A Seminar on USB: Universal Serial Bus By Alok K' Watve M'Tech' IT 2nd yr'

1
A Seminar onUSB Universal Serial BusByAlok
K. WatveM.Tech. (IT) 2nd yr.
2

• Why USB?
• Much higher data rate compared to other data
  communication protocols (such as RS 232, parallel
  ports)
• USB 1.1 - 1.5 Mbps / 12 Mbps
• USB 2.0 - 1.5 Mbps / 12 Mbps / 480 Mbps
• Supports plug and play. Hence easy to operate.
• Allows large number of devices (theoretically
  127) to be connected.
• Support for real time audio/video data
  communication
• Low cost

3
Different types of devices using USB - Storage
devices (pen drives), CDROM drives - Digital
cameras - Speakers, microphones - Printers,
scanners - Keyboards, mice - Mobile phones
4

• History
• Developed by a group of seven companies -
  Compaq, Digital Equipment, IBM, Intel, Microsoft
  and Northern Telecom
• USB 1.0 released in 23rd September 1996
  supported two speeds
• Low speed 1.5 Mbps
• Full speed 12 Mbps
• USB 1.1 released in 15th January1998 No
  functional improvement. Clarification of certain
  timing parameters.
• USB 2.0 released in 27th April 2000
• High speed 480 Mbps
• protocol extension

5
USB Architecture USB host The root of the USB
communication model with which all other USB
devices communicate. USB function USB devices
like printers, cameras etc. which communicate
with the host. A single physical device may have
multiple functions. USB hub a special device
having a USB host or another hub on the upstream
port and a number of USB devices (or hubs) on the
downstream ports. USB follows tree topology in
which host is the root. Each hub is a center of a
local star. The tree can have at the most seven
layers(tiers) including host tier.
6
USB topology
Ref Universal Serial Bus specification revision
2.0 April 27 2000
7
Working of a typical USB system
Sequence of events 1. Bus enumeration 2.
Configuration 3. Communication
8
Logical view of the system
Host 2
Hub
Digital Camera
Speakers
9
Mechanical Specifications Deal with connector
plugs, sockets and wires. Original USB
specification defines only two types of
connectors - 1. Series A plugs are oriented
upstream and receptacles are oriented
downstream. 2. Series B plugs are oriented
downstream and receptacles are oriented
upstream. Recently smaller versions of the plugs
and sockets are designed and added to the
specification. These plugs are to be used in
portable devices like mobile phones.
10
Typical USB plugs and receptacles
11

• Electrical specification
• It defines the signaling, power distribution and
  physical layer specification.
• USB uses NRZ I signaling with bit stuffing
• USB cable contains 4 conductors. Two power
  conductors Vbus and GND. Two signal conductors D
  and D-
• Pull-up resistors are used to detect any USB
  device automatically. They also determine the
  speed (low/full/high) at which the device
  operates.
• For each speed, particular voltages at D and D-
  lines represent idle states. The resistor values
  are adjusted such that on attach , D and D-
  will have the desired voltage levels

12
Example of automatic attach detection for a
full speed device For a full speed device idle
state is represented by D High ( gt 2.8V) D
Low (lt0.3V)
Ref http//www.beyondlogic.org/usbnutshell/usb2.
htm
13
Example of automatic attach detection for a low
speed device For a full speed device idle state
is represented by D High ( gt 2.8V) D Low
(lt0.3V)
Ref http//www.beyondlogic.org/usbnutshell/usb2.
htm
14
Power considerations USB devices can be either
externally powered or bus powered. Bus powered
devices with low power rating cannot draw more
than 100 mA. Bus powered devices with high power
rating can draw at the most 500mA.. Each device
specifies its power rating (in terms of current)
during configuration. If there is no bus activity
for more than 3 ms, USB device goes in suspend
mode during which it can draw up to 500µA. It is
important for the host to keep track of power
ratings of all the functions because if total
power requirement goes beyond the driving
capacity of the host, it will no longer be able
to support all the devices. Host in such cases
either rejects a particular device or puts some
devices in suspend mode.
15
Frames Time slots are divided in frames. For high
speed signaling the frame width is 125µs. For
full speed signaling, the frame width is 1ms.
Frames are especially important in interrupt mode
data transfer. USB host transmits a Start Of
Frame (SOF) packet at the start of each frame.
Although low speed device do not receive any SOF
packet, they can see EOP (End Of Packet) signal
on the bus, which acts as a reference for
synchronization. SOF is used for two purposes
1. It keeps host and functions synchronized
with less than 0.05 error. 2. It prevents hosts
from going into suspend mode in the absence of
data.
16
USB protocol
Client Software
Function
USB system software
USB logical device
USB host controller
USB Bus interface
Logical communication Actual communication
17

• USB data transfer types
• Control transfer used for Control messages e.g.
  SETUP
• Interrupt transfer error free delivery,
  guaranteed latency
• Isochronous transfer no error correction , only
  for FS/HS modes, guaranteed bandwidth, bounded
  latency
• Bulk transfer error-free delivery, no guarantee
  of latency or bandwidth, only for FS/HS modes

Each logical pipe has an end point type that
identifies the transfer type supported by the
pipe. Each pipe is identified by an end point
identifier (ENDP). Every device has to support a
control pipe at address 0 (default pipe). Low
speed devices can have 4 pipes. Full and high
speed devices can have up to 16 pipes
18

• USB transactions
• Each transaction consists of
• A token packet
• An optional data packet
• A handshake packet
• Token packet format
• Data packet format
• Handshake packet format

Sync PID(8) Address(7) End
point(4) CRC(5) EOP
Sync PID(8) Data(0-8192)
CRC(16) EOP
19
In addition to these three packets a special
packet called Start Of Frame (SOF) is sent at the
start of each frame. The format for SOF packet is
as follows High speed devices see an SOF
packet with SAME frame number 8 times per frame
(a frame consists of 8 micro frame).
Sync Frame number(11) EOP
20

• Split transactions and data rate translation
• When a low/full speed device is connected to USB
  host through a hub, special transaction called
  split transaction precedes any transaction
  between the host and the low speed device.
• The sequence of events is as follows
• Start of Split token packet (host to hub)
• FS/LS token
• Data
• Handshake (Complete split packet handshake
  packet)

21
Ref Universal Serial Bus specification revision
2.0 April 27 2000
22
A complete taxonomy of packets and their PIDs
PID 0000 is reserved. PID 1100 is used twice
23
Revisit the example of the USB system
Sequence of events 1. Bus enumeration 2.
Configuration 3. Communication
24

• Device configuration during bus enumeration
• Host reads several descriptors from the device.
  Commonly used descriptors are
• Device descriptors(Only one for each device)
• Configuration descriptor(can be more than one)
• Interface descriptors (can be more than one per
  interface , a device can have multiple
  interfaces)
• Endpoint descriptors (One for each endpoint)
• String descriptors (Description of device in
  human readable form)
• These descriptors give host all the information
  needed to configure a device. Some parameters of
  the descriptors are given by the
  USB-Implementers Forum

25

• Recent developments
• USB OTG(on-the-go)
• Point to point connection
• Low power consumption
• Simplified cabling more connectivity options
• Wireless USB

26

• References
• Universal Serial Bus Specification revision
  2.0 April 2000
• http//www.beyondlogic.org/usbnutshell/
• http//www.sss-mag.com/usb.html
• http//www.usbdeveloper.com/
• USB implementers forum www.usb.org
• www.semiconductors.philips.com/markets/connectivit
  y/wired/usb/products/otg/tutorial/

27
Questions ?