The Intel X79 platform (LGA 2011 socket), which housed the Sandy Bridge-E and Ivy Bridge-E processors, is a classic High-End Desktop (HEDT) architecture.
1. The Power Flow Sequence (Boot State to S0)
When you press the power button on an X79 motherboard, power does not hit all components simultaneously. It follows a strict sequence governed by the Super I/O chip and the chipset.
1. Standby State (S5): The ATX 24-pin cable provides a constant 5VSB (5V Standby) to the motherboard. This powers the Super I/O chip, the BIOS flashback ICs (if present), the network wake-up logic, and the PCH (Platform Controller Hub) standby logic
2. Power Button Press: The front panel power button grounds the PWR_BTN# pin. The Super I/O chip detects this and sends a signal to the X79 PCH.
3. PS_ON# Assertion: The PCH signals the Super I/O to pull the PS_ON# pin (green wire on the 24-pin) to ground.
4. Main PSU Activation: The power supply unit (PSU) clicks on, sending 12V, 5V, and 3.3V to the motherboard via the 24-pin and the 8-pin EPS CPU power connectors.
5. VRM Sequencing: The onboard Voltage Regulator Modules (VRMs) wake up in a specific order:
PCH (Chipset) and Memory voltages stabilize first.
System Agent (VCCSA) and I/O voltages (VTT/VCCIO) are initialized.
Finally, the CPU VCore is supplied by the main CPU VRM.
6. Power Good (PWR_OK): Once the VRMs report stable voltage, a PWRGD (Power Good) signal is sent to the CPU. The CPU releases its reset state, reads the BIOS, and the system POSTs.
2. Critical Power Rails on X79
For an X79 board to function—and especially to overclock—several critical power rails must be perfectly maintained by the motherboard's VRMs.
- Function: Powers the primary compute cores and the L3 Cache.
- Criticality: Extremely high. Sandy Bridge-E CPUs (like the i7-3960X) had a TDP of 130W at stock but could easily pull 250W+ when overclocked. The primary VRM (often 8 to 16 phases on X79) is dedicated strictly to this rail.
- Function: Powers the Integrated Memory Controller (IMC) and the PCI Express controller inside the CPU.
- Criticality: High. Because X79 uses quad-channel memory and 40 PCIe lanes, the System Agent is under massive load. If VCCSA drops, you will experience memory dropouts (e.g., Windows showing 12GB usable instead of 16GB) or GPU crashes.
- Function: Powers the termination logic for the memory bus and the QPI (QuickPath Interconnect) links (if applicable in Xeon variants).
- Criticality: High. Often tied closely to VCCSA in tuning; both need to be carefully balanced to achieve memory overclocks past 1600MHz on DDR3.
- Function: Provides standard operating voltage to the DDR3 modules (typically 1.50V, up to 1.65V for XMP profiles).
- Criticality: High. Unique to X79: Because the memory slots are flanked on both sides of the CPU socket, X79 motherboards feature a split memory VRM topology. There is an independent 2-to-4 phase VRM for the AB channels (left side) and another identical VRM for the CD channels (right side).
- Function: Powers the X79 silicon itself (usually operating at ~1.1V).
- Criticality: Moderate. The X79 chipset only handles SATA, USB, and lower-speed PCIe lanes. It generally does not require voltage tweaking unless pushing extreme BCLK (Base Clock) overclocks.
- Function: Powers the CPU's internal clock generation.
- Criticality: Moderate. On X79, lowering this slightly from its default (~1.8V) could actually lower CPU temperatures, while raising it could help stabilize edge-case CPU multipliers.
