I have a Lenovo laptop. There is a feature called "Always On USB" which allows devices connected to the laptop via USB to be powered over USB even when the Operating system (Windows 10 in my case) shuts down.

I'm curious as to how this works exactly.

I can disable this feature from the BIOS menu. When I disable / enable this feature from the BIOS, what happens exactly?

Does it just do something electrically like completely disconnect a power line to a USB controller chip?

Or else is there some certain flash memory somewhere which holds information relating to the USB or power controller and when we disable/enable the feature, it writes the information there, which gets gets check on shut-down?

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2 Answers

Almost all the BIOS settings get stored in nonvolatile memory; this used to be SRAM with a small battery, attached to the real-time clock. It may be Flash now on your system.

There are a number of possible ways to implement this, and you'd have to check with Lenovo, but the obvious ones involve the parts of the laptop that are still powered when "off". These include:

• battery management processor (these are usually pretty limited)
• board management controller (BMC): more common in desktop PCs, this is used to do things like remotely administer the system from outside
• ACPI controller: the main suspect, this deals with all forms of sleep and power management.

ACPI in particular defines a number of "sleep" states in which the computer may appear "off" but has various subsystems powered up and listening. Often this is the Ethernet card for "wake on LAN".

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There must always be a 5V STBY voltage for sleep and other dynamic modes of wakeup. (WOL, ST, Mouse, KB etc)

This feature must use a non-volatile setting for the USB port to connect to the 5V STBY. This can be set in Software or disabled in BIOS.

"Click the "Start" button (or "Windows" logo ), then access the "Control Panel."

Click on "Classic View" if not already selected, then double-click "Power Options."

Click "Power Manager" on the left to access your ThinkPad battery's properties, then click on the "Global Power Settings" tab.

Click the box next to "Enable Always On USB," then hit "OK." You will now be able to charge devices through the ThinkPad's USB port even when it's turned off."

How can an Engineer not find this answer with the internet?

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