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Backup battery

Hi All,

I am testing RTC(Real Time Clock) on Mistral REV G board with AM3715 processor. The backup battery on Power board has around 2.5V. I change the system time and power cycle the unit. I expect to see that backup battery should keep the new time but it doesn't. I know RTC works since I test it with wakeme application and it works as expected. I found something interesting in the document as the following:

5.4.2.4.3 Backup Events

Table 5-13 lists the events that put the device in BACKUP state.

                                                        Table 5-13. Backup Events
Backup Event                                                        Mode                                   State Transition
Main battery < VMBLO (= 2.7 V typ).                     M, S                                    BACKUP → ON
To stay in BACKUP state, main
battery and backup battery > VBNPR
(= 1.8 V typ).

Does that mean the condition to transit state to BACKUP is (main battery < 2.7V && (main battery and backup battert > 1.8V ))?

My understanding is that backup battery should provide power for the backup domain registers and 32KHz clock. But it seems not that way.

Can anyone explain how the backup battery works with TPS659xx module?

Thanks,

James

P.S. We have a different product which has an external rtc chip and a backup battery.  The real time is kept if I power cycle the device since the backup battery provides power for rtc domain registers and circuitry.