Symptom: Fans Spin for a Split Second, Then Power Down (S5 to S0 Interruption)

Before the user interacts with the machine, a foundational power infrastructure must exist.

Standby LEDs on, but completely unresponsive to power button. Dead CR2032 Battery / Bad RTC Crystal. RSMRST#

Power sequencing is crucial for board bring-up because modern processors and chipsets have strict requirements about which voltages must appear before others — and within what time window.

What or chipset generation (e.g., Intel H61, AMD B550) are you troubleshooting?

Pressing the power button begins the active sequencing. The signal chain is:

With all voltages up, the PCH:

The CMOS battery supplies power to the RTC section inside the Platform Controller Hub (PCH) or Chipset.

The following steps represent the standard logic found in many technician-level technical guides:

The user presses the front-panel power button. This temporarily shorts the high-signal line to the ground, creating a High-to-Low-to-High pulse sent directly to the SIO chip.

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The SIO chip detects that SLP_S3# has gone high. It then pulls the signal (Pin 16 on the 24-pin ATX connector) low to 0V. Grounding PS_ON# tells the PSU to switch from standby mode to full operation, turning on the main power rails: +12V, +5V, and +3.3V . Phase 3: Voltage Regulator Step-Up Sequence

The SIO then sends a signal to the PCH, effectively "asking permission" to boot. Sleep State Release (SLP_S4/SLP_S3)

This phase represents the transition from a sleeping machine to an active system.

Before you even touch the power button, your motherboard is already alive. As soon as the ATX power supply is connected and switched on, the power sequence begins. 1. The Real-Time Clock (RTC) Section

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