Hi Calvin,
OSD335x C-SIP integrates the 24MHz main oscillator clock on OSC0. An external oscillator is required to generate the 32KHz real time clock for the RTC subsystem.
Please see the updated datasheet for available RTC power modes. Although we have separated the VDDS and VDDS_RTC rails that allow for them to be powered independently in an RTC only mode, there could be power up sequencing issues that might not allow for an RTC only mode operation when using just the TPS65217C PMIC in the SiP (See https://e2e.ti.com/support/processors/f/791/t/497615 for more explanation and alternate implementation). We are testing several other alternate implementations of the RTC only mode and we will create an App note with respect to implementation. For now, using an external RTC module is your best bet.
The battery controller you linked should work. Note that V_CTRL needs to be always high to enable 1.8V LDO to power the RTC. VDDS would required a separate LDO that would come up in the correct sequence. Please see: http://www.ti.com/lit/ug/slvu551i/slvu551i.pdf for the power up sequencing.
Best,
Neeraj
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