View source for Real-time clock

{{Short description|Circuit in a computer that maintains accurate time}}
{{Distinguish|Real-time computing|Real-time communication}}
[[Image:Dallas-Semiconductor-DS1287-Real-Time-IC.jpg|thumb|[[Dallas Semiconductor]] DS1287 real-time clock manufactured in 1988]]
[[File:Types of RTC modules.jpg|thumb|Types of hobbyist RTC modules commercially available from China]]

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 computer]]s, [[server (computing)|server]]s and [[embedded system]]s, RTCs are present in almost any electronic device which needs to keep accurate [[time of day]].

==Terminology==
The term ''real-time clock'' is used to avoid confusion with ordinary [[clock signal|hardware clocks]] which are only [[signal]]s 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.

==Purpose==
Although keeping time can be done without an RTC,<ref name="pxjGa">{{cite web |first = Jaakko |last = Ala-Paavola |title = Software interrupt based real time clock source code project for PIC microcontroller |date = 2000-01-16 |access-date = 2007-08-23 |url = http://users.tkk.fi/~jalapaav/Electronics/Pic/Clock/index.html |url-status = dead |archive-url = https://web.archive.org/web/20070717134822/http://users.tkk.fi/~jalapaav/Electronics/Pic/Clock/index.html |archive-date = 2007-07-17}}</ref> using one has benefits:
* Reliably maintains and provides current time through disruptive system states such as [[Hang (computing)|hang]]s, [[Sleep mode|sleep]], [[reboot]]s, or if given sufficient backup power, full [[Shutdown (computing)|shutdown]] and hardware reassembly, without the need to have its time set again.
* Low power consumption<ref name="0xyzn">{{Citation | title = Enabling Timekeeping Function and Prolonging Battery Life in Low Power Systems | publisher = NXP Semiconductors | year =2011 | url =https://www.digikey.com/en/articles/techzone/2011/dec/enabling-timekeeping-function-and-prolonging-battery-life-in-low-power-systems}}</ref>  (important when running from alternate power)
* Frees the main system for time-critical tasks
* Sometimes more accurate than other methods

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.<ref name="R1b3k">{{Patent|US|5893044|Real time clock apparatus for fast acquisition or GPS signals}}</ref> If it has been less than a few hours, then the previous [[ephemeris]] is still usable.

Some motherboards are made without RTCs. The RTC may be omitted out of desire to save money or reduce possible sources of hardware failure.

==Power source==
[[File:Inside the Real-time clock IC vnutri u nei neonka.jpg|thumb|Lithium battery inside the real-time clock IC]]
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]],<ref name="sasjZ">{{Citation | title = New PCF2123 Real Time Clock Sets New Record in Power Efficiency | publisher = futurle | url = http://www.futurlec.com/News/Philips/PCF2123.shtml}}</ref><ref name="QmZme">{{Citation | title =Application Note 3816 | publisher =Maxim/Dallas Semiconductor | year =2006 | url =http://www.maxim-ic.com/appnotes.cfm/an_pk/3816}}</ref> because they are rechargeable and can be [[soldering|soldered]]. The alternate power source can also supply power to [[Nonvolatile BIOS memory|battery backed RAM]].<ref name="CMOS Battery">{{cite web |last = Torres |first = Gabriel |title = Introduction and Lithium Battery |url = http://www.hardwaresecrets.com/article/81 |work = Replacing the Motherboard Battery |publisher = hardwaresecrets.com |access-date = June 20, 2013 |date = 24 November 2004 |url-status = dead |archive-url = https://web.archive.org/web/20131224085334/http://www.hardwaresecrets.com/article/81 |archive-date = 24 December 2013}}</ref>

