Desktop Power Plane Partitioning

Effective power management requires not only the ability to throttle components, but also the ability to turn off subsystems when idle and restore them to operation when required. PC designs implement that requirement by providing multiple controllable voltage sources from the power delivery subsystem, each tied to a distribution network called a power plane. Power sources in power-managed PCs have either two or three types of outputs:

• Main outputs only active when the power supply is switched on.

• Standby outputs active whenever AC power is present and the main outputs are switched off.

• Dual outputs active when selected by the motherboard, even if the main outputs are inactive. Dual mode voltages are present in most ACPI systems supporting the S3 (suspend to RAM) state; you ll see in Chapter 6 that systems can develop dual mode voltages using custom power supplies with switching logic inside the power supply or using standard power supplies complemented by circuitry on the motherboard.

Motherboards use power planes to keep these power supply outputs separate and distribute them. Figure 5.1 shows the typical power plane partitioning in a power-managed PC.