1. USB : Universal Serial Bus
A Seminar on USB: Universal Serial Bus By Alok K. Watve M.Tech. (IT) 2nd yr.
11-Nov-18
USB : Universal Serial Bus

2. USB : Universal Serial Bus
Why USB?
Much higher data rate compared to other data communication protocols (such as RS 232, parallel ports)
USB Mbps / 12 Mbps
USB 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
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USB : Universal Serial Bus

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

4. USB : Universal Serial Bus
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
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USB : Universal Serial Bus

5. USB : Universal Serial Bus
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.
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USB : Universal Serial Bus

6. USB : Universal Serial Bus
USB topology
Ref: Universal Serial Bus specification revision 2.0 April
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USB : Universal Serial Bus

7. USB : Universal Serial Bus
Working of a typical USB system
Speakers
Full speed
Hub
Digital camera
High speed
Key board
Low speed
Sequence of events:
1. Bus enumeration
2. Configuration
3. Communication
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USB : Universal Serial Bus

8. USB : Universal Serial Bus
Logical view of the system
Host 1
Keyboard
Host 2
Hub
Digital Camera
Speakers
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USB : Universal Serial Bus

9. USB : Universal Serial Bus
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.
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USB : Universal Serial Bus

10. USB : Universal Serial Bus
Typical USB plugs and receptacles
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USB : Universal Serial Bus

11. USB : Universal Serial Bus
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
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USB : Universal Serial Bus

12. USB : Universal Serial Bus
Example of automatic ‘attach’ detection for a full speed device:
For a full speed device idle state is represented by
D+ : High ( > 2.8V)
D– : Low (<0.3V)
Ref :
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USB : Universal Serial Bus

13. USB : Universal Serial Bus
Example of automatic ‘attach’ detection for a low speed device:
For a full speed device idle state is represented by
D– : High ( > 2.8V)
D+ : Low (<0.3V)
Ref :
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USB : Universal Serial Bus

14. USB : Universal Serial Bus
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.
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USB : Universal Serial Bus

15. USB : Universal Serial Bus
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.
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USB : Universal Serial Bus

16. USB : Universal Serial Bus
USB protocol
Client Software
Function
USB system software
USB logical device
USB host controller
USB Bus interface
Logical communication
Actual communication
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USB : Universal Serial Bus

17. USB : Universal Serial Bus
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
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USB : Universal Serial Bus

18. USB : Universal Serial Bus
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
Sync PID(8) EOP
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USB : Universal Serial Bus

19. USB : Universal Serial Bus
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
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USB : Universal Serial Bus

20. USB : Universal Serial Bus
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)
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USB : Universal Serial Bus

21. USB : Universal Serial Bus
Ref: Universal Serial Bus specification revision 2.0 April
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USB : Universal Serial Bus

22. USB : Universal Serial Bus
A complete taxonomy of packets and their PIDs
Packet Type
PID
Token
OUT
0001
ACK
0010
Handshake IN
1001
NAK
1010
SOF
0101
STALL
1110
SETUP
1101
NYET
0110
Data
DATA0
0011
PRE
1100
Special
DATA1
1011
ERR
DATA2
0111
SPLIT
1000
MDATA
1111
PING
0100
PID 0000 is reserved. PID 1100 is used twice
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USB : Universal Serial Bus

23. USB : Universal Serial Bus
Revisit the example of the USB system
Speakers
Full speed
Hub
Digital camera
High speed
Key board
Low speed
Sequence of events:
1. Bus enumeration
2. Configuration
3. Communication
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USB : Universal Serial Bus

24. USB : Universal Serial Bus
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-Implementer’s Forum
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USB : Universal Serial Bus

25. USB : Universal Serial Bus
Recent developments
USB OTG(on-the-go)
Point to point connection
Low power consumption
Simplified cabling more connectivity options
Wireless USB
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USB : Universal Serial Bus

26. USB : Universal Serial Bus
References
Universal Serial Bus Specification – revision 2.0 – April 2000
USB implementer’s forum –
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USB : Universal Serial Bus

27. USB : Universal Serial Bus
Questions ?
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USB : Universal Serial Bus