Hi Jimmy,
Which PMIC/SBC are you using?
If FS26, a VCORE_PGOOD signal is available through GPIO2 pin. When GPIO2 is configured as VCORE_PGOOD using the GPIO2_VCORE_PGOOD_OTP bit, GPIO2 is connected to the HS switch (connected internally to VPRE).
VCORE_PGOOD signal may be connected to the WKPU signal of the MCU which requires a 3.3 V level. A resistor divider is needed to adapt the voltage level.
• When VCORE is in the good voltage range, VCORE_PGOOD is High logic level.
• When VCORE is not in the good voltage range or is disabled, VCORE_PGOOD is Low logic level.
During Standby mode, in case of a reset from the MCU, its RESET pin must be connected to the FS26 RSTB pin but also to the FS26 WAKE1 pin, which is configured as a low-level wake-up source, to wake up the SBC.
The total capacitance on the RSTB line should not exceed 1nF.
The MCU EXTWKPU pin must be connected to the FS26 WAKE2 pin, configured as a high-level wake-up source.

BRs, Tomas