PCF2127 devices are "Not Recommended for New Designs", please use the replacement part PCA2131TF
The PCF2127 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 PCF2127
has 512 bytes of general-purpose static RAM, a selectable I2C-bus or SPI-bus, a backup
battery switch-over circuit, a programmable watchdog function, a timestamp function, and
many other features.