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Low-power electronics are electronics, such as
notebook processor A notebook processor is a CPU optimized for laptops. One of the main characteristics differentiating notebook processors from other CPUs is low-power consumption, however, they are not without tradeoffs; they also tend to not perform as well as t ...
s, that have been designed to use less
electric power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions ...
than usual, often at some expense. In the case of notebook processors, this expense is processing power; notebook processors usually consume less power than their desktop counterparts, at the expense of lower processing power.


History


Watches

The earliest attempts to reduce the amount of power required by an electronic device were related to the development of the
wristwatch A watch is a portable timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attached ...
. Electronic watches require electricity as a power source, and some mechanical movements and hybrid electromechanical movements also require electricity. Usually, the electricity is provided by a replaceable
battery Battery most often refers to: * Electric battery, a device that provides electrical power * Battery (crime), a crime involving unlawful physical contact Battery may also refer to: Energy source *Automotive battery, a device to provide power t ...
. The first use of electrical power in watches was as a substitute for the mainspring, to remove the need for winding. The first electrically powered watch, the
Hamilton Electric 500 The Hamilton Watch Company is a Swiss manufacturer of wristwatches based in Bienne, Switzerland. Founded in 1892 as an American firm, the Hamilton Watch Company ended American manufacture in 1969. Through a series of mergers and acquisitions, th ...
, was released in 1957 by the Hamilton Watch Company of
Lancaster, Pennsylvania Lancaster, ( ; pdc, Lengeschder) is a city in and the county seat of Lancaster County, Pennsylvania. It is one of the oldest inland cities in the United States. With a population at the 2020 census of 58,039, it ranks 11th in population amon ...
. The first quartz wristwatches were manufactured in 1976, using analog hands to display the time. Eric A. Vittoz
"The Electronic Watch and Low-Power Circuits"
2008.
Watch batteries (strictly speaking cells, as a battery is composed of multiple cells) are specially designed for their purpose. They are very small and provide tiny amounts of power continuously for very long periods (several years or more). In some cases, replacing the battery requires a trip to a watch repair shop or watch dealer. Rechargeable batteries are used in some
solar-powered watch A solar-powered watch or light-powered watch is a watch that is powered entirely or partly by a solar cell. History A model produced from 1978 by the Riehl Time Corporation was simply described as running on solar power, but having "silicon powe ...
es. The first digital ''electronic'' watch was a
Pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
LED prototype produced in 1970. Digital LED watches were very expensive and out of reach to the common consumer until 1975, when
Texas Instruments Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globa ...
started to mass-produce LED watches inside a plastic case. Most watches with LED displays required that the user press a button to see the time displayed for a few seconds because LEDs used so much power that they could not be kept operating continuously. Watches with LED displays were popular for a few years, but soon the LED displays were superseded by
liquid crystal display A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly but ...
s (LCDs), which used less battery power and were much more convenient in use, with the display always visible and no need to push a button before seeing the time. Only in darkness, you had to press a button to light the display with a tiny light bulb, later illuminating LEDs. Most electronic watches today use 32 kHz quartz oscillators. As of 2013, processors specifically designed for wristwatches are the lowest-power processors manufactured today—often
4-bit In computer architecture, 4-bit integers, or other data units are those that are 4 bits wide. Also, 4-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers, or data buses of that si ...
, 32 kHz processors.


