Once SLP_S4# and SLP_S3# are high, the motherboard enables secondary power rails before the CPU core.
Exclusive Failure Point: If VDDQ is shorted to ground (bad RAM slot or MOSFET), the board will not proceed to Vcore. You will see a 0.5-second fan spin and then nothing.
Symptom: A Gigabyte B660 DS3H board. Press power button. Fans spin for 0.5 seconds, LEDs flash, then stop. Repeats every 3 seconds.
Common Mistake: Blame the PSU. Replace it. Same issue.
Expert Diagnosis (using our sequence):
The power sequence told us the failure happened before Vcore, narrowing the fault to RAM or System Agent rails.
Before a single volt of power reaches the CPU, a hierarchy of control must be established. The motherboard is not a passive board; it is an active circuit managed by two primary conductors:
The power sequence relies heavily on handshakes—signals sent from one chip to another to say, "I am stable. You may proceed."
Now that power is stable, the digital logic must be synchronized and reset. desktop motherboard power sequence pdf exclusive
The Reset Sequence:
POST (Power On Self Test):
When you press the chassis power button, you ground the PWRBTN# pin on the SIO. This signal is a negative logic pulse (active low). The SIO debounces this (typically 16ms to 50ms) and then internally latches the request.
Exclusive Timing: In most desktop boards, the SIO will wait for 250ms after the button is released before initiating Stage 2. This prevents false triggers. Once SLP_S4# and SLP_S3# are high, the motherboard
This phase covers the moment the user presses the power button to the activation of the main power rails.
Sequence Steps:
PSON# Activation: The EC receives the command from the PCH and pulls the PSON# (Power On) signal Low.
PSU Wake-up: The PSON# signal connects to the ATX PSU Green Wire. When this wire is grounded (Low), the PSU wakes up. Exclusive Failure Point: If VDDQ is shorted to