Dears:
The whole equipment works normally when VBAT pin is powered by CR1220-H (3.0V) and connecting the power supply (VDD3V3/ VDD_MCU in SCH as below);
But disconnecting the power supply(VDD3V3), 1.6VDC is detected on Net VDD3V3 while powered by CR1220-H (VBAT=3.0V) ONLY.
Below is the hardware circuit , FYI.
For software,