This document provides an overview of the Universal Serial Bus (USB) standard. It describes the key aspects of USB including its architectural overview as a tiered star topology, electrical and mechanical characteristics of USB connectors, the USB protocol layer and packet types, USB device framework including descriptors and data transfer types, and how USB interfaces are created. The document is intended to explain the fundamental concepts and specifications that define USB.

1. Universal Serial Bus
• SUBMITTED TO:
• DR. KAMRANULLAH
• SUBMITTED BY:
• MUHAMMAD MUZAFFAR KHAN
• KHAWAR NADEEM
• HAIDER ALI
1

2. Outline
 Introduction and Background
 Architectural Overview
 Electrical and Mechanical Characteristics
 Protocol Layer
 USB Device Framework
 USB Host Hardware and Software
 How to Create a USB Interface
 Works Cited
2

3. Introduction and Background
3

4. Introduction
 Industry standard that defines the cables, connectors and communications
protocols used in a bus for connection, communication, and power supply
between computers and electronic devices
 Developed in1996
 By Joint efforts of
 Intel, Microsoft, Digital Equipment Corporation, IBM, NEC, Nortel, Compaq
 USB has superseded and effectively replaced Serial, Parallel & PS/2 ports
4

5. Why USB?
 To standardize connections of electrical devices, while maintaining and
optimizing
 High speed
 Reliability
 Cost of manufacturing
 To make it fundamentally easier to connect external devices to PCs
 by replacing the multitude of connectors at the back of PCs,
 addressing the usability issues of existing interfaces,
 simplifying software configuration of all devices connected to USB,
5

6. Comparison of Speeds of USB
 Low Speed - 1.5Mbits/s (USB version 1.1/1.0)
 Full Speed - 12Mbits/s (USB version 1.1/1.0)
 High Speed - 480Mbits/s (USB version 2.0)
 High Speed – 4.8 Gbit/s (USB version 3.0)
 Super Speed – 10 Gbit/s (USB version 3.1)
6

7. Architectural Overview
7

8. Architectural Overview
 Host-controlled
 i.e. there can only be one host per bus.
 Tiered Star Topology
 similar to that of the 10BaseT Ethernet.
 Requires the use of a hub in arrangement.
8

9. Tiered Star Topology
 Uses 7-bit addressing of devices
 allowing connection of up to 127 devices on a single USB bus
 Allows expandability and ease of use,
 Each device can be handled and removed individually without interrupting
others.
 Allows plug’n’play connectivity which allows for dynamically loadable and
unloadable devices and drivers.
 Plug the device in and the host loads the drivers without needing a reboot or
initiation/termination of connection, etc.
 Unplug the device the absence is automatically detected by the host and the
drivers are unloaded.
9

10. Tiered Star Topology 10

11. Hub
 Most devices come with a hub built-into the main circuitry and expose
only the connector required to be plugged into the host (e.g. mouse,
keyboard, etc.).
 Hubs themselves are considered as USB devices, and may or may not have
some level of ‘intelligence’ circuitry.
 They are mainly required to increase the logical and physical fan-out of the
network.
 Devices attached to the Hub may work as any other USB devices, powered
by either the Hub or any external source
 Client, through software, communicates with every device as if it were
connected directly; i.e. through direct addressing
11

12. Hub 12

13. Electrical and Mechanical
Characteristics
13

14. Mechanical Characteristics
 There are commonly two kinds of USB connectors
 Type A
 Type B
 Many cables are made with Type A connector on one end and Type B
connector on the other.
 This is an attempt to prevent improper physical connections between
devices, as the connectors are not physically interchangeable.
14
Type A Type B

15. Pin Configuration 15
Type A Type B
PIN # Cable Color Signal
1 Red Vbus = +5V
2 White D-
3 Green D+
4 Black Ground

16. Electrical Characteristics
 All USB devices have an upstream connection
 All hosts have a downstream connection to the device,
 These Connections are not interchangeable.
 While Type A connectors on both ends are available in cables, two
computers should not be connected together using a USB cable.
 This is partly because of the fact that one topology can only have one host
 it is also to prevent damage to the interface: If two computers are connected to
each other using one cable, it would cause their 5V supplies to short and cause
damaging current flow. Two computers can, however, share a single peripheral
device using a hub.
16

