The ATX PSU now delivers full power. The sequence is strictly timed to prevent damage.
The desktop motherboard power sequence is a highly logical, deterministic cascade. By understanding the dependencies—how standby power enables the Super I/O, how the Super I/O wakes the PCH, and how the PCH coordinates with the PSU and CPU VRMs—diagnosing complex hardware failures changes from guesswork into a precise, step-by-step science.
The PCH receives VR_READY and the overall system PWR_OK . It then releases the logic reset by pulling PLTRST# high. Finally, the CPU receives CPURST# , which clears its internal registers and commands it to start processing data. 5. Phase 5: The Post-Reset State (Booting & POST) desktop motherboard power sequence pdf exclusive
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
At this point, the main power rails start ramping up. The ATX PSU now delivers full power
This entire sequence typically takes less than a second from button press to BIOS execution.
Understanding the desktop motherboard power sequence is essential for diagnosing hardware failures, as it reveals the precise order of signals and voltages required for a successful boot. This sequence functions like a "handshake" between the Power Supply Unit (PSU), the Super I/O (SIO) chip, and the Platform Controller Hub (PCH). Phase 1: Standby State (G3 to S5) Finally, the CPU receives CPURST# , which clears
If a motherboard is dead or looping, technicians use an oscilloscope or multimeter to check signals in this exact chronological order. Finding where the sequence breaks tells you exactly which circuit is faulty. Signal / Rail Name Expected Voltage Common Symptom if Missing Likely Root Cause +5VSB Completely dead PC; no standby LEDs. Faulty PSU, shorted standby rail capacitor. 2 +3.3VSB_STB No response to power button. Defective linear regulator (LDO) or bad Super I/O. 3 RSMRST# No response to power button. SIO chip corrupt, missing standby power, or PCH defect. 4 PWRBTN# 3.3V → 0V → 3.3V No response to power button when pressed. Broken case power switch or shorted diode on header. 5 SLP_S3# / SLP_S4# Fans spin for half a second then turn off; short-cycling. Corrupted BIOS ROM, short circuit on main 12V/5V rails. 6 PSON# 0V (when active) PSU fan doesn't spin; main rails stay at 0V. Open circuit between SIO and ATX pin 16; bad SIO chip. 7 VCORE 0.8V - 1.4V Fans spin at maximum speed, no display, no POST codes. Blown VRM MOSFET, dead VRM driver, shorted CPU. 8 SYS_PWROK / PW_OK 3.3V / 5.0V System stays on but black screen; no reset release.
To help with real‑world troubleshooting, we've prepared an to this guide. It contains:
If you are using an oscilloscope or multimeter to troubleshoot a dead motherboard, always trace the signals in this exact cronological sequence: Signal Component Expected Voltage Common Failure Symptom +5VSB / +3.3VSB 5.0V / 3.3V Complete dead board; no standby LEDs. 2 RTC_RST# 3.0V - 3.3V Board won't turn on until CMOS battery is pulled. 3 PCH_PWRBTN# →right arrow →right arrow SIO is faulty or power switch circuit is broken. 4 SLP_S3# / SLP_S4# 3.3V (when on) PCH is shorted or missing an internal prerequisite. 5 PS_ON# 0V (when on) Fans don't spin; PSU remains in standby. 6 ATX Rails (+12V, +5V) Nominal Values Short circuit on a major power plane causing PSU shutdown. 7 SYS_PWROK
Before you even touch the power button, the motherboard is already partially alive. This is known as the transitioning to the S5 (Soft Off) state. 1. RTC Circuit Activation