Can I use a battery or super cap to retain RTC content through a power failure?

The short answer is no.  Gainspan does NOT have any kind of a reference design for keeping the RTC alive thru a power failure.  This is the case for all Gainspan products: GS1011M, GS1500M, GS2011M, and GS2100M.  The Gainspan chips are architected under the assumption that the RTC is able to control turning on and off power to the remainder of the chip, using the dc_dc_cntl signal.  When the RTC is going to change the power status of the core logic, it first 1) isolates itself from the core logic to prevent RTC register corruption, and 2) asserts reset to the core logic.  When power is removed without the RTC's knowledge, neither of these steps occurs.  Therefore RTC registers might get corrupted due to its not being isolated from the core, and the core logic might not start up correctly because reset was not asserted.

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