A desktop motherboard power sequence is the rigorous, millisecond-precise order in which voltages and logic signals must activate to transition a system from "Dead" (G3) to "Fully Operational" (S0).

The desktop motherboard power-on sequence consists of a multi-stage process where the SIO chip, chipset, and PSU, starting from a 5VSB standby state, negotiate to initiate main voltage rails (+3.3V, +5V, +12V). Following the detection of a stable Power Good signal, the system triggers the VRM to power the CPU and releases the reset signal to begin BIOS execution. Detailed technical documentation for these sequences can be found at Motherboard Power Sequence Overview | PDF - Scribd

PWRBTN#: The power button pulls a high signal (3.3V) to ground (0V) at the Super I/O.

PSON# Activation: The EC receives the command from the PCH and pulls the PSON# (Power On) signal Low.

SIO pulls the Green wire (PS_ON) to Ground, activating all main rails (+12V, +5V, +3.3V).

Now that the main power is flowing, the board builds its "ladder" of voltages. Laptop Motherboard Power Sequence Guide | PDF - Scribd

Introduction

The PCH then checks its RTC (Real Time Clock) circuit and 32.768kHz crystal. If successful, the PCH waits for the SIO to release the PWRBTN# latch. Then, the PCH drives: