TAILIEUCHUNG - USB Complete fourth- P7

USB Complete fourth- P7: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. | Chapter 2 the received data and take any other required action. On receiving an IN packet if the endpoint has data ready to send to the host the hardware sends the data on the bus and typically triggers an interrupt. Firmware can then do whatever is needed to get ready to send data in the next IN transaction. An endpoint that isn t ready to send or receive data in response to an IN or OUT packet sends a status code. For SuperSpeed transactions the protocol differs as described later in this chapter. Pipes Connecting Endpoints to the Host Before data can transfer the host and device must establish a pipe. A pipe is an association between a device s endpoint and the host controller s software. Host software establishes a pipe with each endpoint address the host wants to communicate with. The host establishes pipes during enumeration. If a user detaches a device from the bus the host removes the no longer needed pipes. The host can also request new pipes or remove unneeded pipes by using control transfers to request an alternate configuration or interface for a device. Every device has a default control pipe that uses endpoint zero. The configuration information received by the host includes an endpoint descriptor for each endpoint that the device wants to use. Each endpoint descriptor contains an endpoint address the type of transfer the endpoint supports the maximum size of data packets and when appropriate the desired interval for transfers. Types of Transfers Devices with varied and differing requirements for transfer rate response time and error correcting can all use USB. Each of the four types of data transfers meets different needs. Each device can support the transfer types that are best suited for its purpose. Table 2-1 summarizes the features and uses of each type. Control transfers are the only type with functions defined by the USB specification. Control transfers enable the host to read information about a device set a device s address and select .

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• sổ tay phần cứng
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