17. Electrical Characteristics
 The parallel data is first serialized, then transmitted, and then parallelized
at the receiving end
 Non-return to zero invert (NRZI) scheme of encoding data
 includes a sync field required to synchronize host and receiver clocks.
17

18. USB – Voltage Levels
 Transmitted Side:
 Differential ‘1’ is transmitted by
 pulling D+ over 2.8V with a 15k ohm resistor pulled to ground
 D- under 0.3V with a 1.5k resistor pulled to 3.6V
 Differential ‘0’ is transmitted by
 D+ less than 0.3V
 D- greater than 2.8V (with the appropriate resistors).
 Receiver Side:
 Differential ‘1’ is received when D+ is 200mV greater than D-
18

19. USB – Logic Levels
 Low Speed/Full Speed Bus=>Characteristic Impedance=90Ω+/- 15%.
 Select Impedance matching series resistors for D+ and D- accordingly.
 The polarity of the signal is inverted depending on the speed of the bus.
 Terms ‘J’ and ‘K’ are used to signify the logic levels.
 In low speed
 J state is a differential 0.
 K state is a differential 1.
 In high speed
 J state is a differential 1.
 k state is a differential 0.
19

20. USB – Logic Levels
 High Speed (480Mbit/s) Mode uses a 17.78mA constant current for
signaling to reduce noise.
 USB device must indicate to the host whether it is using a low speed or
high speed connection.
 For low speed, D- is pulled to 3.3V
 For high speed D+ is pulled to 3.3V
20

21. Classes of USB Functions
 Low-powered Bus Functions
 Derive all the power from Vbus
 Cannot draw more than 1 unit load defined as 100mA
 Work between voltages of 5.25V down to 4.40V
 High-Powered Bus Functions
 Draw 1 unit load until they are configured, after which they can draw 5 unit loads
(500mA max).
 Work between 4.40V and 5.25V.
 Self-Powered Bus Functions
 Drain upto 1 unit load and consume the remaining power from an external source.
 In case the external source should fail, provisions must be in place to prevent more
than 100mA of current draw.
21

22. Caution!!!
 Precautions need to be taken in creation of devices, for which the USB
specification require that to prevent excessive inrush current should be
limited by not having a decoupling capacitance of less than 1uF or more
than 10uF.
 The total power draw of the device of 500mA max must include
 Currents drawn by the pull-up and pull-down resistors of D+ and D-,
 Current wasted by the voltage regulator in stepping down from 5V to 3.3V for
the device
22

23. USB Suspend Mode
 No Activity for 3ms  Suspend Mode
 Rated Suspend Current = 500uA for a device with one unit load.
 Suspend Current includes
 Idle current drawn by resistors and regulator  Design Constraint
 Where battery drain is not an issue, a Keep Alive packet must be sent
periodically to keep device from entering suspend mode.
 The time frames for these are given below.
 A high speed bus will have micro frames sent every 125us +/- 62.5ns
 A full speed bus will have a frame sent each 1ms +/- 500ns.
 A low speed bus will have a keep alive (end of packet) data sent every 1ms in
the absence of data transfer.
23

24. Protocol Layer
24

25. USB Packet Types
 Data is sent in packets Least Significant Bit (LSB) first.
 There are 4 main USB packet types:
 Token
 Data
 Handshake
 Start of Frame
 The packets are then bundled into frames to create a USB message
25

26. USB Packet Fields
 Each packet is constructed from different field types,
 SYNC (synchronize)
 PID (packet ID)
 Address
 Data
 Endpoint
 CRC (cyclic redundancy check)
 EOP (end of packet)
26

