# Low-power electronics

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

A notebook processor is a CPU optimized for notebook computers.

Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second.

## History

### Watches

The earliest attempts to reduce the amount of power required by an electronic device were related to the development of the wristwatch. Electronic watches require electricity as a power source, and some mechanical movements and hybrid electronic-mechanical movements also require electricity. Usually the electricity is provided by a replaceable battery. 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, was released in 1957 by the Hamilton Watch Company of Lancaster, Pennsylvania.

A mainspring is a spiral torsion spring of metal ribbon—commonly spring steel—used as a power source in mechanical watches, some clocks, and other clockwork mechanisms. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it unwinds, until the next winding is needed. The adjectives wind-up and spring-powered refer to mechanisms powered by mainsprings, which also include kitchen timers, music boxes, wind-up toys and clockwork radios.

The Hamilton Watch Company is a Swiss manufacturer of wristwatches based in Bienne, Switzerland. The Hamilton Watch Company had its genesis as an American watch design and manufacturing company, which incorporated in 1892 and produced its first watch in 1893.

Lancaster is a city located in South Central Pennsylvania which serves as the seat of Pennsylvania's Lancaster County and one of the oldest inland towns in the United States. With a population of 59,322, it ranks eighth in population among Pennsylvania's cities. The Lancaster metropolitan area population is 507,766, making it the 101st largest metropolitan area in the U.S. and second largest in the South Central Pennsylvania area.

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 watches.

A solar-powered watch or light-powered watch is a watch that is powered entirely or partly by a solar cell.

The first digital electronic watch, a Pulsar LED prototype in 1970. [1] Digital LED watches were very expensive and out of reach to the common consumer until 1975, when Texas Instruments started to mass-produce LED watches inside a plastic case.

Pulsar is a brand of watch and currently a division of Seiko Watch Corporation of America (SCA). Pulsar was the world's first electronic digital watch. Today Pulsar watches are mostly analog and use the same movements in Seikos such as the 7T62 quartz chronograph movement.

Texas Instruments Incorporated (TI) is an American technology company that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globally. Its headquarters are in Dallas, Texas, United States. TI is one of the top-10 semiconductor companies worldwide, based on sales volume. Texas Instruments's focus is on developing analog chips and embedded processors, which account for more than 80% of their revenue. TI also produces TI digital light processing technology and education technology products including calculators, microcontrollers and multi-core processors. To date, TI has more than 45,000 patents worldwide.

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 displays (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. [2]

As of 2013, processors specifically designed for wristwatches are the lowest-power processors manufactured today—often 4-bit, 32 kHz processors.

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.

In computer architecture, 4-bit integers, memory addresses, or other data units are those that are 4 bits wide. Also, 4-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. A group of four bits is also called a nibble and has 24 = 16 possible values.

### Mobile computing

When personal computers were first developed, power consumption was not an issue. Soon after though, development of portable computers started, and with it, the requirement to run a computer off a battery pack, setting off the search for a compromise between computing power and power consumption. Originally most processors ran both the core and I/O circuits at 5 volts, as in the Intel 8088 used by the first Compaq Portable. 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. By consuming less power, the system will be less expensive to run, but more importantly for portable or mobile systems, it will run much longer on existing battery technology. 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. [3]

## Electronics

### 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. 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 transistor channel length of 6.3 nanometres using conventional semiconductor materials, and devices have been built that used carbon nanotubes as MOSFET gates, giving a channel length of approximately one nanometre. 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 to change state has fallen exponentially with the Moore's law shrinking of process feature size. [4]

An integrated-circuit chip contains many 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 capacitances, 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:

${\displaystyle E_{\mathrm {stored} }={1 \over 2}CU^{2}}$

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 and some other CMOS 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 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 can be reduced by raising the threshold voltage and lowering the supply voltage. Both these changes slowdown 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. [5] 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 power consumption is power gating: [6] 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 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, 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, 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 external temperature (about 4 kT, where T is the device's internal temperature in kelvins 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 circuits. 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, they have been difficult to use 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, 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. [ citation needed ]

### Wireless communication elements

There are a variety of techniques for reducing the amount of battery power required for a desired wireless communication goodput. [7]

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

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 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, 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.

