NXP PCF2129AT/2,518, Real Time Clock (RTC), 128bit RAM Serial-SPI, 20-Pin SO20

The NXP PCF2129 is a CMOS1 real time clock (RTC) and calendar with an integrated temperature compensated crystal (Xtal) oscillator (TCXO) and a 32.768 kHz quartz crystal optimized for very high accuracy and very low power consumption. The PCF2129 has a selectable I2C bus or SPI bus, a backup battery switch over circuit, a programmable watchdog function, a timestamp function, and many other features.

Battery backed output voltage
Battery low detection function