine wave clocks are often differential signals
Differential signaling
Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet , PCI Express and USB...
, because this type of signal has twice the slew rate
Slew rate
In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
, and therefore half the timing uncertainty, of a single-ended signal
Single-ended signalling
Single-ended signaling is the simplest and most commonly used method of transmitting electrical signals over wires. One wire carries a varying voltage that represents the signal, while the other wire is connected to a reference voltage, usually ground....
with the same voltage range. Differential signals radiate less strongly than a single line. Alternatively, a single line shielded by power and ground lines can be used.
In CMOS circuits, gate capacitances are charged and uncharged continually. A capacitor does not dissipate energy, but energy is wasted in the driving transistors. In reversible computing
Reversible computing
Reversible computing is a model of computing where the computational process to some extent is reversible, i.e., time-invertible. A necessary condition for reversibility of a computational model is that the transition function mapping states to their successors at a given later time should be...
, inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s can be used to store this energy and reduce the energy loss, but they tend to be quite large. Alternatively, using a sine wave clock, CMOS transmission gates and energy-saving techniques, the power requirements can be reduced.
Distribution
The most effective way to get the clock signal to every part of a chip that needs it, with the lowest skew, is a metal grid.In a large microprocessor, the power used to drive the clock signal can be over 30% of the total power used by the entire chip.
The clock signal must be propagated with a clock distribution network. This is often done with a recursive H tree
H tree
The H tree is a family of fractal sets whose Hausdorff dimension is equal to 2...
. The whole structure with the gates at the ends and all amplifiers in between have to be loaded and unloaded every cycle. To save energy, unused parts of the tree may be temporarily cut off (clock gating).
See also
- Self-clocking signalSelf-clocking signalIn telecommunications and electronics, a self-clocking signal is one that can be decoded without the need for a separate clock signal or other source of synchronization...
- Four-phase logicFour-phase logicFour-phase logic is a type of, and design methodology for, dynamic logic; it enabled non-specialist engineers to design quite complex ICs, using either PMOS or NMOS processes.-History:R. K...
- JitterJitterJitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
- Programmable Interval TimerProgrammable Interval TimerIn computing and in embedded systems, a programmable interval timer is a counter which triggers an interrupt when it reaches the programmed count.- Common features :...
- Synchronous circuitSynchronous circuitA 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...
- Clock rateClock rateThe clock rate typically refers to the frequency that a CPU is running at.For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry into a corresponding square wave...