USB Universal Serial Bus

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USBUniversal Serial Bus

• Omid Fatemi
• Interface Design

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Motivations

• Connection of the PC to Telephone
• The USB provides a ubiquitous link that can be
  used across a wide range of PC-to-telephone
  interconnects.
• Ease of use
• Hot plug
• Port expansion
• The lack of a bi-directional, low-cost,
  low-to-mid speed peripheral bus has held back the
  creative proliferation of peripherals such as
  telephone/fax/modem adapters, answering machines,
  scanners, PDAs, keyboards, mice, etc.

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USB

• Fast
• Bi-directional
• Isochronous
• low-cost
• dynamically attachable serial interface
• consistent with the requirements of the PC
  platform of today and tomorrow

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Goals for USB

• The following criteria were applied in defining
  the architecture for the USB
• Ease-of-use for PC peripheral expansion
• Low-cost solution that supports transfer rates up
  to 12Mb/s
• Full support for real-time data for voice, audio,
  and compressed video
• Protocol flexibility for mixed-mode isochronous
  data transfers and asynchronous messaging
• Integration in commodity device technology
• Comprehension of various PC configurations and
  form factors
• Provision of a standard interface capable of
  quick diffusion into product
• Enabling of new classes of devices that augment
  the PCs capability.

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Taxonomy of Application Space
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Feature list

• Easy to use for end user
• Single model for cabling and connectors
• Electrical details isolated from end user (e.g.,
  bus terminations)
• Self-identifying peripherals, automatic mapping
  of function to driver, and configuration
• Dynamically attachable and re-configurable
  peripherals
• Wide range of workloads and applications
• Suitable for device bandwidths ranging from a few
  kb/s to several Mb/s
• Supports isochronous as well as asynchronous
  transfer types over the same set of wires
• Supports concurrent operation of many devices
  (multiple connections)
• Supports up to 127 physical devices
• Supports transfer of multiple data and message
  streams between the host and devices
• Allows compound devices (i.e., peripherals
  composed of many functions)
• Lower protocol overhead, resulting in high bus
  utilization

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Feature list (con)

• Isochronous bandwidth
• Guaranteed bandwidth and low latencies
  appropriate for telephony, audio, etc.
• Isochronous workload may use entire bus bandwidth
• Flexibility
• Supports a wide range of packet sizes, which
  allows a range of device buffering options
• Allows a wide range of device data rates by
  accommodating packet buffer size and latencies
• Flow control for buffer handling is built into
  the protocol
• Robustness
• Error handling/fault recovery mechanism is built
  into the protocol
• Dynamic insertion and removal of devices is
  identified in user-perceived real-time
• Supports identification of faulty devices

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Feature list (con)

• Relation with PC industry
• Protocol is simple to implement and integrate
• Consistent with the PC plug-and-play architecture
• Leverages existing operating system interfaces
• Low-cost implementation
• Low-cost sub-channel at 1.5Mb/s
• Optimized for integration in peripheral and host
  hardware
• Suitable for development of low-cost peripherals
• Low-cost cables and connectors
• Uses commodity technologies
• Upgrade path
• Architecture upgradeable to support multiple USB
  Host Controllers in a system

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USB System Description

• USB interconnect
• USB devices
• USB host.

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USB Interconnect

• Bus Topology Connection model between USB
  devices and the host.
• Inter-layer Relationships In terms of a
  capability stack, the USB tasks that are
  performed at each layer in the system.
• Data Flow Models The manner in which data moves
  in the system over the USB between producers and
  consumers.
• USB Schedule The USB provides a shared
  interconnect. Access to the interconnect is
  scheduled in order to support isochronous data
  transfers and to eliminate arbitration overhead.

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BUS Topology
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Physical Interface
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Throughput

• Low speed
• 1.5 Mbps
• Full speed
• 12 Mbps
• High speed (in USB 2.0)
• 480 Mbps
• Dynamic mode switching
• Clock encoding scheme NRZI (Non Return to Zero
  Invert)

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Power Distribution

• Devices
• Bus-powered devices
• Self-powered devices
• Power management
• Host based
• Power events
• Suspend
• Resume

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Bus Protocol

• Polled bus
• Data transfers initiated only by host controller
• Three packets
• Token packet
• Type
• Direction
• Address
• End point number
• Data packet
• Handshake packet
• ACK
• NAK

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Transfer Model

• Pipe
• Stream pipes
• No USB standard format
• Message pipes
• USB format
• Request
• Data
• Status
• End point zero
• Default control pipe
• Always exists

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Data Flow Types

• Control Transfers
• Used to configure a device at attach time and can
  be used for other device-specific purposes,
  including control of other pipes on the device.
• Bulk Data Transfers
• Generated or consumed in relatively large and
  bursty quantities.
• Interrupt Data Transfers
• Used for characters or coordinates with
  human-perceptible echo or feedback response
  characteristics.
• Isochronous Data Transfers
• Occupy a pre-negotiated amount of USB bandwidth
  with a pre-negotiated delivery latency. (Also
  called streaming real time transfers).

