Real-time clock

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Realtime time RTC is the computer clock (most often in the form of integrated circuits) that tracks the current time.

Although this term often refers to devices on personal computers, servers, and embedded systems, RTCs are present in almost all electronic devices that need to keep accurate timing. The common RTC used in single board computers is DS1307.

Video Real-time clock

Terminology

The term real-time is used to avoid confusion with regular hardware clocks that are just signals that regulate digital electronics, and do not count the time in a human unit. RTCs should not be confused with real-time computing, which shares a three-letter acronym but is not directly related to the time of day.

Maps Real-time clock

Destination

Although keeping time can be done without RTC, using one has benefits:

• Low power consumption (important when running from alternate power)
• Unleashes main system for important tasks of time
• Sometimes more accurate than other methods

The GPS receiver can shorten the startup time by comparing the current time, according to its RTC, to the time at which it last had a valid signal. If it has been less than a few hours, then the previous ephemeris can still be used.

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Resources

RTCs often have alternative resources, so they can continue to keep time while the main power source is off or unavailable. These alternative power sources are typically lithium batteries on older systems, but some newer systems use supercapacitors, as they can be recharged and can be soldered. Alternate power sources can also supply power to the battery-backed RAM.

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Time

Most RTCs use crystal oscillators, but some have the option of using the power line frequency. In many cases, the oscillator frequency is 32,768 kHz. This is the same frequency used in quartz clocks and watches, and for the same reason, namely that the exact frequency of 2 ¹⁵ cycles per second, is a convenient level for use with simple binary counter circuits.

Many commercial RTC ICs are accurate to less than 5 parts per million. In practical terms, this is good enough for navigating the celestial, classic task of a chronometer. In 2011, atomic clock Chip scale was found. Although more expensive, they save time in 100 nanoseconds.

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Example

Many integrated circuit manufacturers create RTCs, including Epson, Intersil, IDT, Maxim, NXP Semiconductors, Texas Instruments, STMicroelectronics and Ricoh.

RTC was introduced to PC compatibles by IBM PC/AT in 1984, which used the Motorola MC146818 RTC. Later, Dallas Semiconductor created a compatible RTC, often used on older personal computers, and easily found on the motherboard because of its distinctive black battery cover and silkscreened logo.

In newer systems, RTC is integrated into the southbridge chip.

Some microcontrollers have real-time clocks installed, generally only those that have many other features and peripherals.

RTC Radio-based

Some modern computers receive clock information via digital radio and use it to promote time standards. There are two common methods: Most mobile protocols (eg LTE) directly provide local time at this time. If internet radio is available, the computer can use the network time protocol. Computers used as local time servers sometimes use GPS or ultra-low frequency radio transmissions broadcast by national standards organizations (ie radio clocks).

Historic RTC

Some older computer designs such as Novas and PDP-8 use real-time clocks known for their accuracy, simplicity, flexibility, and low cost. The computer power supply generates pulses at logic voltages for each half wave or any zero crossing from AC power. A wire carries a pulse to the interrupt. The interrupt handler software counts cycles, seconds, etc. In this way, it can provide all hours and calendars.

Clocks also usually form the basis of a computer software timing chain; for example typically a timer is used to redirect tasks in the operating system. The timers used in modern computers provide similar features at lower precision, and can track their requirements for this type of clock. (eg in PDP-8, the parent-based clock, DK8EA model, comes first, and then followed by crystal-based clock, DK8EC.)

Software-based clocks must be set each time the computer is turned on. Initially this was done by a computer operator. When the Internet becomes commonplace, network time protocols are used to automatically set this type of clock.

In Europe, North America and some other networks, these RTCs function because the main AC frequencies are adjusted to have a long-term frequency accuracy and a national standard clock. That is, in the grid, the RTC is superior to the quartz clock and is cheaper.

This RTC design is not practical in portable or grid computers (eg in South Asia) that do not regulate the AC power frequency. Also may think uncomfortable without Internet access to set the clock.

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Clockless CPUs

Some motherboards are made without real-time clocks. Real time clocks are eliminated either from the desire to save money (as in the Raspberry Pi system architecture) or because real time clocks may not be needed at all (as in the Arduino system architecture).

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See also

• High Precision Organizer
• IRQ 8
• Real-time alarm clock
• System time
• Timer
• Clock time
• Time formatting and storage bugs

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References

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External links

• Media related to Real-time clocks on Wikimedia Commons

Source of the article : Wikipedia