The RX8900CE:UB0 is a real-time clock module featuring an I²C-Bus interface with Fast-Mode 400 kHz operation and a built-in frequency-adjusted 32.768 kHz DTCXO for high-stability timekeeping. It operates across an interface voltage range of 2.5 V to 5.5 V, with temperature-compensated operation from 2.0 V to 5.5 V and clock supply capability down to 1.6 V, enabling flexible power management. The module incorporates automatic power switching that monitors VDD voltage and seamlessly switches to backup power supply, along with programmable alarm functions (day, date, hour, minute), wakeup timer interrupts (selectable from 244 µs to 2.8 days), and frequency output options (32.768 kHz, 1024 Hz, or 1 Hz). With a frequency stability of ±5.0 × 10?6 across -40°C to +85°C (equivalent to ±13 seconds per month deviation) and low current consumption (0.70–1.5 µA typical), this module is ideal for battery-backed real-time clock applications in portable devices, IoT systems, and industrial equipment requiring reliable timekeeping with minimal power draw.