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Low-power electronics

Low-power electronics

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Low-power electronics means that the consumption of electric power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

 is deliberately low, e.g. notebook processor
Notebook processor
A notebook processor is a CPU optimized for notebook computers.One of the main characteristics differentiating notebook processors from other CPUs is low-power consumption....


Computing elements

The density and speed of integrated-circuit computing elements have increased exponentially for several decades, following a trend described by Moore's Law
Moore's Law
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....

. 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 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...

A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

 channel length of 6.3 nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

s using conventional semiconductor materials, and devices have been built that used carbon nanotube
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...

s as MOSFET gates, giving a channel length of approximately one nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

. 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 in spite of the fact that the energy consumed by a single CMOS logic gate to change state has fallen exponentially with the Moore's law shrinking of process feature size.
An integrated-circuit chip contains many capacitive
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

 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 capacitances, which involves a change in the amount of stored energy. As the capacitive loads are charged and discharged through resistive
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...

 devices, an amount of energy comparable to that stored in the capacitor is dissipated as heat:

The effect of heat dissipation on state change is to limit the amount of computation that may be performed on 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
Dynamic logic (digital logic)
In integrated circuit design, dynamic logic is a design methodology in combinatorial 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...

, for example – require a minimum clock rate in order to function properly, wasting "dynamic power" even when it has nothing to do. Other circuits – most famously, the RCA 1802
RCA 1802
The RCA CDP1802, also known as the COSMAC , is an 8-bit CMOS microprocessor introduced by RCA in early 1976. It is being by Intersil Corporation as a high-reliability microprocessor...

, but also many later chips such as the WDC 65C02
WDC 65C02
The Western Design Center WDC 65C02 microprocessor is an upgraded CMOS version of the popular NMOS-based MOS Technology 6502 8-bit CPU — the CMOS redesign being made by Bill Mensch of the Western Design Center...

, the 80C85, the Freescale 68HC11
Freescale 68HC11
The 68HC11 is an 8-bit microcontroller family introduced by Motorola in 1985. Now produced by Freescale Semiconductor, it descended from the Motorola 6800 microprocessor. It is a CISC microcontroller...

 and some other CMOS
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...

 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 circuits shrink, subthreshold leakage
Subthreshold leakage
The Subthreshold conduction or the subthreshold leakage or the subthreshold drain current is the current that flows 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 below the threshold voltage. The...

 current becomes more important. This leakage current results in power consumption, even when no switching is taking place (static power consumption); with modern chips, this current is frequently more than 50% of power used by the IC.

Reducing power loss

Loss from subthreshold leakage
Subthreshold leakage
The Subthreshold conduction or the subthreshold leakage or the subthreshold drain current is the current that flows 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 below the threshold voltage. The...

 can be reduced by raising the threshold voltage
Threshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...

 and lowering the supply voltage. Both these changes slow the circuit down significantly, and some modern low-power circuits use dual supply voltages to provide speed on critical parts of the circuit and lower power 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 used to reduce static power consumption is power gating
Power gating
This technique uses high Vt sleep transistors which cut off VDD from a circuit block when the block is not switching. The sleep transistor sizing is an important design parameter. This technique, also known as MTCMOS, or Multi-Threshold CMOS reduces stand-by or leakage power, and also enables Iddq...

: the use of sleep transistors to disable entire blocks when not in use. Systems which 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; power consumption is a key design factor. 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 exist to lowering the power cost of state changes. One is to reduce the operating voltage of the circuit, as in a dual-voltage CPU
Dual-voltage CPU
A dual-voltage CPU uses a split-rail design to allow lower voltages to be used in the processor core while the external Input/Output voltages remain 3.3 volts for backwards compatibility....

, 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
Low-voltage differential signaling, or LVDS, is an electrical digital signaling system that can run at very high speeds over inexpensive twisted-pair copper cables. It was introduced in 1994, and has since become very popular in computers, where it forms part of very high-speed networks and...

, 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), 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
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

 external temperature (about 4 kT, where T is the device's internal temperature in kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s and k is the Boltzmann constant).

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 circuit
Adiabatic circuits are low power circuits which use "reversible logic" to conserve energy. Unlike traditional CMOS circuits, which dissipate energy during switching, adiabatic circuits attempt to conserve charge by following two key rules:...

s. The charge is supplied either from a variable-voltage inductive
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...

 power supply, or by other elements in a reversible-logic
Toffoli gate
In computer science, the Toffoli gate , invented by Tommaso Toffoli, is a universal reversible logic gate, which means that any reversible circuit can be constructed from Toffoli gates...

 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
RC time constant
In an RC circuit, the value of the time constant is equal to the product of the circuit resistance and the circuit capacitance , i.e. \tau = R × C. It is the time required to charge the capacitor, through the resistor, to 63.2 percent of full charge; or to discharge it to 36.8 percent of its...

 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, they have been difficult to use to reduce computation power substantially in practical circuits.

