Real-time clock
From Wikipedia, the free encyclopedia
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 most any electronic device which needs to keep accurate time.
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[edit] Terminology
The term is used to avoid confusion with ordinary hardware clocks which despite their name, do not actually track time, but merely govern digital electronics. RTC should not be confused with real-time computing, which shares its three-letter acronym, but does not directly relate to time of day.
[edit] Purpose
Although keeping time can be done without an RTC, 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 computers RTCs are often inaccurate)
GPS receivers can use RTCs to reduce their startup time by predicting which satellites will be in view.
[edit] 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, because they are rechargeable and can be soldered. The alternate power source can also supply power to battery backed RAM.
[edit] Timing
Virtually all RTCs use a crystal oscillator. 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.
[edit] Examples
Some models of integrated Real-time clock circuits are the DS1307 from Maxim and the PCF8563 from Philips. RTCs in older personal computers were often made by Dallas Semiconductor (now a subsidiary of Maxim), easily found on motherboards because of their distinctive black battery cap and silkscreened logo. In newer systems the RTC is integrated with the southbridge chip.
Some microcontrollers, such as the Philips LPC2106, have a real-time clock built in.
