The ATX PSU now delivers full power. The sequence is strictly timed to prevent damage.
With all power across the board perfectly stable, the structural resets are lifted in a rapid domino effect:
The motherboard's memory PWM controller switches on. It converts +12V or +5V into the native voltage required by the RAM (e.g., 1.2V for DDR4, 1.1V for DDR5).
: This standby voltage powers the Super I/O (SIO) chip or Embedded Controller (EC) and the standby logic circuits inside the PCH. Phase 2: The Super I/O and PCH Handshake
The CPU reset line goes from 0V to high. This transition clears the CPU's internal registers and forces it to look at a specific memory address: (the Reset Vector). The CPU reads the BIOS ROM chip, starts the POST (Power-On Self-Test) routine, and initializes the display. Troubleshooting a Broken Power Sequence desktop motherboard power sequence pdf exclusive
is active-low. The SIO pulls this pin down to 0V (Ground) .
Once all power rails are stable, the system releases the reset signals (Platform Reset, ), allowing the CPU to start executing code from the BIOS.
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.
(Sleep) signals, telling the SIO to pull the system out of its slumber. Green Light: The SIO finally pulls the PSON (Power Supply On) The ATX PSU now delivers full power
The SIO and VRMs combine their status signals to send and SYS_PWROK to the PCH. 2. System Clock Generation
While this guide provides a high-level view, technicians often require a , often found in technical PDF documents (such as boardview files or schematics). A dedicated PDF offers:
The CPU VRM controller utilizes multiple power phases to step down +12V from the 8-pin EPS connector into VCORE (typically between 0.8V and 1.5V) and VCCGT (integrated graphics power).
The desktop motherboard power sequence is a critical process that ensures the proper functioning of a computer system. It involves a series of steps that are executed in a specific order to provide power to various components of the motherboard. Understanding this power sequence is essential for motherboard designers, engineers, and enthusiasts who want to optimize system performance, troubleshoot issues, or design their own motherboards. It converts +12V or +5V into the native
The PCH sends a signal to the PSU to turn on the main rails (3.3V, 5V, 12V) by pulling the PS-ON# line low (green wire).
The system launches the Power-On Self-Test (POST). It initializes the cache as RAM, trains the system memory, inventories the PCIe bus, initializes the GPU, and finally hands off control to the operating system bootloader (e.g., Windows Boot Manager). Motherboard Diagnostic Reference Table
What are the of the board? (e.g., completely dead, fans spin for one second then stop, spins continuously with no display?)
Just because voltages are present does not mean they are clean or stable. This phase ensures the system logic is protected from dirty power. 1. PWR_OK / Power_Good Hardware Handshake
[AC Power In] ➔ [+5VSB / +3.3V_Dual] ➔ [EC/SIO Initialization] ➔ [RTCRST# / Crystal Oscillator] │ [SLP_S3# High] 🡨 [SLP_S4# High] 🡨 [Power Button Pressed (PWRBTN#)] 🡨───────┘ │ ▼ [PS_ON# Low to PSU] ➔ [Main Rails Active (+12V, +5V, +3.3V)] ➔ [PWROK / VRM Enable] │ [CPU Reset Asserted/Deasserted (CPURST#)] 🡨 [All Power Goods Verified] 🡨┘ │ ▼ [BIOS ROM Read via SPI Bus] ➔ [POST Code Sequence Begins] ➔ [S0 Working State]