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Real Time Clock (RTC): Introduction

Real Time Clocks (RTCs) manage the time inside systems. Even when the main power supply is switched off, they continuously operate by being powered from a coin battery or a secondary battery.
In addition to a simple clock function, RTCs play a great role in the high-accuracy time management of systems, such as timestamps at file update, time management of log data, etc.
Although it is possible to check the time with high-performance timers or system timers, those devices are usually turned off during their standby mode due to their large supply current.
Instead of these large-current-consuming devices, RTCs are used because of the capability of long operation with a coin battery or secondary battery even at the CPU's OFF state.

We have a wide lineup of RTCs featuring four- (SPI) to two-line (I2C) interfaces, a backup battery switch-over function, a 32 kHz built-in crystal oscillator, etc.
All of our RTCs include a clock adjust function which correct variations of the crystal oscillator frequency, frequency deviation caused by temperature, and so on.
Other options such as a oscillator halt sensing, a supply voltage monitor, etc. can be selected according to customers' applications, of course with low power operation by low voltage and low supply current taken into consideration.

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Merits of Using RTCs

Lower Supply Current than MCU

RTCs can reduce the cost and the size of a backup circuit and are designed to operate with much lower current and voltage than a time keeping circuit constructed with an MCU.

Back-up Time Measurement (R2051S01)

Backup Device Backup Time
Backup Starting Voltage: 5V Backup Starting Voltage: 3V
Coin Cell Primary Battery (CR2032) - 10 Years or more (Calculated Value)
Electric Double Layered Capacitor (1 F) 130 Days 116 Days
Electric Double Layered Capacitor (0.1 F) 21 Days 15 Days
Aluminum Electrolytic Capacitor (4700 µF) 20 Hrs 12 Hrs 30 Min
Aluminum Electrolytic Capacitor (470 µF) 2 Hrs 1 Hrs 15 Min
Aluminum Electrolytic Capacitor (47 µF) 12 Min 7 Min 30 Sec

Easy Software Development

In order to construct a clock function with an MCU, designers have to program a software which manages the complex calender and time including leap years, long months and short months, combinations of sexagesimal and base 24 scales, etc.
RTCs, on the other hand, are designed specifically for the time management; they include circuits to keep the time and the calendar until 2099, and easy to debug.

Simple Oscillator Circuit

Only an external crystal oscillator is enough to construct an oscillator circuit, for the ambient components except an crystal unit are contained in RTCs.
Thanks to the small number of external components, it is easy to lay out an oscillator circuit.