TAILIEUCHUNG - USB Complete fourth- P47

USB Complete fourth- P47: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 18 Test_Packet Value. 04h. Action. Repetitively transmit the test packet defined by the USB specification. Purpose. Test rise and fall times eye pattern jitter and other dynamic waveform specifications. Test_Force_Enable Value. 05h. Action. Enable downstream-facing hub ports in high-speed mode. Packets arriving at the upstream-facing port are repeated at the port being tested. The disconnect-detect bit can be polled while varying the loading on the port. Purpose. Measure the disconnect-detection threshold. Other Values Test-mode values 06h through 3Fh are reserved for future standard tests. Values C0 FFh are available for vendor-defined tests. All other values are reserved. SuperSpeed SuperSpeed s fast dual-simplex interface and new power-management capabilities require different encoding packet formats and low-level protocols. A SuperSpeed transmitter scrambles and encodes data to be sent on the bus. A SuperSpeed receiver decodes and de-scrambles the received data. Data Scrambling Data scrambling eliminates repetitive patterns in the data. Doing so spreads the radiated EMI over a wider frequency spectrum and helps in meeting FCC requirements. To scramble data to be transmitted a free-running linear feedback shift register implements a polynomial defined in the USB specification. The transmitter XORs the output of the shift register with the data bits. Descrambling uses a complementary mechanism to recover the unscrambled data. Encoding SuperSpeed uses 8b 10b data encoding as specified in ANSI INCITS 230-1994. Other interfaces that use this encoding include PCI Express Gigabit 436 Packets on the Bus Ethernet and IEEE-1394b. The encoding converts each byte value to a 10-bit Data Symbol for transmitting. The encoded data has no more than five ones or zeroes in series and contains equal numbers of ones and zeroes over time. As with USB data frequent transitions enable the receiver to synchronize with the transmitted data without requiring a separate .

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