AM574x IDK lost RTC time after power off

Hi, All:

I found AM574x_IDK comes with 2 RTC.

ls /dev/rtc*                                                                                                   
/dev/rtc   /dev/rtc0  /dev/rtc1

The RTC is work fine and it is able to wake up system.

echo +100 > /sys/class/rtc/rtc0/wakealarm
echo mem  > /sys/power/state

RTC is able to set time.

date -s "2020-10-28 16:55"
Wed Oct 28 16:55:00 UTC 2020
date
Wed Oct 28 16:55:01 UTC 2020
hwclock -w
hwclock                                                                                                        
Wed Oct 28 16:55:09 2020  0.000000 seconds

But, the RTC time lost after power off.
How to add a coins battery to keep time ?

Note:
Linux version:
uname -a                                                                                                       
Linux am57xx-evm 4.19.94-gbe5389fd85 #3 SMP PREEMPT Fri Oct 16 14:25:31 CST 2020 armv7l GNU/Linux

Nancy Wang：

请参考 2.19 RTC部分的说明。
www.ti.com/…/sprack7b.pdf
纽扣电池部分建议去电源管理论坛咨询。
e2echina.ti.com/…/24

greatcat chen：

回复 Nancy Wang:

e2e.ti.com/…/593938Please see the note at the beginning of section 23.1 of the AM572x TRM Rev. H:"Device RTC low-power mode (only RTC active in the device) is not supported in this family of devices."AM574x Technical Reference Manual,也有同樣的描述.e2e.ti.com/…/727193There is not a mode with just the PMIC RTC or just LDO9. The only way to use a back up battery would be to add a supervisor on VCC1 and turn on the backup when the main supply is too low. Unfortunately, this would not work well for a coin cell battery since the current consumption would drain the battery pretty fast.AM574x_IDK 有2個RTC,一個在AM574x,一個在TPS659037.我猜測,兩者,都不能設計成,掉電後不丟失時間,是嗎?