PCF8563 RTC/Calendar ICs NXP Semiconductors PCF8563 RTC/Calendar ICs are CMOS Real-Time Clock (RTC) and calendar integrated circuits optimized for low power consumption. A programmable clock output, interrupt output, and low-voltage detector are also provided. All addresses and data are transferred serially via a two-line bidirectional I2C-bus, and maximum bus speed is 400kbit/s. The register address is incremented automatically after each written or read data byte.