tailieunhanh - USB Complete fourth- P11

USB Complete fourth- P11: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 3 send by sending an ERDY Transaction Packet but still must wait for the host to request data packets. Availability All speeds allow interrupt transfers. Devices aren t required to support interrupt transfers but a device class may require it. For example a HID-class device must support interrupt IN transfers for sending data to the host. Structure A USB interrupt transfer consists of one or more IN transactions or one or more OUT transactions. Transferring data in both directions requires a separate transfer and pipe for each direction. On the bus interrupt transactions are identical to bulk transactions Figure 3-5 and Figure 3-6 with these differences Interrupt transactions have guaranteed maximum latency and thus different scheduling by the host. The host doesn t use the PING protocol in high-speed interrupt transfers. SuperSpeed interrupt transfers don t support Streams. On a SuperSpeed bus after receiving NRDY a host must wait for ERDY before resuming communications with an interrupt endpoint. Waiting is optional for bulk endpoints. An interrupt transfer ends successfully when the expected amount of data has transferred or when a transaction contains less than the endpoint s maximum packet size including zero data bytes. The USB specification doesn t define a protocol for specifying the amount of data in an interrupt transfer. When needed the device and host can use a class-specific or vendor-specific protocol to pass this information. High Speed Differences In an interrupt transfer on a high-speed bus with a low- or full-speed device the host uses split transactions for all of the transfer s transactions. Unlike high-speed bulk OUT transfers high-speed interrupt OUT transfers can t use the PING protocol when a transfer has multiple transactions. 76 A Transfer Type for Every Purpose SuperSpeed Differences The host schedules ACK Transaction Packets to an IN endpoint until the device has sent all of the transfer s data or the device returns a Data .