Dynamic voltage scaling
Encyclopedia
Dynamic voltage scaling is a power management
technique in computer architecture
, where the voltage used in a component is increased or decreased, depending upon circumstances. Dynamic voltage scaling to increase voltage is known as overvolting; dynamic voltage scaling to decrease voltage is known as undervolting. Undervolting is done in order to conserve power
, particularly in laptop
s and other mobile devices, where energy comes from a battery and thus is limited. Overvolting is done in order to increase computer performance, or in rare cases, to increase reliability.
The term "overvolting" is also used to refer to increasing static operating voltage of computer
components to allow operation at higher speed (overclocking
).
-based digital circuits operate using voltages at circuit nodes to represent logical state. The voltage at these nodes switches between a high voltage and a low voltage during normal operation—when the inputs to a logic gate
transition, the transistors making up that gate may toggle the gate's output.
At each node in a circuit is a certain amount of capacitance
. Capacitance can be thought of as a measure of how long it takes for a given current to effect a given voltage change. The capacitance arises from various sources, mainly transistors (primarily gate capacitance and diffusion capacitance
) and wires (coupling capacitance
). Toggling a voltage at a circuit node requires charging or discharging the capacitance at that node; since currents are related to voltage, the time it takes depends on the voltage applied. By applying a higher voltage to the devices in a circuit, the capacitances are charged and discharged more quickly, resulting in faster operation of the circuit and allowing for higher frequency operation.
). It is usually possible to control the voltages supplied to the CPU, RAM, PCI
, and PCI Express
(or AGP
) port through a PC's BIOS.
However, some components do not allow software control of supply voltages, and hardware modification is required by overclockers seeking to overvolt the component for extreme overclocks. Video card
s and motherboard
northbridge
s are components which frequently require hardware modifications to change supply voltages.
These modifications are known as "voltage mods" in the overclocking community.
gates is C·V2·f, where C is the capacitance
being switched per clock cycle, V is voltage
, and f is the switching frequency, so this part of the power consumption decreases quadratically with voltage. The formula is not exact however, as many modern chips are not implemented using 100% CMOS, but also uses pseudo nMOS gates, domino logic etc. Moreover, there is also a static leakage current, which has become more and more accentuated as feature sizes have become smaller (below 90 nanometres) and threshold levels lower.
Accordingly, dynamic voltage scaling is widely used as part of strategies to manage switching power consumption in battery powered devices such as cell phones and laptop computers. Low voltage modes are used in conjunction with lowered clock frequencies to minimize power consumption associated with components such as CPUs and DSPs; only when significant computational power is needed will the voltage and frequency be raised.
Some peripherals also support low voltage operational modes. For example, low power MMC and SD cards can run at 1.8 V as well as at 3.3 V, and driver stacks may conserve power by switching to the lower voltage after detecting a card which supports it.
When leakage current is a significant factor in terms of power consumption, chips are often designed so that portions of them can be powered completely off. This is not usually viewed as being dynamic voltage scaling, because it is not transparent to software. When sections of chips can be turned off, as for example on TI
OMAP3
processors, drivers and other support software need to support that.
) to "high" (VDD) or vice versa - is proportional to the voltage differential in that circuit. Reducing the voltage means that circuits switch slower, reducing the maximum frequency at which that circuit can run. This, in turn, reduces the rate at which program instructions that can be issued, which may increase run time for program segments which are sufficiently CPU-bound.
This again highlights why dynamic voltage scaling is generally done in conjunction with dynamic frequency scaling, at least for CPUs. There are complex tradeoffs to consider, which depend on the particular system, the load presented to it, and power management goals. When quick responses are needed, clocks and voltages might be raised together. Otherwise, they may both be kept low to maximize battery life.
is another power conservation technique that works on the same principles as dynamic voltage scaling. Both dynamic voltage scaling and dynamic frequency scaling can be used to prevent computer system overheating, which can result in program or operating system crashes
, and possibly hardware damage. Reducing the voltage supplied to the CPU below the manufacturer's recommended minimum setting can result in system instability.
. Increases in voltage or frequency may increase system power demands even faster than the CMOS formula indicates, and vice-versa.
There are also longer-term concerns: various adverse device-level effects such as hot carrier injection
and electromigration
occur more rapidly at higher voltages, decreasing the lifespan of overvolted components.
