System Agent voltage ( VCCSA or VCCIO ) is generated to power the memory controller inside the CPU. Phase 5: The Power Good Chain and Reset Sequence
Powers the DDR memory modules and the memory controller interface.
As soon as the power supply (PSU) is plugged in, it sends via the purple wire to the SIO chip.
The power-on sequence for a desktop motherboard is a precise, multi-step process involving specific signals and voltage levels that must occur in a fixed order for the system to boot successfully Standard Power-On Sequence Standby Power (5VSB): desktop motherboard power sequence pdf exclusive
The Super I/O (SIO) chip acts as the gatekeeper of the motherboard’s power infrastructure. 1. SIO Reset and Ready State
Deep sleep/standby voltage required by modern Intel/AMD chipsets to maintain basic configuration data. 3. Real-Time Clock (RTC) Activation
For engineers and advanced users, these references provide the definitive specifications: System Agent voltage ( VCCSA or VCCIO )
This triggers the PSU to generate the main rails. Power Good Confirmation (PWROK)
When you press the power button, your computer doesn't turn on instantly. Instead, it follows a strict, timed, and hierarchical sequence of events. The motherboard's Super I/O (SIO) chip and the PCH (Platform Controller Hub) coordinate this, ensuring that different components receive power in the correct order to prevent hardware damage.
The SIO forwards this wake request to the PCH via the multiplexed signal. The power-on sequence for a desktop motherboard is
Check for shorts on 12V VCORE high-side MOSFETs. Verify RAM and PCH buck output coils. PLTRST# or CPU VCORE missing
The SIO validates this press and forwards the command to the Chipset by dropping the (Power Button Monitor) signal to 0V, signaling the PCH to wake the computer up. 5. Stage 4: Waking Up the Main Power Rails (S5 to S0)
The PCH sends a signal to the SIO to confirm it is awake.
For hardware engineers, diagnostic technicians, and computer repair enthusiasts, understanding this architecture is essential for troubleshooting "No Power," "No POST (Power-On Self-Test)," or intermittent boot loop failures.
Confirmation to the CPU/PCH that all voltages are stable and within spec. PCH → CPU