==Timing==
Most RTCs use a [[crystal oscillator]],<ref name="ST_AN10337">{{Citation | title =Application Note 10337 | publisher =ST Microelectronics | year =2004 | pages = 2 }}</ref><ref name="xm0Sw">{{Citation | title =Application Note U-502 | publisher =Texas Instruments | year =2004 | pages =13 | url =http://focus.ti.com/analog/docs/techdocsabstract.tsp?familyId=475&abstractName=slua051 | access-date =2007-08-24 | archive-date =2009-05-05 | archive-url =https://web.archive.org/web/20090505211057/http://focus.ti.com/analog/docs/techdocsabstract.tsp?familyId=475&abstractName=slua051 | url-status =dead }}</ref> but some have the option of using the [[power line frequency]].<ref name="nzx32">{{Citation | title =Application Note 1994 | publisher =Maxim/Dallas Semiconductor | year =2003 | url =http://www.maxim-ic.com/appnotes.cfm/an_pk/1994}}</ref> The crystal frequency is usually 32.768&nbsp;kHz,<ref name="ST_AN10337" /> the same frequency used in [[Quartz clock|quartz clocks and watches]]. Being exactly 2<sup>15</sup> cycles per second, it is a convenient rate to use with simple binary counter circuits. The low frequency saves power, while remaining above human [[hearing range]].  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 [[Microelectromechanical systems|micromechanical]] resonator on the silicon chip of the RTC. This reduces the size and cost of an RTC by reducing its 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.<ref name="l7w4T">{{cite web |title=Maxim DS3231m |url=https://datasheets.maximintegrated.com/en/ds/DS3231M.pdf |publisher=Maxim Inc. |access-date=26 March 2019}}</ref>

Typical crystal RTC accuracy specifications are from ±100 to ±20&nbsp;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.<ref name="a3SFx">{{cite web|title=Highly Accurate Real-Time Clocks|url=https://www.maximintegrated.com/en/products/digital/real-time-clocks/highly-accurate-real-time-clocks.html|publisher=Maxim Semiconductors|access-date=20 October 2017|archive-date=3 July 2017|archive-url=https://web.archive.org/web/20170703064332/https://www.maximintegrated.com/en/products/digital/real-time-clocks/highly-accurate-real-time-clocks.html|url-status=dead}}</ref><ref name="wm4pV">{{cite web |title=RTC comparison |date=3 February 2017 |first=Dan |last=Drown |url=https://blog.dan.drown.org/rtc-comparison/}}</ref> In practical terms, this is good enough to perform [[celestial navigation]], the classic task of a [[Marine chronometer|chronometer]]. In 2011, [[chip-scale atomic clock]]s became available. Although vastly more expensive and power-hungry (120&nbsp;mW vs. <1 μW), they keep time within 50 parts per trillion ({{val|5|e=-11}}).<ref name="yYcMJ">{{cite web|title=Chip Scale Atomic Clock|url=https://www.microsemi.com/product-directory/clocks-frequency-references/3824-chip-scale-atomic-clock-csac|website=Microsemi|access-date=20 October 2017|archive-date=8 October 2018|archive-url=https://web.archive.org/web/20181008214109/https://www.microsemi.com/product-directory/clocks-frequency-references/3824-chip-scale-atomic-clock-csac|url-status=dead}}</ref>

==Examples==
[[File:Realtimeclock Motherboard Baby AT crop.jpg|thumb|right|[[Dallas Semiconductor]] (DS1387) real-time clock from an older PC. This version also contains a battery-backed SRAM.]]
[[File:DS1307 1438A4 +102BN.jpeg|thumb|right|Dallas DS1307 RTC chip in DIP-8 package]]

Many [[integrated circuit]] manufacturers make RTCs, including [[Epson]], [[Intersil]], [[Integrated Device Technology|IDT]], [[Maxim Integrated Products|Maxim]], [[NXP Semiconductors]], [[Texas Instruments]], [[STMicroelectronics]] and [[Ricoh]]. A common RTC used in [[single-board computer]]s is the [[Maxim Integrated]] DS1307.

The RTC was introduced to PC compatibles by the [[IBM PC/AT]] in 1984, which used a [[Motorola]] MC146818 RTC.<ref name="JPtT3">{{cite book | title = IBM PC AT Technical Reference | publisher = International Business Machines Corporation | year = 1984 | chapter = Real-time Clock/Complementary Metal Oxide Semiconductor (RT/CMOS) RAM Information | page = System Board 1–45 | url = http://bitsavers.trailing-edge.com/pdf/ibm/pc/at/1502494_PC_AT_Technical_Reference_Mar84.pdf}}</ref><ref name="KWwl1">{{cite book | title = MC146818A REAL-TIME CLOCK PLUS RAM (RTC) | publisher = Motorola Inc. | year = 1984 | url = https://web.stanford.edu/class/cs140/projects/pintos/specs/mc146818a.pdf}}</ref> Later, [[Dallas Semiconductor]] made compatible RTCs, which were often used in older [[personal computer]]s, and are easily found on [[motherboard]]s because of their distinctive black battery cap and [[silkscreen]]ed logo. A standard CMOS interface is available for the PC RTC.<ref>{{cite web |title=CMOS RTC - Real Time Clock and Memory (ports 70h & 71h) :: HelpPC 2.10 - Quick Reference Utility :: NetCore2K.net |url=https://helppc.netcore2k.net/hardware/rtc |website=helppc.netcore2k.net}}</ref>

