Link Wakeup from L2 Low Power State

When a device's Link is in the L2 low power state, its main power is turned off though V_aux is still applied. A device returns to the full-on L0 power state by one of two methods. Either the device signals a wakeup event or Power Management software triggers the wakeup procedure "Waking Non-Communicating Links" on page 642.

Device Signals Wakeup

The powered down device (device(s) are in D3_Cold with V_aux valid) whose Link is in L2 state, is able to signal a wakeup event either through signaling a Beacon (see "Beacon Signaling" on page 469) upstream towards the root complex or by asserting WAKE#. The wakeup event ultimately results in the power controller re-applying power and clock to the device (or group of devices) who signaled the wakeup. The power controller also causes (either autonomously or under software control) a PERST# Reset to the device or group of devices whose power and clock has been re-applied. If the device does not support PERST#, it must autonomously generate its own Fundamental Reset when it senses main power re-applied to it. Upon exit from Fundamental Reset, the device proceeds with Link training and initialization. The device, which is now in the D0_unintialized state, can now send a PM_PME TLP message upstream to the root complex to inform Power Management software of the wakeup event.

Power Management Software Generates Wakeup Event

Power Management software can wake up a device or group of devices (device(s) are in D3_Cold) whose Link is in L2 state. Power Management software causes the power controller to re-apply power and clock to the device or group of devices. The power controller also causes (either autonomously or under software control) a PERST# Reset. If the device does not support PERST#, it must autonomously generate its own Fundamental Reset when it senses main power re-applied to it. Upon exit from Fundamental Reset, the device proceeds with Link training and initialization. The device is now in the D0_unintialized state. It will have to be configured to bring it to the D0 state.