# Dynamic voltage scaling

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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 laptops and other mobile devices, [1] where energy comes from a battery and thus is limited, or in rare cases, to increase reliability. Overvolting is done in order to increase computer performance.

## Contents

The term "overvolting" is also used to refer to increasing static operating voltage of computer components to allow operation at higher speed (overclocking).

## Background

MOSFET-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 operationwhen 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 produce 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.

## Methods

Many modern components allow voltage regulation to be controlled through software (for example, through the BIOS). 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 cards and motherboard northbridges 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 CMOS gates is ${\displaystyle C\cdot V^{2}\cdot f}$, where C is the capacitance being switched per clock cycle, V is the supply voltage, and f is the switching frequency, [2] 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 use special memory circuits, dynamic logic such as 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.

## Program execution speed

The speed at which a digital circuit can switch states - that is, to go from "low" (VSS) 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.

## Implementations

The 167-processor AsAP 2 chip enables individual processors to make extremely fast (on the order of 1-2ns) and locally controlled changes to their own supply voltages. Processors connect their local power grid to either a higher (VddHi) or lower (VddLow) supply voltage, or can be cut off entirely from either grid to dramatically cut leakage power.

Another approach uses per-core on-chip switching regulators for dynamic voltage and frequency scaling (DVFS). [3]

## Operating system API

Unix system provides a userspace governor, allowing to modify the cpu frequencies (though limited to hardware capabilities).

## System stability

Dynamic frequency scaling 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.

## Temperature

The efficiency of some electrical components, such as voltage regulators, decreases with increasing temperature, so the power used may increase with temperature causing thermal runaway. Increases in voltage or frequency may increase system power demands even faster than the CMOS formula indicates, and vice versa. [4] [5]

## 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 and electromigration occur more rapidly at higher voltages, decreasing the lifespan of overvolted components.

## Related Research Articles

Complementary metal–oxide–semiconductor (CMOS), also known as complementary-symmetry metal–oxide–semiconductor (COS-MOS), is a type of MOSFET fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFETs for logic functions. CMOS technology is used for constructing integrated circuit (IC) chips, including microprocessors, microcontrollers, memory chips, and other digital logic circuits. CMOS technology is also used for analog circuits such as image sensors, data converters, RF circuits, and highly integrated transceivers for many types of communication.

In computing, overclocking is the practice of increasing the clock rate of a computer to exceed that certified by the manufacturer. Commonly operating voltage is also increased to maintain a component's operational stability at accelerated speeds. Semiconductor devices operated at higher frequencies and voltages increase power consumption and heat. An overclocked device may be unreliable or fail completely if the additional heat load is not removed or power delivery components cannot meet increased power demands. Many device warranties state that overclocking and/or over-specification voids any warranty.

Central processing unit power dissipation or CPU power dissipation is the process in which central processing units (CPUs) consume electrical energy, and dissipate this energy in the form of heat due to the resistance in the electronic circuits.

Underclocking, also known as downclocking, is modifying a computer or electronic circuit's timing settings to run at a lower clock rate than is specified. Underclocking is used to reduce a computer's power consumption, increase battery life, reduce heat emission, and it may also increase the system's stability and compatibility. Underclocking may be implemented by the factory, but many computers and components may be underclocked by the end user.

In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage-dependent capacitance of a reverse-biased p–n junction.

Power management is a feature of some electrical appliances, especially copiers, computers, CPUs, GPUs 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 around a standard called ACPI. This supersedes APM. All recent (consumer) computers have ACPI support.

In digital electronics, the fan-out is the number of gate inputs that the output of a logic gate drives.

A voltage multiplier is an electrical circuit that converts AC electrical power from a lower voltage to a higher DC voltage, typically using a network of capacitors and diodes.

In computer engineering, a logic family may refer to one of two related concepts. A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compatible logic levels and power supply characteristics within a family. Many logic families were produced as individual components, each containing one or a few related basic logical functions, which could be used as "building-blocks" to create systems or as so-called "glue" to interconnect more complex integrated circuits. A "logic family" may also refer to a set of techniques used to implement logic within VLSI integrated circuits such as central processors, memories, or other complex functions. Some such logic families use static techniques to minimize design complexity. Other such logic families, such as domino logic, use clocked dynamic techniques to minimize size, power consumption and delay.