Mobile computing

When
personal computers A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tec ...
were first developed, power consumption was not an issue. With the development of portable computers however, the requirement to run a computer off a battery pack necessitated the search for a compromise between
computing power In computing, computer performance is the amount of useful work accomplished by a computer system. Outside of specific contexts, computer performance is estimated in terms of accuracy, efficiency and speed of executing computer program instructio ...
and power consumption. Originally most
processors A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...
ran both the core and I/O circuits at 5 volts, as in the Intel 8088 used by the first
Compaq Portable The Compaq Portable was an early portable computer which was one of the first IBM PC compatible systems. It was Compaq Computer Corporation's first product, to be followed by others in the Compaq Portable series and later Compaq Deskpro serie ...
. It was later reduced to 3.5, 3.3, and 2.5 volts to lower power consumption. For example, the Pentium P5 core voltage decreased from 5V in 1993, to 2.5V in 1997. With lower voltage comes lower overall power consumption, making a system less expensive to run on any existing battery technology and able to function for longer. This is crucially important for portable or mobile systems. The emphasis on battery operation has driven many of the advances in lowering processor voltage because this has a significant effect on battery life. The second major benefit is that with less voltage and therefore less power consumption, there will be less heat produced. Processors that run cooler can be packed into systems more tightly and will last longer. The third major benefit is that a processor running cooler on less power can be made to run faster. Lowering the voltage has been one of the key factors in allowing the clock rate of processors to go higher and higher.


Electronics


Computing elements

The density and speed of integrated-circuit computing elements has increased exponentially for several decades, following a trend described by
Moore's Law Moore's law is the observation that the number of transistors in a dense integrated circuit (IC) doubles about every two years. Moore's law is an observation and projection of a historical trend. Rather than a law of physics, it is an empi ...
. While it is generally accepted that this exponential improvement trend will end, it is unclear exactly how dense and fast integrated circuits will get by the time this point is reached. Working devices have been demonstrated which were fabricated with a
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
channel length of 6.3
nanometre 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
s using conventional semiconductor materials, and devices have been built that use
carbon nanotube A scanning tunneling microscopy image of a single-walled carbon nanotube Rotating single-walled zigzag carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers. ''Single-wall carbon na ...
s as MOSFET gates, giving a channel length of approximately one
nanometre 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
. The density and computing power of integrated circuits are limited primarily by power-dissipation concerns. The overall power consumption of a new personal computer has been increasing at about 22% growth per year. This increase in consumption comes even though the energy consumed by a single CMOS logic gate in order to change its state has fallen exponentially in accordance with Moore's law, by virtue of shrinkage. Paul DeMone. "The Incredible Shrinking CPU: Peril of Proliferating Power". 2004

An integrated-circuit chip contains many capacitor, capacitive loads, formed both intentionally (as with gate-to-channel capacitance) and unintentionally (between conductors which are near each other but not electrically connected). Changing the state of the circuit causes a change in the voltage across these
parasitic capacitance Parasitic capacitance is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. When two electrical conductors at different voltages ...
s, which involves a change in the amount of stored energy. As the capacitive loads are charged and discharged through resistive devices, an amount of energy comparable to that stored in the capacitor is dissipated as heat: : E_\mathrm = C U^2 The effect of heat dissipation on state change is to limit the amount of computation that may be performed within a given power budget. While device shrinkage can reduce some parasitic capacitances, the number of devices on an integrated circuit chip has increased more than enough to compensate for reduced capacitance in each individual device. Some circuits – dynamic logic, for example – require a minimum clock rate in order to function properly, wasting "dynamic power" even when they do not perform useful computations. Other circuits – most prominently, the RCA 1802, but also several later chips such as the WDC 65C02, the Intel 80C85, the
Freescale 68HC11 The 68HC11 (6811 or HC11 for short) is an 8-bit microcontroller (µC) family introduced by Motorola in 1984. Now produced by NXP Semiconductors, it descended from the Motorola 6800 microprocessor by way of the 6801. The 68HC11 devices are mo ...
and some other
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSF ...
chips – use "fully static logic" that has no minimum clock rate, but can "stop the clock" and hold their state indefinitely. When the clock is stopped, such circuits use no dynamic power but they still have a small, static power consumption caused by leakage current. As circuit dimensions shrink,
subthreshold leakage Subthreshold conduction or subthreshold leakage or subthreshold drain current is the current between the source and drain of a MOSFET when the transistor is in subthreshold region, or weak-inversion region, that is, for gate-to-source voltages ...
current becomes more prominent. This leakage current results in power consumption, even when no switching is taking place (static power consumption). In modern chips, this current generally accounts for half the power consumed by the IC.


