The NXP PCF2123 is a CMOS1 real time clock (RTC) and calendar optimized for low power applications. Data is transferred serially via a Serial Peripheral Interface (SPI bus) with a maximum data rate of 6.25 Mbit or s. An alarm and timer function is also available providing the possibility to generate a wake up signal on an interrupt pin. An offset register allows fine tuning of the clock.

Internal power-on reset (POR)
Open-drain interrupt and clock output pins
Programmable offset register for frequency adjustment