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. 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.

Terminology

The term 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.

Purpose

Although keeping time can be done without an RTC [cite web
first =Jaakko
last =Ala-Paavola
coauthors =
title = Software interrupt based real time clock source code project for PIC microcontroller
date = 2000-01-16
accessdate = 2007-08-23
url = http://users.tkk.fi/~jalapaav/Electronics/Pic/Clock/index.html] , using one has benefits:
* Low power consumption (important when running from alternate power)
* Frees the main system for time-critical tasks
* Sometimes more accurate than other methods (although personal computer RTC's are often inaccurate)

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. [Patent|US|5893044|Real time clock apparatus for fast acquisition or GPS signals] If it has been less than a few hours then the previous ephemeris is still usable.

Power source

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 [Citation
title =Application Note 3816
publisher =Maxim/Dallas Semiconductor
year =2006
url =http://www.maxim-ic.com/appnotes.cfm/an_pk/3816] , because they are rechargeable and can be soldered. The alternate power source can also supply power to battery backed RAM.

Timing

Most RTCs use a crystal oscillatorCitation
title =Application Note 10337
publisher =ST Microelectronics
year =2004
pages = 2
url =http://www.st.com/stonline/products/literature/an/10337.htm] [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] , but some use the power line frequency [Citation
title =Application Note 1994
publisher =Maxim/Dallas Semiconductor
year =2003
url =http://www.maxim-ic.com/appnotes.cfm/an_pk/1994] . In many cases the oscillator's frequency is 32.768 kHz. This is the same frequency used in quartz clocks and watches, and for the same reasons, namely that the frequency is exactly 2¹⁵ cycles per second, which is a convenient rate to use with simple binary counter circuits.

Examples

Many integrated circuit manufacturers make RTCs, including Intersil, Maxim, Philips, Texas Instruments and STMicroelectronics. RTCs in older personal computers were often made by Dallas Semiconductor, now a subsidiary of Maxim, and are easily found on motherboards because of their distinctive black battery cap and silkscreened logo. In newer systems the RTC is integrated into the southbridge chip. [cite web
title = ULi M1573 Southbridge Specifications
publisher = AMDboard.com
url = http://www.amdboard.com/ali_uli_m1573.html
accessdate = 2007-08-23 ]

Some microcontrollers have a real-time clock built in, generally only the ones with many other features and peripherals.

See also

* System time
* Timer
* RTC Alarm

References