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Real time clock with seconds5/27/2023 When the "time" variable exceeds a constant, usually a power of two, the nominal, calculated clock time (say, for 1/50 of a second) is subtracted from "time", and the clock's timing-chain software is invoked to count fractions of seconds, seconds, etc. However, it uses math to adjust the timing chain for accuracy: 50 Hz, to mimic a historic RTC (see below). The hardware timer can produce a periodic interrupt, e.g. Software can do the math to make these into accurate RTCs. These have inaccurate absolute timing (more than 100 parts per million) that is yet very repeatable (often less than 1 ppm). Most computers have one or more hardware timers that use timing signals from quartz crystals or ceramic resonators. The following system is well-known to embedded systems programmers, who sometimes must construct RTCs in systems that lack them. Computers used as local time servers occasionally use GPS or ultra-low frequency radio transmissions broadcast by a national standards organization (i.e. If an internet radio is available, a computer may use the network time protocol. LTE) directly provide the current local time. There are two common methods: Most cell phone protocols (e.g. Some modern computers receive clock information by digital radio and use it to promote time-standards. Some microcontrollers have a real-time clock built in, generally only the ones with many other features and peripherals. In newer computer systems, the RTC is integrated into the southbridge chip. Later, Dallas Semiconductor made compatible RTCs, which were often used in older personal computers, and are easily found on motherboards because of their distinctive black battery cap and silkscreened logo. The RTC was introduced to PC compatibles by the IBM PC/AT in 1984, which used a Motorola MC146818 RTC. A common RTC used in single-board computers is the Maxim Integrated DS1307. Many integrated circuit manufacturers make RTCs, including Epson, Intersil, IDT, Maxim, NXP Semiconductors, Texas Instruments, STMicroelectronics and Ricoh. <1 μW), they keep time within 50 parts per trillion ( 5 ×10 −11). Although vastly more expensive and power-hungry (120 mW vs. In 2011, chip-scale atomic clocks became available. In practical terms, this is good enough to perform celestial navigation, the classic task of a chronometer. Typical crystal RTC accuracy specifications are from ☑00 to ☒0 parts per million (8.6 to 1.7 seconds per day), but temperature-compensated RTC ICs are available accurate to less than 5 parts per million. So, these compensate for temperature changes using an electronic thermometer and electronic logic. Micromechanical resonators are much more sensitive to temperature than quartz resonators. This reduces the size and cost of an RTC by reducing its parts count. Some RTCs use a micromechanical resonator on the silicon chip of the RTC. The quartz tuning fork of these crystals does not change size much from temperature, so temperature does not change its frequency much. The low frequency saves power, while remaining above human hearing range. Being exactly 2 15 cycles per second, it is a convenient rate to use with simple binary counter circuits. The crystal frequency is usually 32.768 kHz, the same frequency used in quartz clocks and watches. Most RTCs use a crystal oscillator, but some have the option of using the power line frequency. The alternate power source can also supply power to battery backed RAM. This alternate source of power is normally a lithium battery in older systems, but some newer systems use a supercapacitor, because they are rechargeable and can be soldered. RTCs often have an alternate source of power, so they can continue to keep time while the primary source of power is off or unavailable. Lithium battery inside the real-time clock IC The real time clock is omitted either out of the desire to save money (as in the Raspberry Pi system architecture) or because real time clocks may not be needed at all (as in the Arduino system architecture ). Some motherboards are made without real time clocks. If it has been less than a few hours, then the previous ephemeris is still usable.
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