want to use the RTC at all?
For lowest power consumption, it is best to follow the advice of EE-281:
Hardware Design Checklist for the Blackfin Processors:
Real-Time Clock (RTC)
Does your design require a battery for continuous time information? If not,
RTC power to VDD_IO.
Ensure that an additional shunt resistor (1 MΩ to 10 MΩ) is present in the RTC
crystal circuit. If the RTC is not used, pull the RTXI pin low.
Check this site for the latest version EE-281: