PCF8523 Real-Time Clock (RTC) & Calendar NXP Semiconductors PCF8523 Real-Time Clock (RTC) and Calendar is optimized for low power consumption, with automatic switching to battery on main power loss. Data is transferred serially via an I2C-bus with a maximum data rate of 1000kbit/s. Alarm and timer functions are available with the possibility to generate a wake-up signal on an interrupt pin. An offset register allows fine-tuning of the clock.