PCF2129T Accurate Real Time Clock NXP Semiconductors PCF2129T Accurate Real Time Clock is a CMOS Real Time Clock (RTC) and Calendar with an integrated Temperature Compensated Crystal (Xtal) Oscillator (TCXO) and a 32.768kHz quartz crystal optimized for very high accuracy and very low power consumption. The PCF2129T has a selectable I2C-bus or SPI-bus, a backup battery switch-over circuit, a programmable watchdog function, a timestamp function, and many other features. Typical applications include electronic metering, precision timekeeping, and GPS equipment.