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USB Devices

• All have endpoint zero
• Hub
• Function
• Compound

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Typical Configuration
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Host Responsibility

• Detecting the attachment and removal of USB
  devices
• Managing control flow between the host and USB
  devices
• Managing data flow between the host and USB
  devices
• Collecting status and activity statistics
• Providing power to attached USB devices.

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Implementer Viewpoints

• USB Physical Device A piece of hardware on the
  end of a USB cable that performs some useful end
  user function.
• Client Software Software that executes on the
  host, corresponding to a USB device. This client
  software is typically supplied with the operating
  system or provided along with the USB device.
• USB System Software Software that supports the
  USB in a particular operating system. The USB
  System Software is typically supplied with the
  operating system, independently of particular USB
  devices or client software.
• USB Host Controller (Host Side Bus Interface)
  The hardware and software that allows USB devices
  to be attached to a host.

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USB Implementation Areas
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Physical / Logical Bus Topology
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USB Host / Device View
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USB Communication Flow
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Control Transfer Format

• Setup packet
• Data packet
• Acknowledge packet
• USB defined structure
• Setup packet 8 bytes
• Maximum data payload size
• Full speed 8, 16, 32 or 64 bytes
• Low speed 8 bytes

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Control Transfer Constraints

• Best effort
• 10 frame time for control
• Use also remaining 90 if no interrupt or
  isochronous transfer

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Full-Speed Control Transfer Limits
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Low Speed Control Transfer Limits
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Isochronous Transfers

• Guaranteed access to USB bandwidth with bounded
  latency
• Guaranteed constant data rate through the pipe as
  long as data is provided to the pipe
• In the case of a delivery failure due to error,
  no retrying of the attempt to deliver the data.
• Stream pipe
• Uni-directional
• Maximum data payload 1023 bytes
• Only in Full speed.
• No more 90 of frame for isochronous, interrupt

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Isochronous Limits
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Interrupt Transfers

• Guaranteed maximum service period for the pipe.
• Retry of transfer attempts at the next period, in
  the case of occasional delivery failure due to
  error on the bus.
• Max. Payload size
• Full speed 64 bytes
• Low speed 8 bytes
• If there is sufficient bus time, for requested
  payload size the pipe is established.

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Full Speed Interrupt Limits
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Low Speed Interrupt Limits
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Bulk Transfers

• Large amount of data.
• Access to the USB on a bandwidth-available basis.
• Retry of transfers, in the case of occasional
  delivery failure due to errors on the bus.
• Guaranteed delivery of data, but no guarantee of
  bandwidth or latency.
• Stream pipe
• Only in full speed
• Max. Payload only 8, 16, 32 or 64 bytes

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Bulk Transfer Limits
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Connectors
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Full Speed CMOS Driver
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Evaluation Setup
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NRZI Encoding
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Bit Stuffing
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Flow Diagram for Bit Stuffing
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Signaling Levels
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Signals

• Low and Full speed devices
• A differential 1 is transmitted by pulling D
  over 2.8V with a 15K ohm resistor pulled to
  ground and D- under 0.3V with a 1.5K ohm resistor
  pulled to 3.6V.
• A differential 0 on the other hand is a D-
  greater than 2.8V and a D less than 0.3V with
  the same appropriate pull down/up resistors.
• The receiver defines a differential 1 as D
  200mV greater than D- and a differential 0 as
  D 200mV less thanD-.
• The polarity of the signal is inverted depending
  on the speed of the bus. Therefore the terms J
  and K states are used in signifying the logic
  levels. In low speed a J state is a
  differential 0. In high speed a J state I s a
  differential 1.

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Differential and Single Ended

• Differential signals mostly for data
• Certain bus states by single ended signals on D,
  D- or both.
• For example a single ended zero or SE0 can be
  used to signify a reset if held more than 10ms.
• SE0 is holding down both D- and D below .3V
• Low and Full speed
• 90 Ohms /- 15

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Standard Chapters
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Selected Problems