USB

1
USB

• USB Universal Serial Bus
• Industry-led open standard
• USB Implementers Forum USB-IF
• Intel, Microsoft, HP, Compaq
• Evolution
• USB 1.0 (1996)
• USB 1.1 (1998)
• USB 2.0 (2000)

2
Why USB?

• Shortcomings of existing PC IO
• Scarcity of PC HW resources (IRQ and DMA
  channels, IO space)
• Serial Port/Parallel Port Single point-to-point
  connections
• Diversity of connectors and cables.
• Non hot-pluggable. Non-PnP
• Poor data bandwidth
• Cost of cables / connectors
• No power delivery through interface

3
Why serial? Why not parallel?

• Advantages of serial buses
• Signal IntegrityFewer coinciding signal
  transitions. Less crosstalk. Less noise. Less
  inter-signal skew.
• CostDrivers, connectors, cables, receivers
• Disadvantage of serial buses
• Higher clock-rates required for given bandwidth

4
External Bus Options
BANDWIDTH
APPLICATIONS
ATTRIBUTES
DEVICE COST
STD FEATURE
Very Low cost Ease of Use Lots of fanout
LOW 10 - 100Kb/s
5-25
1997
Input Devices Control Functions
USB
MEDIUM 200K - 10Mb/s
15-150
1997
Telephony/ Modem Audio, Scanner
Low cost Guaranteed latency
1394A for CE
HIGH 100 - 400Mb/s
100-500
TBD
Entertainment, A/V Imaging
Peer-to-peer Multiple channels
1394B Gigabit
COMPUTE 1 Gb/s
200-500
1999
Primary Disk Home Backbone
Very High bandwidth Fiber capability
USB Focus on Low Cost,High Volume Applications
USB Ver 2.0 Supports 480Mb/s
5
PC Connectivity Vision
USB in 1996 Initially introduced as an
incremental connector for new applications.
USB
Keyboard
Serial Port
Sound/Game Ports
Modem
LAN
Mouse
Parallel Port
Graphics Port
SCSI Port
USB Future The PC evolves into a simpler, easier
to use appliance.
Telephony, Modem, Kyb, Mouse, Joystick, Still/
Motion Camera, Digital Audio, Backup
Store, Printer, Scanner, Wireless Adaptors
Graphics Port
LAN
6
Universal Serial Bus (USB) - Introduction

• Complex, hierarchical standard
• Requires software layers both on the host
  computer and on the USB device
• Serial Protocol and Physical Link
• Hierarchy PC is the host
• Upstream points towards the host
• Downstream points away from the host

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Introduction (2)

• Data transmitted serially
• Data transmitted differentially on a pair of
  wires (D and D-)
• 2 other wires are used to supply power to USB
  devices
• USB devices may be Bus Powered or Self-Powered
• USB includes specs on power consumption and
  voltage drops etc

8
2 Types of USB Cable

• High Speed Cables
• Shielded, jacketed - use twisted pair wiring
• Support max data rates of 12Mb/s
• Support Cable lengths of 5m
• Low Speed cables
• Not shielded, pairs not twisted, cheaper
• Support 1.5Mb/s
• Support Cable lengths of 3m

9
USB Connectors

• Connectors
• 4-Position with shielded housing
• Positive Retention
• Blind Mating Capabilities
• Cables
• 28 AWG twisted pair for signalling
• 20-28 AWG pair for power
• Shielding for fully rated segments

10
Type A B connections
Type A Connector connects to Upstream Ports Type
B Connector connects to Downstream Ports Each USB
Cable has a Type A Type B Connector
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Power Distribution

• Significant capability of USB
• Eliminate wall adaptors
• Hubs may be self-powered or bus-powered
• Two current levels 100 500 mA
• Overcurrent protection for safety
• Wire gauge options 20-28 AWG

12
Data Signalling

• Bi-directional, half-duplex link
• Embedded clock and dataNRZI Non Return to
  Zero, InvertedNRZI with Bit Stuffing
• Differential signal pair

