PCA2129T device is "Not Recommended for New Designs", please use the replacement part PCA2131TF
The PCA2129 is a CMOS 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 PCA2129
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.