3.2.4 USB Device Classifications

Learn to classify USB devices according to how they receive power.

Self-Powered

Devices that rely on their own power supply (e.g., they are plugged into an AC outlet) are self-powered devices (sometimes called active devices). USB 2.0 devices that draw more than 500 mA of power are required to be self-powered; USB 3.0 devices that draw more than 900 mA of power are required to be self-powered.

Bus Powered

USB cables have wires to carry both power and data. Bus-powered (sometimes called passive) devices get their power via the USB cable. Bus-powered devices are classified as low-powered or high-powered devices, depending on the amount of power they draw from the USB bus.

• Low powered devices use 100 mA or less
• High-powered devices use between 100 and 500 mA (up to 900 mA for USB 3.0)

Like USB devices, USB hubs can be bus-powered or self-powered. You can connect only low-powered or self-powered devices to a bus-powered hub. You cannot connect high-powered devices to a bus-powered hub. Therefore, self-powered hubs that provide 500 mA per port are recommended to ensure an adequate power supply to all bus-powered devices connected to the hub.

Make sure to install the software driver of the USB before attaching it to the system for automatic detection and configuration.

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