USB System Architecture

USB Driver (USBD)

The USB Driver is involved in the following functions:

•Configuration Management

•Bus Management (tracking and allocating bus bandwidth)

•Data Transfer Management

•Providing Client Services (the USBDI)

Each of these USBD functions is described below.

Configuration Management

Host software must support automatic configuration of USB devices. The actual software elements involved in configuration and the sequence of steps taken to perform the configuration vary. Configuration begins with the root hub and proceeds one port at a time until all devices have been detected and configured.

Configuration activities rely on access to the default control endpoint within each device. The USBD initializes the default control pipe before configuration begins. This pipe is used repeatedly during the configuration process. The actual software components involved in configuration vary depending on the implementation; however, conceptually the components involved can be thought of as the:

•hub client driver

•configuration software

•USBD

USB Elements Requiring Configuration

USB configuration involves configuring three distinct elements:

•Device Configuration — Device configuration involves accessing the device’s descriptors, determining the USB resources required by the device (i.e., power and bus bandwidth), allocating these resources by establishing communications pipes for each endpoint, and assigning the configuration value for the selected configuration. Device configuration is discussed in Part Five of this book.

•USB Configuration — The communications pipes established during device

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