Power management
Power management is a feature of some electrical appliances, especially copiers, computers and computer peripherals such as monitors and printers, that turns off the power or switches the system to a low-power state when inactive. In computing this is known as PC power management and is built...
technique in computer architecture
Computer architecture
In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....
, where the voltage used in a component is increased or decreased, depending upon circumstances. Dynamic voltage scaling to increase voltage is known as overvolting; dynamic voltage scaling to decrease voltage is known as undervolting. Undervolting is done in order to conserve power
Energy conservation
Energy conservation refers to efforts made to reduce energy consumption. Energy conservation can be achieved through increased efficient energy use, in conjunction with decreased energy consumption and/or reduced consumption from conventional energy sources...
, particularly in laptop
Laptop
A laptop, also called a notebook, is a personal computer for mobile use. A laptop integrates most of the typical components of a desktop computer, including a display, a keyboard, a pointing device and speakers into a single unit...
s and other mobile devices, where energy comes from a battery and thus is limited. Overvolting is done in order to increase computer performance, or in rare cases, to increase reliability.
The term "overvolting" is also used to refer to increasing static operating voltage of computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
components to allow operation at higher speed (overclocking
Overclocking
Overclocking is the process of operating a computer component at a higher clock rate than it was designed for or was specified by the manufacturer, but some manufacturers purposely underclock their components to improve battery life. Many people just overclock or 'rightclock' their hardware to...
).
Background
MOSFETMOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
-based digital circuits operate using voltages at circuit nodes to represent logical state. The voltage at these nodes switches between a high voltage and a low voltage during normal operation—when the inputs to a logic gate
Logic gate
A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...
transition, the transistors making up that gate may toggle the gate's output.
At each node in a circuit is a certain amount of capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
. Capacitance can be thought of as a measure of how long it takes for a given current to effect a given voltage change. The capacitance arises from various sources, mainly transistors (primarily gate capacitance and diffusion capacitance
Diffusion capacitance
Diffusion capacitance is the capacitance due to transport of charge carriers between two terminals of a device, for example, the diffusion of carriers from anode to cathode in forward bias mode of a diode or from emitter to base for a transistor...
) and wires (coupling capacitance
Coupling (electronics)
In electronics and telecommunication, coupling is the desirable or undesirable transfer of energy from one medium, such as a metallic wire or an optical fiber, to another medium, including fortuitous transfer....
). Toggling a voltage at a circuit node requires charging or discharging the capacitance at that node; since currents are related to voltage, the time it takes depends on the voltage applied. By applying a higher voltage to the devices in a circuit, the capacitances are charged and discharged more quickly, resulting in faster operation of the circuit and allowing for higher frequency operation.
Methods
Many modern components allow voltage regulation to be controlled through software (for example, through the BIOSBIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
). It is usually possible to control the voltages supplied to the CPU, RAM, PCI
Peripheral Component Interconnect
Conventional PCI is a computer bus for attaching hardware devices in a computer...
, and PCI Express
PCI Express
PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...
(or AGP
Accelerated Graphics Port
The Accelerated Graphics Port is a high-speed point-to-point channel for attaching a video card to a computer's motherboard, primarily to assist in the acceleration of 3D computer graphics. Since 2004 AGP has been progressively phased out in favor of PCI Express...
) port through a PC's BIOS.
However, some components do not allow software control of supply voltages, and hardware modification is required by overclockers seeking to overvolt the component for extreme overclocks. Video card
Video card
A video card, Graphics Card, or Graphics adapter is an expansion card which generates output images to a display. Most video cards offer various functions such as accelerated rendering of 3D scenes and 2D graphics, MPEG-2/MPEG-4 decoding, TV output, or the ability to connect multiple monitors...
s and motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
northbridge
Northbridge (computing)
The northbridge has historically been one of the two chips in the core logic chipset on a PC motherboard, the other being the southbridge. Increasingly these functions have migrated to the CPU chip itself, beginning with memory and graphics controllers. For Intel Sandy Bridge and AMD Fusion...
s are components which frequently require hardware modifications to change supply voltages.
These modifications are known as "voltage mods" in the overclocking community.
