Low-power electronics

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. 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. The first digital electronic watch, a Pulsar LED prototype in 1970. 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. 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 Watches with LED displays were popular for a few years, but soon the LED displays were superseded by liquid crystal displays (LCDs), which used less battery power and were much more convenient in use, with the display always visible and no need to push a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. As of 2013, processors specifically designed for wristwatches are the lowest-power processors manufactured today—often 4-bit, 32 kHz processors. 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. 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.