Universal Serial Bus Motivation
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- Detecting Device Attachment and Speed
- Detection
- NRZI Encoding
- Encoder
- Bit Stuffing
- Differential Pair Signaling
Signaling EnvironmentUSB serial data is NRZI encoded prior to transmission Non-Return to Zero Inverted Signaling is differential Helps to Mitigate affects of noise on lines Ensure data integrity Configured as follows: 
Detecting Device Attachment and SpeedPrior to transmission to client device Host software must detect presence of device on bus USB designed to automatically detect presence (absence) of device When attached to (removed from) USB Same mechanism used to determine speed of client device Low speed or full speed DetectionDetection accomplished by monitoring differential data lines After cable power applied to port When no device attached Pull down resistors on D+ and D- ensure both data lines near ground USB devices must provide pull-up resistor On either D+ or D- Based upon speed of device D+ for full speed D- for low speed C  onnections appear as When device attached Current flows through voltage divider Pull-down is 15 K Pull-up is 1.5 K Signal on data line increases to about 90% of Vcc When hub detects One of data lines approaching Vcc Other near ground Knows device attached By knowing which line is high Can ascertain which kind of device has been attached Thus when D+ or D- rises above 2.0 Vdc for greater than 2.5 sec Device detected as attached Removal detected when both lines Fall below 0.8 Vdc for greater than 2.5 sec When device attached or removed Status bits set or reset to indicate state Host software polls hub periodically to check for attachment or detachment
Encoded using NRZI encoding Helps to ensure high data integrity without requiring clock Such scheme has been used for many years S  cheme shown as Data Transitions in data stream represent 0s No transitions represent 1s
To correctly sample bits Data stream must be sampled within data window To detect whether transition has occurred since last bit time DecoderDecoder samples data stream during each bit time To check for transitions Transitions in data stream Permit decoder to maintain sync with incoming data Observe Long series of consecutive 1s Results in no transitions Eventually receiver will loose sync Solution Use bit stuffing Bit StuffingForces transitions into NRZI data stream When 6 consecutive 1s encountered Ensures receiver detects transition Every 7th bit time Enables receiver to maintain sync
Noise reduction etc As implemented in USB Uses half duplex scheme Device on either end can receive or transmit Only one direction at a time Half duplex scheme Requires drivers be placed into high impedance state When not transmitting data Two single ended receivers also used to detect bus states E  xample drivers look like the following Directory: 475 475 -> Annual ambassadors signing day 475 -> A. Steps in the Policy Analysis Process 475 -> John p affatica 7660 W. Massey Dr. Littleton, co 80128 475 -> Project proposal 475 -> Pianist Aaron Diehl To Receive Jazz Journalists Association Award At Atlanta Jazz Festival 475 -> Northwestern nj: Appalachian Ridge & Valley and Highlands Regions 475 -> Pattern of Abnormal Pap smears in developing countries: a report from a large referral hospital in Saudi Arabia using the revised 2001 Bethesda System Dr Layla s abdullah, md, frcpc 475 -> “For all the children to whom we entrust the future.” Download 205.96 Kb. Share with your friends:
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