Than 3V_ALW or 3VPCU provide power supply to the EC , BIOS IC , South Bridge ( SB ) , clock generator and oscylator on ALW voltage .
USB or switch voltage button (3.3V) will appear after Bios and EC powered. some of laptop motherboard will found 17V switch voltage on one of pin switch button but after switch -on voltage is changed to be 3.3 V.This is normal.
Press power button ( short to ground ) then 3.3VSB will turn into 0V and back to 3.3V.After this, signal being sent to the EC ( Embedded controller NPCE 885LAODX ) signal names is NBSWON with other brands of EC signal name which may be SW_ON, EC_ON or whatever name of the signal as long as signal coming from SW1 sent to EC , this voltage must respond in the same manner of testing power button , BIOS IC and firmware connected to SPI interface to the EC .
Signal confirmation to ensure EC BIOS working properly is RSMRST for3.3V.RSMRST# signal which sometimes appear after switching on power button.Modern Motherboard has 2 or 3 IC BIOS. Identified EC BIOS which is usually on schema with name EC BIOS.
Other important signal from EC is DNBSWON # ,this is PWR_BTN signal to switch on South bridge sent by the EC after receiving NBSWON # signal from power button.
DNBSWON 3.3V must be present otherwise it would not switch on , after this signal appears then the SB will sent SUSCLK signal back to the EC and turn on VS as trigger for all switching gate driver as VR_ON , Suson , MAINON , S5_ON or SLP_S3 , SLP_S4 and SLP_S5 . and for all the Vs voltage to the Device .
