DS3231 USE AS TIMER PCThe 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. Sometimes more accurate than other methodsĪ GPS receiver can shorten its startup time by comparing the current time, according to its RTC, with the time at which it last had a valid signal.Frees the main system for time-critical tasks.Low power consumption (important when running from alternate power).RTC should not be confused with real-time computing, which shares its three-letter acronym but does not directly relate to time of day.Īlthough keeping time can be done without an RTC, using one has benefits: The term real-time clock is used to avoid confusion with ordinary hardware clocks which are only signals that govern digital electronics, and do not count time in human units.
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