Real-time clock with integrated TCXO — no external crystal needed
The NXP PCA2129T/Q900/2,51 is an I2C real-time clock (RTC) that integrates a temperature-compensated crystal oscillator (TCXO) and crystal on-chip, so you can drop it onto a 16-SOIC footprint without sourcing a separate 32.768 kHz resonator. It keeps time in HH:MM:SS (12/24 hr) and YY-MM-DD-dd formats, with alarm, leap-year compensation, square-wave output, and a watchdog timer built in. The supply range spans 1.8 V to 4.2 V for both the main VDD and the battery backup input, so it runs off a 3.0 V or 3.6 V coin cell as easily as a 3.3 V logic rail. Timekeeping current is a maximum of 1.5 µA at 3.3 V — low enough to keep the calendar alive for years on a CR2032.