Section 10: Wake Events

The Big Picture: Who Can Knock on the Door?

A sleeping person wakes for some knocks and ignores others: a fire alarm, yes; a passing car, no. A sleeping PC is the same. Only certain wake events are allowed to wake it, and Windows lets you see and control the list.

This matters because a PC that wakes by itself in the night (or never sleeps) usually has a wake event or a power request you did not know about.

10.1 What Can Wake a Sleeping PC

How it works inside: each wake-capable device uses standby power to watch for its event (Chapter 4's +5 V standby rail). When the event happens, the device raises a wake signal. The firmware and acpi.sys (Section 1) tell Windows which device woke the PC, using the _PRW data in the tables.

10.2 Wake-on-LAN

What it is: Wake-on-LAN (WoL) lets another computer switch a sleeping PC on across the network. The sending computer transmits a special message called a magic packet: a block of data that contains the target network card's MAC address (its hardware ID, Chapter 27) repeated many times. A network card on standby power listens for it. If the address matches, it wakes the PC.

Other PC / phone app                       Sleeping PC (standby power)
     |   magic packet (MAC repeated)          network card is listening
     |------------------------------------->  match -> WAKE SIGNAL
                                              PC wakes (S3/S4/S5)

What you need (all three):

Safety note: Wake-on-LAN has no password in its basic form, so it works only on trusted networks. If you do not use it, leaving it off is a good habit.

10.3 Wake Timers

What it is: A wake timer is a clock alarm set by a program or Windows to wake the PC at a given time.

Typical sources:

The power plan has a setting Allow wake timers (Section 7) with values like Enable, Disable, or Important wake timers only. If your PC wakes at the same time every night, a wake timer is the likely cause.

See them (read-only):

powercfg /waketimers

It lists active wake timers: which program set it and when it will fire, or says there are none. You may need to run it in an administrator window on some PCs (hedge).

10.4 Device-Initiated Wake (USB, Keyboard)

What it is: A device can wake the PC itself. For example, pressing a key on a USB keyboard sends activity on the USB port; the USB controller tells the system to wake.

Control it per device: Device Manager (Chapter 14) → open the device → Power Management tab → untick "Allow this device to wake the computer" (this is a setting change; for the lab, only look). USB selective suspend (Section 8) is separate: it lets idle ports doze, but it does not decide who may wake the PC.

10.5 Finding Out What Woke the PC (Read-Only Commands)

All of these only read. They do not change anything.

A simple detective path:

PC woke by itself?
   |
   +--> powercfg /lastwake         (who/what woke it)
   +--> powercfg /waketimers       (a scheduled alarm?)
   +--> powercfg /devicequery wake_armed   (which devices may wake it)
   |
   v
Then fix the cause: untick a device's wake option, or turn off the
task's "wake the computer" option, or set Allow wake timers = Disable.

Summary