13
USB Bandwidth

• USB 1.0 / 1.1
• 12 Mb/s Full Speed (FS) bit rate
• 1.5 Mb/s Low Speed (LS) bit rate
• USB 2.0 (May 2000)
• additionally 480 Mb/s High Speed (HS)
• Applications USB HDD. Video

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Connections and Terminations
R2
D
D
F.S./L.S. USB Transceiver
F.S. USB Transceiver
R1
D-
(45O Outputs)
ZO 90O15 5 Meters Max.
(45O Outputs)
D-
Hub Port 0 or Full Speed Function
R1
Host or Hub Port
R1 15KO5 R2 1.5KO5
D
D
F.S./L.S. USB Transceiver
L.S. USB Transceiver
R1
R2
D-
(45O Outputs)
3 Meters Max.
(45O Outputs)
D-
R1 15KO5 R2 1.5KO5
R1
Host or Hub Port
Low Speed Function
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USB Transceiver

• Differential Driver
• Slew rate controlled
• SE0 drive capability
• Differential Receiver
• Sensitivity lt200mV
• Common mode range lt 1.0V to gt 3.0V
• Single-Ended Receivers
• Threshold 0.6V - 1.5V

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Tiered Star Topology
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Tiered Star Topology

• Tiered Star Topology
• Distributed connectivity points
• Up to 6 Tiers
• Up to 5m cable length per segment
• Up to 127 Devices

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Hosts, Hubs, Devices

• Only one host per system usually the PC
• Host (Root Hub) is the USB system master
• Controls and schedules all communications
• Hubs are communication nodes that interconnect
  devices
• Peripherals controlled by the USB bus are slaves
  that respond to host commands
• Peripherals are called USB devices or functions

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Hubs

• USB Devices themselves
• Bridges that increase logical and physical
  fan-out of the network
• Single Upstream connection
• To Root Hub (host) or next hub nearer host
• One or more downstream connections
• Hubs detect connection/removal of USB devices
  Topology Changes

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USB Hub Function

• Port Control
• Connection detect
• Port Enable/ Disable
• Reset/ Resume Signaling
• Data Switch
• Signal Regeneration
• Robustness/ Recovery
• Power Distribution

21
Enumeration

• Process that occurs when a new peripheral is
  connected to the USB network
• Host communicates with the peripheral to find out
  what device driver is needed
• Host supplies a device address during enumeration
• Hence no switches to set up addresses

22
Hubs in logical connection

• After Enumeration the hub becomes logically
  invisible to the application
• Application code sees a logical connection
  directly from the host to each USB device

23
USB Pipes

• USB Communication uses the idea of a Pipe
• The USB connection consists of a big pipe
  (12Mb/s) and up to 127 small pipes
• Each of the small pipes connects to a USB device
• Each small pipe can connect to a smaller pipe
  (tiny pipe) up to 16 tiny pipes per small pipe

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USB Pipes Analogy With Physical Pipe
25
Addresses and Endpoints

• USB Token has 7 address bits per USB device
• Address 0 USB Default address is used for all
  devices when first attached
• Hence 127 USB Devices (small pipes)
• 4 Endpoint bits in a USB token allow up to 16
  pairs of tiny pipes per small pipe
• Endpoint 0 just for Control transfers
• USB Tokens also identify IN or OUT direction
• 15 bi-directional sources or receivers of data
  per USB Device identified by Endpoints

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USB Pipes and Endpoints

• All USB transfers occur through virtual pipes
  associated with Endpoints
• Each Pipe may have different bandwidth and may
  require different types of data transfer
• Each endpoint associated with a descriptor
• Type of Data Transfer
• Max size of data packets
• Interval for transfer, bandwidth needed etc

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Endpoints

• Device may have a number of endpoints
• E.g. Different Endpoint (and pipe) for
• Data
• File transfers low error rate necessary
• Voice
• Error tolerant but not delay tolerant
• Control
• Low error rate but lower data rate

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Basic USB Model
Host
Device
Interconnect
Capability
Function
Client SW
USB Logical Device
Provides common device abstraction
USB Sys SW
USB Bus
USB Bus
Physical Interface, Signalling
Interface
Interface
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Three layers in USB model

