Chapter 4: USB Cable Power Distribution

Self-Powered Devices

A self-powered device must provide the 1.5KΩ on its D+ line, indicating that it’s a full-speed device. Note that a self-powered device can be a low-speed device, however, the author seriously doubts that any will be built.

Power During Configuration

A device’s bus interface may be powered from bus power or solely from its local supply. This impacts whether a device will be detected when it is attached and whether its descriptors can be read. The following sections describe each case.

Locally Powered Bus Interface

A self-powered device may power its function controller from its local supply. Consequently, if the device is not plugged into AC, the 1.5KΩ resistor on the D+ line may not have Vcc available and the devices, when attached, will not be detected by the system.

It is also possible that the pullup resistor be powered by the bus, but that the device controller be powered by the local supply. In this instance, device attachment is detectable, but attempts by the host to read the device descriptors will fail. Specifically, host generated accesses to the default endpoint will result in no response from the device. The host detects the error condition but has no idea what caused the failure.

The dotted line in Figure 4-6 on page 90, running between the local supply and the device controller electronics, illustrates the option of powering the bus interface using the local power supply.

Hybrid Powered Device

A self-powered device may use bus power for its function controller, but must limit current draw to 100ma. The dotted line illustrating the bus power lines in Figure 4-6 indicates that bus power use is optional. Obtaining power from the bus makes it possible to detect the hub’s attachment and read its descriptors. Power for the function, however, is obtained from the local supply. This is termed a “hybrid” powered device.

89