Another good app note from Microchip, a guidance in using the Microchip RTCC family of devices (PDF):
This application note provides assistance and
guidance in using the Microchip RTCC family of
devices. This application note covers both the I
2C™
(MCP794XX) and SPI (MCP795XXX) family of
devices. These recommendations are not meant as
requirements, however, their adoption will lead to a
more robust overall design. The following topics are
discussed:
• Basic Design Considerations
•VCC Supply
• Backup Supply options
• Input Pins
• Output Pins
• Crystal Selection
• Recommended Schematics (Appendix B-D)