TAILIEUCHUNG - USB Complete fourth- P8

USB Complete fourth- P8: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 Split Transactions A USB hub communicates with a USB host at high speed unless a USB hub is between the host and hub. When a low- or full-speed device is attached to a USB hub the hub converts between speeds as needed. But speed conversion isn t all a hub does to manage multiple speeds. High speed is 40x faster than full speed and 320x faster than low speed. It doesn t make sense for the entire bus to wait while a hub exchanges low- or full-speed data with a device. The solution is split transactions. A USB host uses split transactions when communicating with a low- or full-speed device on a high-speed bus. What would be a single transaction at low or full speed usually requires two types of split transactions one or more start-split transactions to send information to the device and one or more complete-split transactions to receive information from the device. The exception is isochronous OUT transactions which don t use complete-split transactions because the device has nothing to send. Split transactions require more transactions to complete a transfer but make better use of bus time because they minimize the time spent waiting for a low-or full-speed device to transfer data. The components responsible for performing split transactions are the USB host controller and a USB hub that has an upstream connection to a high-speed bus segment and a downstream connection to a low full-speed bus segment. The transactions at the device are identical whether the host is using split transactions or not. At the host device drivers and application software don t have to know or care whether the host is using split transactions because the protocol is handled at a lower level. Chapter 15 has more about how the host and hubs manage split transactions. Ensuring Successful Transfers USB transfers use status and control codes and error-checking to help ensure that data gets to its destination as quickly as possible and without errors. .

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