Chapter 4: USB Cable Power Distribution

Figure 4-3: Low-Power USB Function

High-Power Devices

High-power devices consume more than 100ma but cannot draw more than 500ma of current from the bus. High-power devices can be designed with their own local power supply to eliminate illegal configurations.

Power During Configuration

Figure 4-4 illustrates a bus-powered device connected to a fully rated port. When bus power is initially applied to a USB device, it must not consume more than a single unit of power, or 100ma, until the device is configured. Consequently, the application of power is staged such that power is applied only to the function controller until the device has been configured. Once configured, maximum current can be drawn by the device.

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USB System Architecture

Insufficient Port Power

If a high-power device that is powered solely by the bus is connected to a buspowered hub, there may not be sufficient current for the device to work properly. Host software must detect the amount of current that the high-power function requires by reading its configuration descriptor. If the current requirements of the device exceed the current available at the port, the device should not be configured, and the deficiency should be reported to the user. Table 4-3 on page 86 shows a portion of the configuration descriptor definition that defines the maximum bus power consumed by a device once it’s configured (shaded area).

Note that a high-power device may provide an alternate configuration that reduces the total power consumed to 100ma, thereby ensuring compatibility with any USB port. The low-power configuration may however, lead to reduced performance and/or functionality when compared with the full-power configuration.

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Chapter 4: USB Cable Power Distribution

Figure 4-4: Bus-Powered Function (High Power)

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