Finally, there are several techniques used to reduce the number of state changes associated with a given computation. For clocked- logic circuits the technique of clock gating
Clock gating
Clock gating is a power-saving technique used in many synchronous circuits-Description:Clock gating is a popular technique used in many synchronous circuits for reducing dynamic power dissipation. Clock gating saves power by adding more logic to a circuit to prune the clock tree...

 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 varying 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 is the application level throughput, i.e. the number of useful information bits, delivered by the network to a certain destination, per unit of time. The amount of data considered excludes protocol overhead bits as well as retransmitted data packets...

Some wireless mesh network
Wireless mesh network
A wireless mesh network is a communications network made up of radio nodes organized in a mesh topology. Wireless mesh networks often consist of mesh clients, mesh routers and gateways.The mesh clients are often laptops, cell phones and other wireless devices while the mesh routers forward traffic...

s 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.


If current trends continue, "Energy costs, now about 10% of the average IT budget, could 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 some instant.
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
CPU power dissipation
Central processing unit power dissipation or CPU power dissipation is the process in which central processing units consume electrical energy, and dissipate this energy both by the action of the switching devices contained in the CPU and by the energy lost in the form of heat due to the impedance...

 at the maximum frequency, maximum workload, and worst-case environment.
To reduce weight and cost, many laptop computers systems choose to use a much lighter, lower-cost cooling system designed around a much lower Thermal Design Power
Thermal Design Power
The thermal design power , sometimes called thermal design point, refers to the maximum amount of power the cooling system in a computer is required to dissipate. For example, a laptop's CPU cooling system may be designed for a 20 watt TDP, which means that it can dissipate up to 20 watts of heat...

, 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.


  • Transmeta
    Transmeta Corporation was a US-based corporation that licensed low power semiconductor intellectual property. Transmeta originally produced very long instruction word code morphing microprocessors, with a focus on reducing power consumption in electronic devices. It was founded in 1995 by Bob...

  • Acorn RISC Machine
    ARM architecture
    ARM is a 32-bit reduced instruction set computer instruction set architecture developed by ARM Holdings. It was named the Advanced RISC Machine, and before that, the Acorn RISC Machine. The ARM architecture is the most widely used 32-bit ISA in numbers produced...

  • AMULET microprocessor
    AMULET microprocessor
    AMULET is a series of microprocessors that implement the ARM processor architecture. Developed by the group under the University of Manchester's computer science school , AMULET is unique from other ARM implementations in being an asynchronous microprocessor, not making use of a square wave clock...

  • Microchip nanoWatt XLP Microcontrollers
    Microchip Technology
    Microchip Technology is an American manufacturer of microcontroller, memory and analog semiconductors. Its products include microcontrollers , Serial EEPROM devices, Serial SRAM devices, KEELOQ devices, radio frequency devices, thermal, power and battery management analog devices, as well as...

  • TI's MSP430 Microcontrollers
  • EFM32 Microcontrollers
    The EFM32 is a family of 32-bit RISC Flash microcontrollers designed by Energy Micro AS based on ARM Cortex-M0 or M3 processor core. EFM32 is a mixed-signal microcontroller focusing on supporting ultra low power battery operated solutions...

See also

  • CPU power dissipation
    CPU power dissipation
    Central processing unit power dissipation or CPU power dissipation is the process in which central processing units consume electrical energy, and dissipate this energy both by the action of the switching devices contained in the CPU and by the energy lost in the form of heat due to the impedance...

  • Cool Chips (symposium)
    Cool Chips (symposium)
    Cool Chips is an international symposium which is held every year since 1998. The symposium is sponsored by Technical Committees on Microprocessors and Microcomputers and Computer Architecture of the IEEE Computer Society. The focus of the symposium is on presentation and discussion of advancements...

  • Common Power Format
    Common Power Format
    The Si2 Common Power Format, or CPF is a file format for specifying power-saving techniques early in the design process. In the design of integrated circuits, saving power is a primary goal, and designers are forced to use sophisticated techniques such as clock gating, multi-voltage logic, and...

  • Performance per watt
    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 consumed....

  • Power management
    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...

  • Green computing
    Green computing
    Green computing or green IT, refers to environmentally sustainable computing or IT. In the article Harnessing Green IT: Principles and Practices, San Murugesan defines the field of green computing as "the study and practice of designing, manufacturing, using, and disposing of computers, servers,...

  • Ultra Low Power
    Ultra Low Power
    An ultra low power, or ULP device, is an electronic gadget that has milli- or micro-watt power consumption.Some examples of ultra-low power devices:* Pacemakers* Hearing aids-See also:* low-power electronics* Ultra-Low Power Bluetooth*...

  • Dynamic frequency scaling
    Dynamic 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...

  • 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...

  • Underclocking
    Underclocking, also known as downclocking, is the practice of modifying a synchronous circuit's timing settings to run at a lower clock rate than it was specified to operate at. It may be said to be the computer equivalent of driving a car below the speed limit...

  • Dynamic voltage scaling
    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; dynamic voltage scaling to decrease voltage is known...

  • Overvolting
  • Undervolting

External links