Reducing power loss

Loss from
subthreshold leakage Subthreshold conduction or subthreshold leakage or subthreshold drain current is the current between the source and drain of a MOSFET when the transistor is in subthreshold region, or weak-inversion region, that is, for gate-to-source voltages ...
can be reduced by raising the
threshold voltage The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important s ...
and lowering the supply voltage. Both these changes slow down the circuit significantly. To address this issue, some modern low-power circuits use dual supply voltages to improve speed on critical paths of the circuit and lower power consumption on non-critical paths. Some circuits even use different transistors (with different threshold voltages) in different parts of the circuit, in an attempt to further reduce power consumption without significant performance loss. Another method that is used to reduce power consumption is
power gating Power gating is a technique used in integrated circuit design to reduce power consumption, by shutting off the current to blocks of the circuit that are not in use. In addition to reducing stand-by or leakage power, power gating has the benefit ...
: the use of sleep transistors to disable entire blocks when not in use. Systems that are dormant for long periods of time and "wake up" to perform a periodic activity are often in an isolated location monitoring an activity. These systems are generally battery- or solar-powered and hence, reducing power consumption is a key design issue for these systems. By shutting down a functional but leaky block until it is used, leakage current can be reduced significantly. For some embedded systems that only function for short periods at a time, this can dramatically reduce power consumption. Two other approaches also exist to lower the power overhead of state changes. One is to reduce the operating voltage of the circuit, as in a
dual-voltage CPU The CPU core voltage (''VCORE'') is the power supply voltage supplied to the CPU (which is a digital circuit), 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 prod ...
, or to reduce the voltage change involved in a state change (making a state change only, changing node voltage by a fraction of the supply voltage—
low voltage differential signaling Low-voltage differential signaling (LVDS), also known as TIA/EIA-644, is a technical standard that specifies electrical characteristics of a differential, serial signaling standard. LVDS operates at low power and can run at very high speeds ...
, for example). This approach is limited by thermal noise within the circuit. There is a characteristic voltage (proportional to the device temperature and to the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constan ...
), which the state switching voltage must exceed in order for the circuit to be resistant to noise. This is typically on the order of 50–100 mV, for devices rated to 100
degrees Celsius The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The d ...
external temperature (about 4 ''kT'', where ''T'' is the device's internal temperature in
Kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ...
s and ''k'' is the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constan ...
). The second approach is to attempt to provide charge to the capacitive loads through paths that are not primarily resistive. This is the principle behind
adiabatic circuit Adiabatic circuits are low-power electronic circuits which use " reversible logic" to conserve energy. The term " adiabatic" refers to an ideal thermodynamic process in which no heat or mass is exchanged with the surrounding environment, alludi ...
s. The charge is supplied either from a variable-voltage inductive power supply or by other elements in a reversible-logic circuit. In both cases, the charge transfer must be primarily regulated by the non-resistive load. As a practical rule of thumb, this means the change rate of a signal must be slower than that dictated by the RC time constant of the circuit being driven. In other words, the price of reduced power consumption per unit computation is a reduced absolute speed of computation. In practice, although adiabatic circuits have been built, it has been difficult for them to reduce computation power substantially in practical circuits. Finally, there are several techniques for reducing the number of state changes associated with a given computation. For clocked-logic circuits, the clock gating technique is used, to avoid changing the state of functional blocks that are not required for a given operation. As a more extreme alternative, the asynchronous logic approach implements circuits in such a way that a specific externally supplied clock is not required. While both of these techniques are used to different extents in integrated circuit design, the limit of practical applicability for each appears to have been reached.