Undervolting
Undervolting is reducing the voltage of a component, usually the processor, reducing temperature and cooling requirements, and possibly allowing a fan to be omitted.Power
The switching power dissipated by a chip using static CMOSCMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
gates is C·V2·f, where C is the capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
being switched per clock cycle, V is voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
, and f is the switching frequency, so this part of the power consumption decreases quadratically with voltage. The formula is not exact however, as many modern chips are not implemented using 100% CMOS, but also uses pseudo nMOS gates, domino logic etc. Moreover, there is also a static leakage current, which has become more and more accentuated as feature sizes have become smaller (below 90 nanometres) and threshold levels lower.
Accordingly, dynamic voltage scaling is widely used as part of strategies to manage switching power consumption in battery powered devices such as cell phones and laptop computers. Low voltage modes are used in conjunction with lowered clock frequencies to minimize power consumption associated with components such as CPUs and DSPs; only when significant computational power is needed will the voltage and frequency be raised.
Some peripherals also support low voltage operational modes. For example, low power MMC and SD cards can run at 1.8 V as well as at 3.3 V, and driver stacks may conserve power by switching to the lower voltage after detecting a card which supports it.
When leakage current is a significant factor in terms of power consumption, chips are often designed so that portions of them can be powered completely off. This is not usually viewed as being dynamic voltage scaling, because it is not transparent to software. When sections of chips can be turned off, as for example on TI
Texas Instruments
Texas Instruments Inc. , widely known as TI, is an American company based in Dallas, Texas, United States, which develops and commercializes semiconductor and computer technology...
OMAP3
OMAP
OMAP developed by Texas Instruments is a category of proprietary system on chips for portable and mobile multimedia applications. OMAP devices generally include a general-purpose ARM architecture processor core plus one or more specialized co-processors...
processors, drivers and other support software need to support that.
Program execution speed
The speed at which a digital circuit can switch states - that is, to go from "low" (VSSVSS
-Organizations:* Vernon Secondary School, a high school in Vernon, B.C.* Valley Stream South High School in Valley Stream, New York* Vaughan Secondary School, a public highschool located in Thornhill, Ontario operated by the York Region District School Board...
) to "high" (VDD) or vice versa - is proportional to the voltage differential in that circuit. Reducing the voltage means that circuits switch slower, reducing the maximum frequency at which that circuit can run. This, in turn, reduces the rate at which program instructions that can be issued, which may increase run time for program segments which are sufficiently CPU-bound.
This again highlights why dynamic voltage scaling is generally done in conjunction with dynamic frequency scaling, at least for CPUs. There are complex tradeoffs to consider, which depend on the particular system, the load presented to it, and power management goals. When quick responses are needed, clocks and voltages might be raised together. Otherwise, they may both be kept low to maximize battery life.
System stability
Dynamic frequency scalingDynamic frequency scaling
Dynamic frequency scaling is a technique in computer architecture whereby the frequency of a microprocessor can be automatically adjusted "on the fly," either to conserve power or to reduce the amount of heat generated by the chip...
is another power conservation technique that works on the same principles as dynamic voltage scaling. Both dynamic voltage scaling and dynamic frequency scaling can be used to prevent computer system overheating, which can result in program or operating system crashes
Crash (computing)
A crash in computing is a condition where a computer or a program, either an application or part of the operating system, ceases to function properly, often exiting after encountering errors. Often the offending program may appear to freeze or hang until a crash reporting service documents...
, and possibly hardware damage. Reducing the voltage supplied to the CPU below the manufacturer's recommended minimum setting can result in system instability.
Temperature
The efficiency of some electrical components, such as voltage regulators, decreases with increasing temperature, so the power used may increase with temperature, and causing increasing power usage, causing thermal runawayThermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...
. Increases in voltage or frequency may increase system power demands even faster than the CMOS formula indicates, and vice-versa.
Caveats
The primary caveat of overvolting is increased heat: the power dissipated by a circuit increases with the square of the voltage applied, so even small voltage increases significantly affect power. At higher temperatures, transistor performance is adversely affected, and at some threshold, the performance reduction due to the heat exceeds the potential gains from the higher voltages. Overheating and damage to circuits can occur very quickly when using high voltages.There are also longer-term concerns: various adverse device-level effects such as hot carrier injection
Hot carrier injection
Hot carrier injection is a phenomenon in solid-state electronic devices where an electron or a “hole” gains sufficient kinetic energy to overcome a potential barrier necessary to break an interface state. The term "hot" refers to the effective temperature used to model carrier density, not to the...
and electromigration
Electromigration
Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direct current densities are used, such as in...
occur more rapidly at higher voltages, decreasing the lifespan of overvolted components.