• Layers
• Function
• USB Device
• USB Bus Interface Layers
• Comments
• Based on 7-layer OSI Model
• Entities at the same layer have a virtual
  peer-to-peer connection
• Entities at lower layers provide services to
  entities at next higher layer
• Progressively hides details from higher layers

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USB Host Layers

• USB Bus Interface
• Physical interface to wire
• Manages low-level protocol
• USB System Software
• Provides interfaces to driver layers
• Manages standard device objects
• Client Software
• E.g. device drivers
• Manages capability

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USB Device Layers

• USB Bus Interface
• Physical interface to wire
• Manages low-level protocol
• USB Logical Device
• Defines common view of device by host
• Manages high-level protocol
• Function
• Represents capability delivered bythe device

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Detailed Host/Device View
33
Client Software lt-gt Function
Client Software
Host
Buffers
Data Flows
Pipes
Endpoints
USB Device
Interface
34
USB Data Transfer Types

• USB supports four transfer types
• Control
• Bulk
• Interrupt
• Isochronous

35
Transfer Type Control

• Control
• Exchange configuration, set-up and command
  information between the device and the host
• CRCs are used for error checking as error-free
  transmission is critical
• Re-transmission initiated when errors are
  detected
• Control endpoints are implemented on Endpoint 0
  pair
• Control transfer has 2 or 3 stages setup stage,
  optional data stage, status stage

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Transfer Type Bulk

• Bulk Transfers
• Large amounts of data where data is not
  time-critical
• Error free transfer important hence CRC
  error-checking implemented
• Claim unused bandwidth when nothing more
  important is going on
• Typical applications scanners, printers

37
Transfer Type Interrupt

• Interrupt
• Not interrupts in normal sense!
• Unidirectional - only inputs to the host
• Small data transfers that occur infrequently
• Host polls polls interrupt endpoints to see if
  they have any data to send
• Full speed devices 1ms - 255ms polling
• Low speed devices 10ms - 255ms
• Error checking validates the data
• Mice and keyboards

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Transfer Type Isochronous

• Isochronous
• Unidirectional or bidirectional
• For time sensitive information, e.g. audio or
  video streaming
• System must be able to tolerate some errors
• No time for error checking
• Guaranteed access to USB with bounded latency
• No Retries
• Max packet size for isochronous transfer is
  1023B/frame
• Max isochronous bandwidth is 8.184Mb/s

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Communication Layers (1)

• Frame
• Time between SOF tokens 1ms
• Consists of a series of transactions
• Transfer
• One or more transactions to move data between
  client software and the function
• Packet
• Bundle of data organised in a set format for
  transmission
• Types Token, Start of Frame, Data, Handshake,
  HS/LS Preamble
• Typically has Control information, Data to be
  transferred and Error detection and correction

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Communication Layers (2)

• Transaction
• Grouping of no more than 3 packets
• Host initiates a token packet according to
  schedule
• PID identifies transaction type

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USB Packets 4 types

• Packet
• Bundle of data organised in a set format for
  transmission
• Typically has Control information, Data tobe
  transferred and Error detection and correction

• Token
• SOF (Start of Frame)
• Data
• Handshake
• HS/LS Preamble (optional)
• All packets have
• Sync field to lock the Phase Locked Loop
• Followed by Packet Identifier (PID)
• Also address, frame number or data

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Token Packet
43
Packet Identifier
44
SOF Packet
SOF packets are sent by the host once every
1.00ms /- 0.05ms
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Data Packet
Devices respond to requests from the Host Host
sends an IN and device endpoint responds with a
packet OR Host sends an OUT followed by a data
packet Bulk, Control, Interrupt host responds
with an Acknowledge Isochronous no handshake
packet
46
Handshake Packet
Special Packet
Host sends a PRE (preamble) packet when it wants
to communicate with 1.5Mb/s devices
47
USB Frame Model
48
USB and the PC

• Windows 95 no USB support until OSR2.1
• No longer officially supported
• Windows 98 contains USB drivers for common
  applications
• Windows NT no USB support
• Windows 2000/XP provides USB support for common
  applications