Wireless communication elements

There are a variety of techniques for reducing the amount of battery power required for a desired wireless communication
goodput In computer networks, goodput (a portmanteau of good and throughput) is the application-level throughput of a communication; i.e. the number of useful information bits delivered by the network to a certain destination per unit of time. The amoun ...
. Some wireless mesh networks use "smart" low power broadcasting techniques that reduce the battery power required to transmit. This can be achieved by using power aware protocols and joint power control systems.


Costs

In 2007, about 10% of the average IT budget was spent on energy, and energy costs for IT were expected to rise to 50% by 2010. The weight and cost of power supply and cooling systems generally depends on the maximum possible power that could be used at any one time. There are two ways to prevent a system from being permanently damaged by excessive heat. Most desktop computers design power and cooling systems around the worst-case CPU power dissipation at the maximum frequency, maximum workload, and worst-case environment. To reduce weight and cost, many laptop computers choose to use a much lighter, lower-cost cooling system designed around a much lower Thermal Design Power, that is somewhat above expected maximum frequency, typical workload, and typical environment. Typically such systems reduce (throttle) the clock rate when the CPU die temperature gets too hot, reducing the power dissipated to a level that the cooling system can handle.


Examples

*
Transmeta Transmeta Corporation was an American fabless semiconductor company based in Santa Clara, California. It developed low power x86 compatible microprocessors based on a VLIW core and a software layer called Code Morphing Software. Code Morphing S ...
* Acorn RISC Machine (ARM) * AMULET microprocessor * Microchip nanoWatt XLP PIC microcontrollers * Texas Instruments MSP430 microcontrollers * Energy Micro/Silicon Labs EFM32 microcontrollers * STMicroelectronics STM32 microcontrollers * Atmel/Microchip SAM L microcontrollers * IoT Pixel


See also

* CPU power dissipation * Common Power Format *
Data organization for low power Power consumption in relation to physical size of electronic hardware has increased as the components have become smaller and more densely packed. Coupled with high operating frequencies, this has led to unacceptable levels of power dissipation. Mem ...
* IT energy management *
Performance per watt In computing, performance per watt is a measure of the energy efficiency of a particular computer architecture or computer hardware. Literally, it measures the rate of computation that can be delivered by a computer for every watt of power con ...
* Power management *
Green computing Green computing, green IT, or ICT sustainability, is the study and practice of environmentally sustainable computing or IT. The goals of green computing are similar to green chemistry: reduce the use of hazardous materials, maximize energy effic ...
* Dynamic frequency scaling * Overclocking *
Underclocking 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, red ...
*
Dynamic voltage scaling 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; ...
* Overvolting *
Undervolting 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; ...
*
Operand isolation In electronic low power digital synchronous circuit design, operand isolation is a technique for minimizing the energy overhead associated with redundant operations by selectively blocking the propagation of switching activity through the circuit.< ...
*
Glitch removal Glitch removal is the elimination of glitchesunnecessary signal transitions without functionalityfrom electronic circuits. Power dissipation of a gate occurs in two ways: static power dissipation and dynamic power dissipation. Glitch power comes un ...
* Autonomous peripheral operation


References


Further reading

*
draft version 2013-09-25 -->
(455 pages)


External links


"High-level design synthesis of a low power, VLIW processor for the IS-54 VSELP Speech Encoder"
by Russell Henning and Chaitali Chakrabarti (NB. Implies that, in general, if the algorithm to run is known, hardware designed to specifically run that algorithm will use less power than general-purpose hardware running that algorithm at the same speed.)
CRISP: A Scalable VLIW Processor for Low Power Multimedia Systems
by Francisco Barat 2005
A Loop Accelerator for Low Power Embedded VLIW Processors
by Binu Mathew and Al Davis

by Jack Ganssle * K. Roy and S. Prasad, ''Low-Power CMOS VLSI Circuit Design'', John Wiley & Sons, Inc., , 2000, 359 pages. * K-S. Yeo and K. Roy, ''Low-Voltage Low-Power VLSI Subsystems'', McGraw-Hill 2004, , 294 pages. {{Electronic systems Electric power Electronics and the environment