Hi @rahuj.1
During system STANDBY, the VDD (usually STPMIC1 buck3) must be kept powered in order to allow wake-up from STM32MP15x RTC/TAMP IP or WKUP pins (which will make PWR_ON pin to go high which ask STPMIC1 to restore supplies which has been shutdown). Other supplies are usually shutdown if not requested by peripherals (managed by Linux).
If you want to fully shutdown the system, this is possible by asking STPMIC1 to completely shutdown, but then, only STPMIC1 could power-on again the system (from STPMC1 PONKEYn or WAKEUP pins).
See also https://wiki.st.com/stm32mpu/wiki/Regulator_overview
and https://wiki.st.com/stm32mpu/wiki/How_to_define_your_low-power_strategy
Regards.
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