In newer computer systems, the RTC is integrated into the [[Southbridge (computing)|southbridge]] chip.<ref name="hASAE">{{cite web | title = ULi M1573 Southbridge Specifications | publisher = AMDboard.com | url = http://www.amdboard.com/ali_uli_m1573.html | access-date = 2007-08-23 | archive-date = 2015-08-11 | archive-url = https://web.archive.org/web/20150811072959/http://www.amdboard.com/ali_uli_m1573.html | url-status = dead }}</ref><ref>[https://www.intel.com/Assets/PDF/datasheet/290562.pdf 82430FX PCISET Data Set]</ref>

Some [[microcontroller]]s have a real-time clock built in, generally only the ones with many other features and [[peripheral]]s.

===Radio-based RTCs===
Some modern computers receive clock information by digital radio and use it to promote time-standards. There are two common methods: Most cell phone protocols (e.g. [[LTE (telecommunication)|LTE]]) directly provide the current local time.  If an internet radio is available, a computer may use the [[network time protocol]]. Computers used as local time servers occasionally use [[GPS]]<ref name="nhueQ">{{cite web|title=GPS Clock Synchronization|date=9 December 2020|url=https://safran-navigation-timing.com/guide-to-gps-gnss-clock-synchronization/|publisher=Safran (formerly Orolia)|access-date=6 January 2021}}</ref> or ultra-low frequency radio transmissions broadcast by a national standards organization (i.e. a [[radio clock]]<ref name="LZ0mo">{{cite web|title=Product: USB Radio Clock|url=https://www.meinbergglobal.com/english/products/usb-wwvb-clock.htm|publisher=Meinburg|access-date=20 October 2017|archive-date=12 November 2020|archive-url=https://web.archive.org/web/20201112035820/https://www.meinbergglobal.com/english/products/usb-wwvb-clock.htm|url-status=dead}}</ref>).

===Software-based RTCs===
{{mos|section|date=May 2025}}
The following system is well-known to [[embedded system]]s programmers, who sometimes must construct RTCs in systems that lack them. Most computers have one or more hardware timers that use timing signals from [[quartz crystal]]s or [[ceramic resonator]]s. These have inaccurate absolute timing (more than 100 parts per million) that is yet very repeatable (often less than 1&nbsp;ppm). Software can do the math to make these into accurate RTCs. The hardware timer can produce a periodic interrupt, e.g. 50&nbsp;[[Hz]], to mimic a historic RTC (see below).  However, it uses math to adjust the timing chain for accuracy:

time = time + rate.

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. With 32-[[bit]] variables for time and rate, the mathematical resolution of "rate" can exceed one part per billion. The clock remains accurate because it will occasionally skip a fraction of a second, or increment by two fractions. The tiny skip ("[[jitter]]") is imperceptible for almost all real uses of an RTC.

The complexity with this system is determining the instantaneous corrected value for the variable "rate".  The simplest system tracks RTC time and reference time between two settings of the clock, and divides reference time by RTC time to find "rate". Internet time is often accurate to less than 20 milliseconds, so 8000 or more seconds (2.2 or more hours) of separation between settings can usually divide the forty milliseconds (or less) of error to less than 5 parts per million to get chronometer-like accuracy. The main complexity with this system is converting dates and times to counts of seconds, but methods are well known.<ref name="FQjlV">{{cite web |title=Calendrical Applications |url=http://aa.usno.navy.mil/faq/docs/JD_Formula.php |website=U.S. Naval Observatory |publisher=U.S. Navy |archive-url=https://web.archive.org/web/20160404103428/http://aa.usno.navy.mil/faq/docs/JD_Formula.php |access-date=7 November 2019|archive-date=2016-04-04}}</ref>

If the RTC runs when a unit is off, usually the RTC will run at two rates, one when the unit is on and another when off. This is because the temperature and power-supply voltage in each state is consistent. To adjust for these states, the software calculates two rates. First, software records the RTC time, reference time, on seconds and off seconds for the two intervals between the last three times that the clock is set. Using this, it can measure the accuracy of the two intervals, with each interval having a different distribution of on and off seconds. The rate math solves two [[linear equation]]s to calculate two rates, one for on and the other for off.

