TAILIEUCHUNG - USB Complete fourth- P31

USB Complete fourth- P31:This book focuses on Windows programming for PCs, but other computers and operating systems also have USB support, including Linux and Apple Computer’s Macintosh. Some real-time kernels also support USB. | 11 Human Interface Devices Using Control and Interrupt Transfers The human interface device HID class was one of the first USB classes supported under Windows. On PCs running Windows 98 or later applications can communicate with HIDs using the drivers built into the operating system. Because the HID class supports exchanging data for application-specific purposes many special-purpose devices use the HID class. Chapter 7 introduced the class. This chapter shows how to determine whether a device can use the human-interface class introduces HID-specific requests and discusses HID firmware options. Chapter 12 describes the reports that HIDs use to exchange information and Chapter 13 shows how to access HIDs from applications. 277 Chapter 11 What is a HID The name human interface device suggests that HIDs interact directly with people and many HIDs do just that. A mouse detects when someone moves it or presses a key. A host may send data that translates to an effect that a user senses on a joystick. Besides keyboards mice and joysticks devices with HID interfaces include remote controls telephone keypads game controls such as data gloves and steering wheels barcode readers and UPS units. Devices with physical control panels can use a HID interface to send control-panel input to the host. Devices with virtual control panels on the host can use a HID interface to send control-panel data to the device. A virtual control panel can be cheaper to implement than traditional physical controls on a device. A HID doesn t have to have a human interface. The device just needs to be able to function within the limits of the HID class specification. These are the major abilities and limits of HID-class devices All data exchanged resides in fixed-length structures called reports. The host sends and receives data by sending and requesting reports in control or interrupt transfers. The report format is flexible and can handle just about any type of data. A HID must have an interrupt IN

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