27. USB Packet Fields
 Synchronize (Sync):
 All packets must start with a sync field.
 Used to synchronize clock rate between device and host.
 It is 8 bits for low speed and 32 bits for full/high speed
connection.
 Packet ID (PID):
 Packet ID identifies the type of packet being sent.
 It has 4 bits for the value, and another 4 bits of the
inverted value to prevent errors.
 Address:
 7 bit field
 Specifies the device the packet is intended for out of 127
devices that can be connected to a single bus.
27

28. USB Packet Fields
 Endpoint:
 4-bit long,
 Allows for further flexibility in addressing.
 Can also be split for IN or OUT data.
 Cyclic Redundancy Checks (CRC):
 Performed on the data with the packet payload.
 All token packets have 5 bit CRC while data packets have 16 bit CRC.
 End of Packet (EOP):
 Signaled by a ‘0’ for approximately 2 bit times followed by a J for 1 bit time.
28

29. Token Packet Format
 In
 Informs the USB device that host wishes to read information.
 Out
 Informs the USB device that the host wishes to send information.
 Setup
 Used to begin control transfers
 Token packets have the following format:
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Sync PID ADDR ENDP CRC EOP

30. Data Packet Format
 Maximum data payload size for low-speed devices is 8 bytes.
 Maximum data payload size for full-speed devices is 1023 bytes.
 Maximum data payload size for full-speed devices is 1024 bytes.
 Payloads must be sent in multiples of bytes.
 Data packets have the following format:
30
Sync PID Data CRC EOP

31. Handshake Packet Format
 There are three types of packets which consist simply of PID.
 ACK:
 Acknowledgement that the packet has been successfully received.
 NACK:
 Reports that the device can not send or receive data. Also used to interrupt
when no data is available to send.
 STALL
 The device is in a state that requires intervention from the host.
31
Sync PID EOP

32. Start of Frame Packet Format
 Data is split into frames before being transmitted.
 The start of frame packet is used to signify the frame number to the host.
 The frame number field is 11 bits long.
32
Sync PID Frame Number CRC EOP

33. Device Framework
33

34. Virtual Pipes – Logic Channels
 USB device communication is based on Pipes or Logic Channels that connect
the peripheral's Endpoints with the host.
 Virtual Pipe is a Logical Connection from the host controller to a device.
 Since it is a logical connection, the pipe may be opened or closed.
 There are two types of pipes:
 Message Pipe:
 A message pipe is bi-directional and is used for control transfers. Message pipes are
typically used for short, simple commands to the device.
 Stream Pipe:
 A stream pipe is a unidirectional pipe connected to a unidirectional endpoint that
transfers data.
34

35. End-point
 The point of connection on the device.
 IN and OUT endpoints are different, not multiplexed.
 An endpoint is defined and numbered by the device during the time
period after physical connection, also known as enumeration time.
 The endpoint is permanent.
 An endpoint is addressable in the token packet.
 There are four bits used to address an endpoint, which translates to a
device having up to 15 endpoints
 endpoint 0 is reserved for control transfers throughout the duration the device
is connected to the bus, every peripheral device must support it
35

36. 36

37. Descriptors
 Data Structure to inform the host about the endpoint's configuration and
expectations.
 When establishing communications with the peripheral, each endpoint
returns a descriptor,
 Descriptors include
 transfer type,
 max size of data packets,
 interval for data transfers,
 the bandwidth needed.
37

38. Types of Descriptors
 Device Descriptor:
 This contains information that applies globally to the device, such as
 serial number,
 product ID,
 vendor ID, etc.
 This information is used to load device drivers on the host end.
 Configuration Descriptor:
 A Device Descriptor can have one or more Configuration Descriptors.
 These are used to inform the host about
 How the device is powered (bus powered, self powered, etc.),
 The maximum power consumption,
 What interfaces are available for the setup.
 The host can choose to read the entire hierarchy or just the Configuration Descriptor.
38

