1.The definition of RTC real-time clock
In an embedded system, it usually use RTC to provide reliable system time, including  minutes and seconds, year, month, day, and so on, but the system can work properly in the off state it  (usually the battery backup power supply), the peripheral also do not need too much auxiliary circuit,  it is typical to only to be a high precision 32.768KHz crystal, such as resistors and capacitors.
In brief, the main power supply of  electronic device is power off, the memory of the time still need to keep working, this is the RTC real-time clock, it usually need a spare button battery power supply.

2. The basic characteristics of RTC real time clock
RTC unit can be powered by a spare button battery, so even if the system power is off, it can also continue to work. RTC real time clock can be sent to the BCD code data through the STRB/LDRB command to CPU 8 bit. These BCD data include seconds, minutes, time, date, week, month, and year. The RTC unit through an external 32.768KHz crystal oscillator provides the clock. RTC has the function of timing alarm. 
Function of RTC controller:
- 1, clock data using the BCD encoding
- 2, able to leap year date of automatic processing
- 3, with alarm function, when the system is in a shutdown state, can generate alarm interrupt;
- 4, it is independent of the power input
- 5, providing millisecond clock interrupt, the interrupt can be used as an embedded operating system kernel clock;

3. Commonly used in RTC real-time clock button battery
RTC real time clock is usually used 3V button cell, For example , PC computer and notebook RTC real-time clock is used 2032 buttons battery, small household electrical control board RTC  are commonly used 2450 button batteries. Small digital devices such as DVD, DVR, LED controller and other commonly used 1220 button battery. 3V button battery is divided into  (non rechargeable) and two batteries (rechargeable), according to the purpose of the product ,it can be used different characteristics of the battery.