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A real-time clock (RTC) is an electronic device (most often in the form of an integrated circuit) that measures the passage of 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 of day .
Clock synchronization is a topic in computer science and engineering that aims to coordinate otherwise independent clocks. Even when initially set accurately, real clocks will differ after some amount of time due to clock drift, caused by clocks counting time at slightly different rates. There are several problems that occur as a result of ...
Commercial rubidium clocks are less accurate than caesium atomic clocks, which serve as primary frequency standards, so a rubidium clock is usually used as a secondary frequency standard. Commercial rubidium frequency standards operate by disciplining a crystal oscillator to the rubidium hyperfine transition of 6.8 GHz ( 6 834 682 610 .904 Hz ).
A real time clock alarm is a feature that can be used to allow a computer to 'wake up' after shut down to execute tasks every day or on a certain day. It can sometimes be found in the 'Power Management' section of a motherboard's BIOS/UEFI setup. Wake On LAN, Wake on ring, and IPMI functions could also be used to start a computer after it is ...
In addition to clock skew due to static differences in the clock latency from the clock source to each clocked register, no clock signal is perfectly periodic, so that the clock period or clock cycle time varies even at a single component, and this variation is known as clock jitter. At a particular point in a clock distribution network, jitter ...
Digital clocks typically use the 50 or 60 hertz oscillation of AC power or a 32,768 hertz crystal oscillator as in a quartz clock to keep time. Most digital clocks display the hour of the day in 24-hour format; in the United States and a few other countries, a commonly used hour sequence option is 12-hour format (with some indication of AM or PM).
Clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to drift, causing eventual divergence unless resynchronized.
Real-time programs must guarantee response within specified time constraints, often referred to as "deadlines". [2] The term "real-time" is also used in simulation to mean that the simulation's clock runs at the same speed as a real clock. Real-time responses are often understood to be in the order of milliseconds, and sometimes microseconds.