TAILIEUCHUNG - USB Complete fourth- P9

USB Complete fourth- P9: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 Table 2-8 The Data Packet Header provides the Data Packet s length and other information. Reserved fields not shown. Field Bits Function Type 5 Data Packet Header 01000 Route String or reserved 20 In downstream communications used by hubs to route a packet to the correct port. Otherwise reserved. Device Address 7 The device that is the source or receiver of the Data Packet. Sequence Number 5 Identifies the Data Packet. End of Burst EOB non-isochronous or Last Packet Flag LPF isochronous 1 For non-isochronous IN endpoints identifies the last packet in a burst. For non-isochronous OUT endpoints zero. For isochronous endpoints identifies the last packet in a service interval. Direction 1 0 host to device 1 device to host. Endpoint Number 4 The endpoint that is the source or receiver of the Data Packet. Setup 1 Set by the host when the Data Packet is a Setup packet. Data Length 16 The number of data bytes in the Data Packet Payload. Stream ID or reserved 16 For bulk endpoints can identify a stream. Packets Pending 1 Set by the host when it has another packet scheduled for the target endpoint. CRC-16 16 For error checking. Link Control Word 16 For link management. Bulk and interrupt endpoints increment the Sequence Number for every transaction resetting to zero on rollover or after completing a Set Configuration Set Interface or Clear Feature ENDPOINT_HALT request. In isochronous transfers the Sequence Number resets to zero at the start of a service interval and increments on each additional Data Packet within the service interval. The endpoint descriptor specifies the length of a service interval and the maximum number of Data Packets per service interval. On detecting an error in a received Data Packet the host or device sends an ACK Transaction Packet with the Retry bit set and the Sequence Number of the packet that contained the error. The sender of the Data Packet must then resend all sent Data Packets beginning with that Sequence Number. 56 Inside USB .

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