General description
The PCF2128 is a ready to run CMOS real time clock/calendar with an integrated temperature compensated crystal oscillator (TCXO). In timekeeping applications the high accuracy of the PCF2128 allows it to be used as a replacement for costly and higher powered long wave receivers or GPS receivers. A programmable battery switch-over circuit enables an uninterruptible power supply and consequently continuous timekeeping. The PCF2128 additionally features 512 bytes of general purpose RAM, a programmable watchdog, a time stamp facility and a voltage monitoring facility. Programming is possible using either an SPI or an I2C-bus interface.

■ Integration of a 32.768 kHz quartz crystal in the same package as the RTC
■ temperature compensated crystal oscillator (TCXO) with integrated capacitors.
■ accuracy:
   typically 3 ppm from−20 °C to +70 °C,
   typically 5 ppm from −40 °C to +85 °C
■ provides year, month, day, weekday, hours, minutes and seconds
■ programmable alarm function with interrupt capability
■ programmable countdown timer with interrupt capability
■ programmable watchdog timer with interrupt and reset capability
■ timestamp function with interrupt capability
■ battery backup input pin and switch-over circuitry
■ extra power fail detection with input and output pins
■ battery low detection
■ battery backed output voltage pin
■ 512 bytes of general purpose static RAM
■ 1 second or 1 minute interrupt output
■ oscillator stop detection
■ two line bi-directional 1 MHz fast mode plus I2C interface
■ timestamp input
■ power-on reset
■ 3 line SPI interface with separate data input and output (maximum speed 6.5 Mbits/s)
■ programmable square wave output pin
■ I2C-bus slave address: read A3H and write A2H
■ clock operating voltage: <tbd> to 5.5 V
■ low backup current; typical 0.95 μA at VDD = 3.0 V and Tamb = 25 °C
■ selectable I2C and SPI interface