Universal Serial Bus Motivation

Device Framework or Layers

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Device Framework or Layers

Protocol stack supports 3 logical layers

Describe relationship between

Host hardware and software

Corresponding view exists in each USB device

Separate layers provided to

Simplify understanding of USB communication

Let’s examine

USB Bus Interface Layer

Provides for low level transfer of data over USB

This is the physical layer in stack

Consists of

  • Physical connection

  • Electrical signaling environment

  • Packet transfer mechanism

Layer represents actual transfer of data across USB cable

Between host system and USB devices
Host side comprises

Host controller and hub

Client side comprises

USB interface within the device

USB Device Layer

USB Device layer

Represents portion of USB

Comprehends actual USB

Communication mechanism

Nature of transfers required by USB functional device
Layer comprises

USB system software on host side

Logical view of USB device on client side

USB views logical device as collection of endpoints

That comprise a functional interface
USB system software provides services needed

To interface client software with its USB function

Has specific knowledge of USB transfer mechanisms

Must allocate bus bandwidth to collection of USB devices

Logical USB device represents collection of endpoints

Through which client communicates with its function

System software views endpoints via device descriptors

Such descriptors parsed during configuration

To obtain transfer characteristics of given device
Descriptors and system software’s knowledge of USB transfer mechanism

Permit bandwidth to be reserved for each functional device

When it’s configured
System software performs variety of functions

  • Device attachment and detachment detection

  • Device configuration

  • Bandwidth allocation

  • Managing control flow between client and device

  • Managing data flow between client and device

  • Collecting status and transaction statistics

  • Transaction scheduling

  • Controlling electrical interface

Function Layer

Layer represents relationship between

Client software

Devices functional interface
Each interface

Consists of particular class of device

Class driver designed to manipulate
USB client software cannot access associated function directly

Such scheme contrasts with PCI, ISA, etc

Result of not being mapped directly into memory or I/O space

Device drivers must use USB programming interface

To access associated device
USB clients view USB device as

Interface which they know how to manipulate

USB system software must report to clients

Interface type

Other device characteristics

USB Peripheral Connection


Provides single type of connector

For attaching peripheral to system

Supports two different speeds of device

  • Low speed devices - 1.5 Mbyte / sec

  • High speed devices - 12 Mbyte / sec

All USB devices attach via USB hub

Hub provides one or more ports
Hub may have either high or low speed device attached

Device’s speed detected when attached to hub

Each port must support

High and low speed devices

Unless has permanently attached device
When transaction initiated by host

All high speed devices and all hubs see transaction

Each transaction has address field

Identifies targeted device or hub

Low speed devices only see low-speed transactions

Preceded by high speed preamble

Directs all hubs to enable low speed ports
Sub uses differential signaling

For serial communication between root hub and USB devices

Power supplied to USB devices

USB cable

Local power supply

Associated with the device

The Physical Environment

Connectors designed to permit any USB peripheral device

To be attached to hub port

Low-speed Cables

Intended only for 1.5Mbyte/sec signaling

Maximum cable length cannot exceed 3 meters

Also referred to as

Sub-channel cable

As noted only intended for 1.5 Mb/sec signaling

Sued in sub-channel applications where wider bandwidth not required

Differential signaling pair

May be non-twisted 28 AWG stranded conductors

Used to reduce effects of noise

Low speed cables do not require shielding

High-speed Cables

Require twisted shielded pair


Cable length is 5 meters

Propagation delay is 30 ns over length of cable

When operating in range of 1-16 MHz

If cannot be met

Cable must be shortened

Except for differences noted above

Low and high speed cables are identical

Cables and Cable Power

Cable power is 5 Vdc

Can be used to power peripheral devices

Provides up to 500 ma

May be as little as 100 ma

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