Hi QiangweiBiao,
For i.MX8MM RTC, if you want to keep the RTC clock after shutdown, it needs 2 voltages and a 32.768K clock:
1. NVCC_SNVS_1V8
2. VDD_SNVS_0V8
3. CLK_32K_OUT
Therefore, you need to check whether the output of LDO1 and LDO2 is normal and whether the output of C32K_OUT is normal after you remove 5V.
[In addition]
Considering that the power consumption of the RTC inside the CPU is larger than that of a separate RTC chip, it is strongly recommended that you use an independent RTC chip. NXP has many types of RTC chips, and you can choose one that meets your requirements.
Have a nice day!
B.R,
weidong