39. Types of Descriptors
 Interface Descriptor:
 These help to more finely group the endpoints,
 also contain information to help the host select a driver for communication
with the USB device.
 An interface descriptor defines one or more endpoints.
 Endpoint Descriptor:
 The endpoint descriptor is the last leaf in the configuration hierarchy
 It defines the bandwidth requirements, transfer type, and transfer direction of
an endpoint.
 For transfer direction, an endpoint is either a source (IN) or sink (OUT) of the
USB device.
39

40. Descriptors Hierarchy
 Every USB device has a hierarchy of descriptors, which are used to inform
the host about the type of device.
40

41. Descriptors Hierarchy
 Each device can only have one Device Descriptor.
 Multiple Configuration Descriptors can be used to group device functions;
however only one Configuration Descriptor can be active at a time.
 These are further divided into multiple Interface Descriptors, but similar to
Configuration Descriptors, only one can be active at a time.
 The Interface Descriptors can be used to group multiple endpoints for a
single function.
41

42. Data Transfer Types
 Control
 Isochronous
 Bulk
 Interrupt
42

43. Data Transfer Types
 Control:
 Control transfers are used to exchange configuration and commands between
the device and the host.
 Isochronous:
 Used by time-critical, streaming devices
 such as speakers and video cameras
 Fixed bandwidth and a Free connection to the host.
 These have almost guaranteed access to the USB bus.
 Data streams between device and host are real-time and hence error correction
is not used.
43

44. Data Transfer Types
 Bulk:
 Where timing is not critical
 Such as Printers and Scanners.
 Data is sent in bigger packets.
 Error correct is used on these packets.
 Interrupt:
 Data that need immediate attention.
 Such as mouse or a keyboard
 Small amount of Data Exchange
 Error checking validates the data.
44

45. Data Transfer Types
 The USB divides the available bandwidth into frames, and the host controls
the frames.
 Frames contain 1,500 bytes, and a new frame starts every millisecond.
 During a frame, isochronous and interrupt devices get a slot so they are
guaranteed the bandwidth they need.
 Bulk and control transfers use whatever space is left.
45

46. USB Host Hardware
46

47. USB Hardware Classes
 The host controller is part of the computer hardware.
 Control is managed by a device known as a Hardware Controller Device
(HCD) which is defined by the hardware implementer.
 USB defines class codes used to identify a device's functionality and to
load a device driver based on that functionality.
 This enables a device driver writer to support devices from different
manufacturers that comply with a given class code.
47

48. USB Device Classes
 The USB-IF Device Working Group defines a discrete number of device classes.
 The idea was to simplify software development by specifying a minimum set of
functionality and characteristics that is shared by a group of devices and interfaces.
 Devices of the same class can all use the same USB driver.
 This saves the end-user the time of installing a driver for every single USB device that is
connected to their host PC.
 Input devices such as mice, keyboards and joysticks are all part of the HID (Human Interface
Device) class.
 Mass Storage class covers removable hard drives and keychain flash disks
 However, a device does not necessarily need to belong to a specific device class. In
these cases, the USB device will require its own USB driver that the host PC must load to
make the functionality available to the host.
48

49. How to Create a USB Interface?
49

50. Microcontrollers
 Designing an USB Interface needs both hardware and software
competence
 Many commonly used microcontrollers support USB interfacing directly
 Microcontrollers allow the designer a higher level of abstraction when
dealing with the creation of a USB interface.
 Such controllers usually have dedicated pins which may be used at the
+5V, GND, V+ and V- for the USB pins readily.
 Commonly found cheaper microcontrollers are slow compared to the data
transfer rates of the USB protocol.
 Even the low speed USB interface (1.5 Mbit/sec) may be too high for some
controllers. A simple calculation goes as follows:
50

51. Speed Incompatibility Problem!
Microcontroller
 Say a microcontroller has 6M
instructions per second (at 24MHz
and 4 cycles/instruction),
 i.e. it needs 6 cycles to complete per
second.
USB
 For one bit of low speed USB
processing (i.e. 1.5 Mbit/sec) it would
require 6/1.5 = 4 clock cycles.
 This is simply too slow for USB, and
this is exactly the case with, for
example, the popular controller
PIC16F84.
51

