The PCF2128 is a ready to run CMOS real time clock/calendar with an integrated temperature compensated crystal oscillator (TCXO). In timekeeping applications the high accuracy of the PCF2128 allows it to be used as a replacement for costly and higher powered long wave receivers or GPS receivers. A programmable battery switch-over circuit enables an uninterruptible power supply and consequently continuous timekeeping. The PCF2128 additionally features 512 bytes of general purpose RAM, a programmable watchdog, a time stamp facility and a voltage monitoring facility. Programming is possible using either an SPI or an I2C-bus interface.