## Related Research Articles

Processor design is the design engineering task of creating a processor, a key component of computer hardware. It is a subfield of computer engineering and electronics engineering (fabrication). The design process involves choosing an instruction set and a certain execution paradigm and results in a microarchitecture, which might be described in e.g. VHDL or Verilog. For microprocessor design, this description is then manufactured employing some of the various semiconductor device fabrication processes, resulting in a die which is bonded onto a chip carrier. This chip carrier is then soldered onto, or inserted into a socket on, a printed circuit board (PCB).

The metal–oxide–semiconductor field-effect transistor, also known as the metal–oxide–silicon transistor (MOS), is a type of field-effect transistor (FET) that is fabricated by the controlled oxidation of silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. The MOSFET was invented by Mohamed Atalla and Dawon Kahng at Bell Labs in 1959, and is the most widely used semiconductor device in the world.

Complementary metal–oxide–semiconductor (CMOS) is a type of MOSFET semiconductor device fabrication process used for constructing integrated circuits (ICs). CMOS technology is used in microprocessors, microcontrollers, memory chips, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication. CMOS was invented by Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor in 1963, and was an adaptation of the PMOS and NMOS processes originally developed by MOSFET inventors Mohamed Atalla and Dawon Kahng in 1960. CMOS became the dominant MOSFET fabrication process for VLSI devices since it overtook NMOS in the 1980s.

Static random-access memory is a type of semiconductor random-access memory (RAM) that uses bistable latching circuitry (flip-flop) to store each bit. SRAM exhibits data remanence, but it is still volatile in the conventional sense that data is eventually lost when the memory is not powered.

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.

Power management is a feature of some electrical appliances, especially copiers, computers, 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 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.

Emission-aware programming is a design philosophy aiming to reduce the amount of electromagnetic radiation emitted by electronic devices through proper design of the software executed by the device, rather than changing the hardware.

Multi-threshold CMOS (MTCMOS) is a variation of CMOS chip technology which has transistors with multiple threshold voltages (Vth) in order to optimize delay or power. The Vth of a MOSFET is the gate voltage where an inversion layer forms at the interface between the insulating layer (oxide) and the substrate (body) of the transistor. Low Vth devices switch faster, and are therefore useful on critical delay paths to minimize clock periods. The penalty is that low Vth devices have substantially higher static leakage power. High Vth devices are used on non-critical paths to reduce static leakage power without incurring a delay penalty. Typical high Vth devices reduce static leakage by 10 times compared with low Vth devices.

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.

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, 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.

Adiabatic circuits are low power circuits which use "reversible logic" to conserve energy.

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.

A nanoelectromechanical (NEM) relay is an electrically actuatedswitch that is built on the nanometer scale using semiconductor fabrication techniques. They are designed to operate in replacement, or in conjunction, with traditional semiconductor logic. While the mechanical nature of NEM relays makes them switch much slower than solid-state relays, they have many advantageous properties, such as zero current leakage and low power consumption, which make them potentially useful in next generation computing.

Finite state machines (FSMs) are widely used to implement control logic in various applications such as microprocessors, digital transmission, digital filters and digital signal processing. Even for designs containing a good number of datapath elements, the controller occupies a sizeable portion. As the devices are mostly portable and hand-held, reducing power dissipation has emerged as the primary concern of today’s VLSI designers. While the datapath elements can be shut down when they are not being used, controllers are always active. As a result, the controller consumes a good amount of system power. Thus, power-efficient synthesis of FSM has come up as a very important problem domain, attracting a lot of research. The synthesis method must be able to reduce both dynamic power and leakage power consumed by the circuit.

## References

1. "All in Good Time: HILCO EC director donates prototype of world's first working digital watch to Smithsonian". Texas Co-op Power. Feb 2012. Retrieved 2012-07-21.
2. : W. Boller, M. Donati, J. Fingerle, P. Wild, Illuminating Arrangement for a Field-Effect Liquid-Crystal Display as well as Fabrication and Application of the Illuminating Arrangement, filed 15 October 1976.
3. Paul DeMone. "The Incredible Shrinking CPU: Peril of Proliferating Power". 2004.
4. "A Survey Of Architectural Techniques for Near-Threshold Computing", S. Mittal, ACM JETC, 2015
5. K. Roy, et. al., "Leakage current mechanisms and leakage reduction techniques in deep-submicrometer CMOS circuits", Proceedings of the IEEE, 2003.
6. "How to use optional wireless power-save protocols to dramatically reduce power consumption" by Bill McFarland 2008.