52. Development Boards
 Prototyping and development boards such as the
 TI Beagleboard,
 NXP mbed,
 Arduino
 provide the designer with an even higher level of abstraction and make
USB interfacing almost trivial to hobbyists and designers who may not be
extremely proficient at writing complicated code.
 These development boards also come with higher quality components (the
BeagleBoard has an ARM core processor) which may be used for high
speed USB connections as well.
52

53. Device Drivers
 The host is what initiates the USB traffic. When the device is plugged in, the host detects
and connects to the peripheral hardware.
 The descriptor is what informs the host of the device specifications such as the vendor
ID and product ID (VID/PID).
 The USB devices can identify themselves as specific classes and subclasses of devices to
use the framework and drivers which already available by the USB Implementers Forum
(see more at www.usb.org) which have been implemented across many operating
systems.
 This prevents the designer from going through the sizable and needless effort of writing
his/her own device driver to interface with the host.
 USB devices that comply with a specific USB class enable cross-vendor and cross-
platform compatible USB devices.
 The USB specification specifies hundreds of device classes that enable the generic
implementation of, for example, Ethernet dongles, mixing desks, or flash disks and
enable operating systems to provide generic drivers for these classes.
53

54. Device Drivers
 There are cases where the USB device does not have any OS support and it should
interact with a user program directly. In that case, a generic driver such as the open-
source libusb driver that allows an application program to communicate with any USB
device can be used.
 The device will be advertised as vendor-specific. Through the libusb interface the user
program can detect a device with a VID and PID that it wants to interact with, claim an
interface, open an endpoint, and send IN and OUT requests to that endpoint.
 For connection of USB between a device and a computer, the operating system
dependent drivers are usually available. For instance, Microsoft Corporation has a
database of information pertaining to USB connection between many different types of
devices and their Windows operating system, which can be used for assistance in the
project.
 An excellent collection of info, sample source code, and interfacing using a PIC
microcontroller and generic PC USB drivers can be found their website.
54

55. Work Cited
 Stevens, Tim. USB 3.0 SuperSpeed... Engadget. [Online] January 10, 2010.
http://www.engadget.com/2010/01/09/usb-3-0-superspeed-gone-wild-at-ces-2010-
trumps-even-your-new-s/.
 Melissa J. Perenson. SuperSpeed USB 3.0: More Details Emerge. PCWorld. [Online]
January 6, 2009. http://www.pcworld.com/article/156494/superspeed_usb.html.
 Cohen, Peter. USB 3.1 vs Thunderbolt - the battle is on! iMore. [Online] August Thursday,
2013. http://www.imore.com/usb-31-spec-finalized-will-match-thunderbolt-1-speeds.
 EE Herald. Online course on Embedded Systems. EE Herald. [Online]
http://www.eeherald.com/section/design-guide/esmod14.html.
 Peacock, Craig. USB In A Nutshell. BeyondLogic. [Online] September Friday, 2010.
http://www.beyondlogic.org/usbnutshell/usb1.shtml.
 FTDI Chip. USB Data Packet Structure Version 1.0. FTDI Chip. [Online]
http://www.ftdichip.com/Support/Documents/TechnicalNotes/TN_116_USB%20Data%20
Structure.pdf.
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56. Work Cited
 Universal Serial Bus. Wikipedia. [Online] http://en.wikipedia.org/wiki/USB.
 USB Background. Total Phase. [Online] [Cited: February 16, 2014.]
http://www.totalphase.com/support/articles/200349256/.
 Muller, Henk. How To Create And Program USB Devices. Electronic Design.
[Online] July 27, 2012. http://electronicdesign.com/boards/how-create-
and-program-usb-devices.
 Microsoft Corporation. USB Host Drivers (Compact 2013). Microsoft
Developer Network. [Online] http://msdn.microsoft.com/en-
us/library/gg156249.aspx.
 picusb. Google Code . [Online] http://code.google.com/p/picusb/.
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