Another approach measures the temperature of the oscillator with an electronic thermometer, (e.g. a [[thermistor]] and [[analog-to-digital converter]]) and uses a [[polynomial]] to calculate "rate" about once per minute. These require a calibration that measures the frequency at several temperatures, and then a [[linear regression]] to find the equation of temperature. The most common quartz crystals in a system are SC-cut crystals, and their rates over temperature can be characterized with a 3rd-degree polynomial. So, to calibrate these, the frequency is measured at four temperatures. The common tuning-fork-style crystals used in watches and many RTC components have parabolic (2nd-degree) equations of temperature, and can be calibrated with only 3 measurements. [[MEMS]] oscillators vary, from 3rd degree to fifth degree polynomials, depending on their mechanical design, and so need from four to six calibration measurements. Something like this approach might be used in commercial RTC ICs, but the actual methods of efficient high-speed manufacturing are proprietary.

===Historic RTCs===
Some [[computer]] designs such as the smaller [[IBM System/360]],<ref name=A22-6821-7>{{cite manual
 | author     = IBM
 | title      = IBM System/360 Principles of Operation
 | id         = A22-6821-7
 | ref        = PoOps
 | date       = September 1968
 | version    = Eighth Edition
 | url        = http://bitsavers.org/pdf/ibm/360/princOps/A22-6821-7_360PrincOpsDec67.pdf
 | mode       = cs2
 }} Revised by {{cite manual
 | author = IBM
 | title       = ibid.
 | id         = GN22-0354
 | date       = May 12, 1970
 | mode       = cs2
 }} and {{cite manual
 | author     = IBM
 | title      = ibid.
 | id         = GN22-0361
 | date       = June 8, 1970
 | mode       = cs2
 }}</ref>
[[PDP-8]]<ref name="UeNm8">{{cite web|last1=Digital Equipment Corp.|title=PDP-8/E Small Computer Handbook, 19|url=https://www.grc.com/pdp-8/docs/PDP-8_Small_Computer_Handbook_1972.pdf|publisher=Gibson Research|access-date=12 November 2016|pages=7–25, the DK8EA}}</ref> and [[Data General Nova]] used a real-time clock that was accurate, simple and low cost. In Europe, North America and some other grids, the frequency of the AC mains is adjusted to the long-term frequency accuracy of the national standards. In those grids, clocks using AC mains can keep perfect time without adjustment. Such clocks are not practical in portable computers or grids (e.g. in South Asia) that do not regulate the frequency of AC mains.

These computers' power supplies use a transformer or resistor divider to produce a sine wave at logic voltages. This signal is conditioned by a zero crossing detector, either using a [[linear amplifier]], or a [[schmitt trigger]]. The result is a square wave with single, fast edges at the mains frequency. This logic signal triggers an interrupt. The interrupt handler software usually counts cycles, seconds, etc. In this way, it can provide an entire clock and calendar. In the IBM 360, the interrupt updates a 64-bit count of microseconds utilized by standardized systems software. The clock's jitter error is half if the clock interrupts for each zero crossing, instead of each cycle.

The clock also usually formed the basis of computers' software timing chains; e.g. it was usually the timer used to switch tasks in an operating system. Counting timers used in modern computers provide similar features at lower precision, and may trace their requirements to this type of clock. (e.g. in the PDP-8, the mains-based clock, model DK8EA, came first, and was later followed by a crystal-based clock, DK8EC.)

A software-based clock must be set each time its computer is turned on. Originally this was done by computer operators. When the Internet became commonplace, [[network time protocol]]s were used to automatically set clocks of this type.

==See also==
* [[Crystal oscillator frequencies]]
* [[High Precision Event Timer]]
* [[Interrupt request#Slave PIC|IRQ 8]]
* [[Real-time clock alarm]]
* [[System time]]
* [[Time formatting and storage bugs]]
* [[Timer]]
* [[Wall-clock time]]

==References==
{{Reflist}}

==External links==
* {{Commons category-inline|Real-time clocks}}

{{Authority control}}

[[Category:Computer real-time clocks| ]]
[[Category:Clocks]]
[[Category:Integrated circuits]]