Real-time clock

A real-time clock (RTC) is a computer clock (most often in the form of an integrated circuit) that keeps track of the current time. A real-time clock (RTC) is a computer clock (most often in the form of an integrated circuit) that keeps track of the current time. Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time. 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. RTC should not be confused with real-time computing, which shares its three-letter acronym but does not directly relate to time of day. Although keeping time can be done without an RTC, using one has benefits: A 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. If it has been less than a few hours, then the previous ephemeris is still usable. 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. 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. The alternate power source can also supply power to battery backed RAM. Most RTCs use a crystal oscillator, but some have the option of using the power line frequency. In many cases, the oscillator's frequency is 32.768 kHz. This is the same frequency used in quartz clocks and watches. This frequency is exactly 215 cycles per second. It is a convenient rate to use with simple binary counter circuits. Also, it is too high for humans to hear. The quartz tuning fork of these crystals does not change size much from temperature, so temperature does not change its frequency much. Some RTCs use a micromechanical resonator on the silicon chip of the RTC. This reduces the cost of an RTC by reducing its complexity and parts count. Micromechanical resonators are much more sensitive to temperature than quartz resonators. So, these compensate for temperature changes using an electronic thermometer and electronic logic. Many commercial RTC ICs are accurate to less than 5 parts per million. In practical terms, this is good enough to perform celestial navigation, the classic task of a chronometer. In 2011, Chip-scale atomic clocks were invented. Although more expensive, they keep time within 50 picoseconds (5x10^-11).