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Both my mouse and keyboard are backlit.

Normally the backlight is on when the peripherals are connected to a powered on computer and off when the peripherals are disconnected or are connected to a powered off computer.

But, in case of my newly purchased desktop, I noticed something surprising: even though it is switched off, the backlight blinks intermittently. Not sure how often yet, but not too often - at least not more often than every half and hour or so.

Does the motherboard sometimes give power to USB ports briefly even if the computer is off? Why does it happen?

The motherboard in question is ASRock H410M-HDV. The mouse was previously connected to various laptops - no such things were happening then.

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Normally the backlight is on when the peripherals are connected to a powered on computer and off when the peripherals are disconnected or are connected to a powered off computer.

This is incorrect. Modern desktops (1) never turn certain power rails off (unless they are mechanically disconected from AC power outlet or by a back switch), and (2) bus power on USB peripherals is never turned off as well (laptops may have a slightly different power policy). So, USB ports are "always ON".

The "always-on" parts of PC mainboard run on low-frequency clock and consume relatively low power. This low-power "embedded controller" is used to sense PC buttons, and to wake up the PC when keyboard or mouse are moved or touched. Without power being supplied all time, this function would be impossible. Embedded WiFi also might cause periodic pings.

However, when going to "computer [soft] off", USB peripherals are placed into SUSPEND mode. It could be "selective SUSPEND", or "global SUSPEND". In SUSPEND, USB peripheral goes into low-power state, and only a "wake-up" function is available. Usually any connect of disconnect on USB ports also generates the "wake-up" event.

If your peripherals blink some lights periodically, it only means that the OS is doing some background bookkeeping, probably collecting some "telemetry", or checking for pripherals presence. Embedded WiFi also might cause periodic pings. If the USB wakes up and goes back to sleep, it is likely some OS service/task is causing this.

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  • If the USB wakes up and goes back to sleep, it is likely some OS service/task is causing this. Is the OS active when the computer is supposed to be off? I thought at least this was down, only stuff like UEFI / Intel Management Engine / etc was active?
    – gaazkam
    Commented Jan 24, 2021 at 2:33
  • @gaazkam OS is not "active", but some services may set up wakeup timers. And "Intel Management Engine" could be activated by some obscure User Experience service. To test this theory, you need either to set up some hardware analyser, or activate and examine USB-related Event Trace Log (ETL), google for "USB trace log Microsoft" docs.microsoft.com/en-us/windows-hardware/drivers/usbcon/…
    – Ale..chenski
    Commented Jan 25, 2021 at 7:26

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