ST's serial real-time clock devices combine the RTC function with the supervisor function for different master devices such as microprocessors and NVRAMs.
The features related to supervising a microprocessor include a programmable watchdog, an early power-fail warning circuit and the power-on reset/low-voltage detect. To protect an NVRAM, they can generate an automatic battery switchover and a write-protect signal, a power-fail deselect voltage and the battery-low monitor flag. The programmable alarm with repeat mode may be used in the battery back-up mode.
Battery with Crystal Option
ST's removable/replaceable SNAPHAT® battery top contains a Lithium coin cell battery and a 32KHz crystal. It is compatible with all of ST's real-time clocks and TIMEKEEPER® devices in the surface mount SOH-28 and SOH-44 packages.
The SNAPHAT top is designed to be "snapped on" after the SOIC is surface mounted on the PC board. Thus the two piece solution prevents the battery and crystal from being exposed to the high temperatures of the surface mount process.
To evaluate the battery capacity consumption and lifetime: Real-time clocks and TIMEKEEPERs battery life calculation