18 Configuration

Process

The Previous Chapter

The previous chapter introduced the concept of split transactions that allow high-speed hubs to support lowand full-speed devices without sacrificing large amounts of bus time to access the slower devices. The operation of the transaction translator was also described, along with the various forms of split transaction and the specific sequences employed by each.

This Chapter

This chapter provides an overview of the configuration process. Each of the major steps involved in USB device enumeration are defined and discussed.

The Next Chapter

This chapter discusses configuration of USB devices that are attached to any USB port. The process is virtually the same for devices of any speed. Device descriptors and other characteristics and features that relate to configuring the device are also detailed and discussed.

Overview

Host software is responsible for detecting and configuring all devices attached to the root hub. The process of identifying and configuring a USB device is commonly referred to as USB device enumeration. Device enumeration begins at the root hub. Each hub port must be reset and enabled in turn. When powered, the hub determines if a low-, full-, or high-speed device is attached or no device at all. If a device is present, status bits within the root hub are set to reflect device attachment. Software recognizes that a device is attached, enables the port, resets the USB device, assigns a unique address, and completes the configuration. This operation is performed for each port until all devices attached to the root hub have been identified and configured.

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USB System Architecture

The discussion in this chapter presumes that software has already initialized the USB host controller and that it is capable of generating USB transactions.

Different operating systems will define different software components involved in USB device configuration and will perform configuration in their own particular sequence. The following discussions identify the primary steps involved in the configuration process, but should not be interpreted as the specific sequence that is followed by a given host software solution. The following list specifies actions taken by host software and the root hub when configuring a device that is connected to a root hub port:

•Host requests power be applied to the ports, if not already powered.

•Hub detects device attachment and device speed, and sets status bits.

•Host polls hub and identifies that a device is attached.

•Host issues reset to port/USB device (minimum of 10ms). Chirp is performed during reset if a high-speed device is attached.

•Host checks hub status again if the device originally reported full-speed operation. This is done to see if the device is now operating at high speed.

•USB device now answers to default address (zero).

•Host performs GetDescriptor requests to fetch the standard descriptors that contain configuration information. These descriptors are parsed by software to determine the characteristics of the device. The configuration information includes bus power and bus bandwidth requirements, and device class information.

•Host assigns unique address to USB device.

•Host verifies that the USB resources needed by the device are available.

•Host issues a configuration value to the USB device specifying how it’s to be used. When the configuration value is received, the device assumes the characteristics that it reported via the descriptors. The device is now ready to be accessed by client software and can draw the amount of bus power described in the configuration.

The operations performed above are primarily accomplished via control transfers. Software uses each device’s control endpoint (endpoint zero) to access its descriptors and to configure the device. (The mechanisms used for performing control transfers to a device are described in the following chapters.)

The following sections provide an overview of the configuration process. More detail regarding device configuration can be found in the following chapters.

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