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TB054 Datasheet(PDF) 2 Page - Microchip Technology |
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TB054 Datasheet(HTML) 2 Page - Microchip Technology |
2 / 10 page TB054 DS91054C-page 2 2004 Microchip Technology Inc. TYPES Descriptors general to USB are discussed in Chapter 9 of the USB Specification v1.1. The general descriptors that pertain to all USB devices are the following: • Device descriptor – describes the most general information about a USB device. For instance, it communicates to the host the product and vendor ID numbers. It tells the host how many configura- tions the device has, and how many configuration descriptors the host must request from the device. A device can have only one device descriptor. • Configuration descriptor – describes information about a specific device configuration. Included in the configuration descriptor, is the number of interfaces under the specific configuration. In essence, an interface is a feature, therefore, a configuration can be viewed as a collection of peripheral features. Multiple configurations can exist for one device. For instance, if a Power Save mode is desired for a part, it may have two configurations, Normal mode and Power Save mode. • Interface descriptor – tells how many endpoints a device feature uses. It also declares the class identity of a device. For the game pad, the class identity is HID. • Endpoint descriptor – describes the properties of an endpoint. These properties are namely whether the endpoint is an IN or OUT endpoint and what the endpoint number is. Every endpoint specified in an interface has its own endpoint descriptor. Descriptors specific to the HID class are discussed in the “Device Class Definition for HID ” document. The HID descriptors that pertain to describing the game pad are the following: • HID descriptor – for a Human Interface Device, a HID descriptor immediately follows the interface descriptor and precedes the endpoint descrip- tor(s). This is done because the HID descriptor may be associated with more than one endpoint, and as a result, is higher in the “descriptor hierar- chy” than endpoint descriptors. A HID descriptor identifies additional descriptors specific to the HID class, namely report descriptors and physical descriptors. Report descriptor – specifies the data format for a device. The information that each bit represents in a data packet is defined in the report descriptor. For the game pad, certain bits correspond to the logic output of its buttons. Other bit groupings correspond to the X and Y-axis positions of the directional pad. Note: The game pad example does not have a physical descriptor associated with it. Physical descriptors are never necessary, they are used to describe the physical aspects and appearance of a device mainly for an engineer's reference. Note: The USB Implementers Forum (www.usb.org) is the main web location for obtaining the USB Specification, Device Class Definition for HID, HID Usage Tables, and all other defining documents pertaining to USB. “USB Complete”, written by Jan Axelson, presents the USB protocol in a easy to understand format. |
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