The CPU core voltage (VCORE) is the power supply voltage supplied to the CPU, GPU, or other device containing a processing core. The amount of power a CPU uses, and thus the amount of heat it dissipates, is the product of this voltage and the current it draws. In modern CPUs, which are CMOS circuits, the current is almost proportional to the clock speed, the CPU drawing almost no current between clock cycles.

In integrated circuits, depletion-load NMOS is a form of digital logic family that uses only a single power supply voltage, unlike earlier nMOS logic families that needed more than one different power supply voltage. Although manufacturing these integrated circuits required additional processing steps, improved switching speed and the elimination of the extra power supply made this logic family the preferred choice for many microprocessors and other logic elements.

Power optimization is the use of electronic design automation tools to optimize (reduce) the power consumption of a digital design, such as that of an integrated circuit, while preserving the functionality.

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 integrated circuits, electrical power is distributed to the components of the chip over a network of conductors on the chip. Power network design includes the analysis and design of such networks. As in all engineering, this involves tradeoffs - the network must have adequate performance, be sufficiently reliable, but should not use more resources than required.

Dynamic frequency scaling is a technique in computer architecture whereby the frequency of a microprocessor can be automatically adjusted "on the fly" depending on the actual needs, to conserve power and reduce the amount of heat generated by the chip. Dynamic frequency scaling helps preserve battery on mobile devices and decrease cooling cost and noise on quiet computing settings, or can be useful as a security measure for overheated systems. Dynamic frequency scaling is used in all ranges of computing systems, ranging from mobile systems to data centers to reduce the power at the times of low workload.

Low-power electronics are electronics, such as notebook processors, that have been designed to use less electric power.

Power management integrated circuits are integrated circuits for power management. Although PMIC refers to a wide range of chips, most include several DC/DC converters or their control part. A PMIC is often included in battery-operated devices such as mobile phones and portable media players to decrease the amount of space required.

In electronics, pass transistor logic (PTL) describes several logic families used in the design of integrated circuits. It reduces the count of transistors used to make different logic gates, by eliminating redundant transistors. Transistors are used as switches to pass logic levels between nodes of a circuit, instead of as switches connected directly to supply voltages. This reduces the number of active devices, but has the disadvantage that the difference of the voltage between high and low logic levels decreases at each stage. Each transistor in series is less saturated at its output than at its input. If several devices are chained in series in a logic path, a conventionally constructed gate may be required to restore the signal voltage to the full value. By contrast, conventional CMOS logic switches transistors so the output connects to one of the power supply rails, so logic voltage levels in a sequential chain do not decrease. Simulation of circuits may be required to ensure adequate performance.

Glitch removal is the elimination of glitches—unnecessary signal transitions without functionality—from electronic circuits. Power dissipation of a gate occurs in two ways: static power dissipation and dynamic power dissipation. Glitch power comes under dynamic dissipation in the circuit and is directly proportional to switching activity. Glitch power dissipation is 20%-70% of total power dissipation and hence glitching should be eliminated for low power design.

Adaptive Voltage Scaling (AVS) is a closed-loop dynamic power minimization technique that adjusts the voltage supplied to a computer chip to match the chip's power needs during operation. Many computer chips, especially those in mobile devices or Internet of things devices are constrained by the power available and face varying workloads. In other situations a chip may be constrained by the amount of heat it is allowed to generate. In addition, individual chips can vary in their efficiency due to many factors, including minor differences in manufacturing conditions. AVS allows the voltage supplied to the chip, and therefore its power consumption, to be continuously adjusted to be appropriate to the workload and the parameters of the specific chip. This is accomplished by integrating a device that monitors the performance of the chip into the chip, which then provides information to a power controller.

## References

1. S. Mittal, "A survey of techniques for improving energy efficiency in embedded computing systems", IJCAET, 6(4), 440–459, 2014.
2. J. M. Rabaey. Digital Integrated Circuits. Prentice Hall, 1996.
3. Wonyoung Kim, Meeta S. Gupta, Gu-Yeon Wei and David Brooks. "System Level Analysis of Fast, Per-Core DVFS using On-Chip Switching Regulators". 2008.
4. Mike Chin. "Asus EN9600GT Silent Edition Graphics Card". Silent PC Review. p. 5. Retrieved 2008-04-21.
5. MIke Chin. "80 Plus expands podium for Bronze, Silver & Gold". Silent PC Review. Retrieved 2008-04-21.