Transfers
USB supports 4 transfer types
Used for devices typically though of as interrupt driven
In legacy PC applications
Example
Keyboard
USB does not support interrupt
Therefor such devices must be periodically polled
Polling rate clearly critical
Data must not be lost
Not so frequent that bus bandwidth is wasted
Used for transferring large blocks of data
Typically such transfers have no periodic requirement
Example
Print job
Such transfers require constant delivery rate
Applications must ensure rate matching
Between sender and receiver
Example
USB microphone and speaker
Want to ensure no data distortion
Results from transfer
Used to transfer specific requests to USB device
Most commonly used during device configuration
Transactions
Typically consist of three phases
Token packet phase
Data packet phase
Handshake phase
Token Packet Phase
Each transaction begins with token phase
Defines type of transaction
Device address also included in this phase
When transaction targets specific device
Two kinds of tokens
Stand alone
Not followed by additional packets
Others
Followed by one or two additional packets
Data Packet Phase
Payload associated with the transfer
Can carry maximum of
1023 bytes during each single transaction
However max payload depends upon
Transfer type being performed
Handshake Packet Phase
All USB transfers implemented
To guarantee data delivery
Except isochronous
No handshake phase
Provides feedback to sender of data
If transaction occurred without errors
If errors occur
Retries attempted
Packets
Packet is mechanism used to perform all USB transactions
Format given as follows
S
ynchronization Sequence
Sequence consists of 8 bits
7 consecutive 0s
1 single 1
Recall 0s encode the transitions on differential data lines
Synchronization sequence also alerts receiver
Data packet on way
Will immediately follow sync sequence
Packet Identifiers
Define purpose and content of given packet
Grouped into 3 major categories
Token packets
Data packets
Handshake packets
Special packets
Currently only special packet is preamble
Used to signal low speed transactions
Format and length of packet depends upon its type
4 bytes
Data packets
Variable length
Depends upon transfer type
Bulk transfers
Limited to 64 bytes durning each transaction
Isochronous transfers
Set at 1024 bytes
Configuration Process
Host software is responsible for
Detecting and configuring
All devices attached to root hub
Process commonly referred to as USB device enumeration
Enumeration begins at root hub
Each hub port powered in turn
When powered
Hub determines if
Low, full speed, no device attached
If device present
Status bits within hub set to indicate device present
Port enabled
Issues USB reset to device
Assigns unique address
Completes configuration
Process repeated until all devices attached to root hub
Identified and configured
Configuration Root Hub
Host software begins enumeration by configuring root hub
Recall two designs have been defined
Open Host Controller
Universal Host Controller
Each provides same basic functionality
Individual Device Isolation
Initially each attached device isolated from hub
Since all ports initially disabled
Host software detects presence of device when power applied
Recall earlier discussion
Next each attached device issued reset command
Reset to Address Zero
Resetting device forces it to respond to
Default device address
Zero
Following reset
Every USB device responds to address zero
Using such a scheme
Configuration software can read every device’s descriptor
At same default address
Address Assignment
During configuration
Each device assigned unique address
Device will respond to that address tehreafter
With such a scheme
Possibility of address contention eliminated
Configuration Verification
During configuration host software probes each device
Must determine
Endpoints associated with device
If endpoints can be accommodated based upon remaining free bandwidth
If bus power required by device can be accommodated
Devices may have one or more configurations
Each configuration descriptor
Represents different set of resources that can be chosen
Host software ensures that all required resource requests can be satisfied
If cannot - configuration refused
Power Requirements
Host software must verify
Bus power required by device
Can be supplied by hub port
During configuration
Device specified to consume no more than 100 mA of bus current
Max bus power needed specified in configuration descriptor
Bus Bandwidth
Host software must verify that bus bandwidth required by device
Can be satisfied
Each configuration
Defines set of endpoints
Each endpoint must specify amount to USB bandwidth it requires
If sufficient bandwidth available
Communication pipe set up
Such set up reserves required bandwidth
After successfully allocating bandwidth to each endpoint within device
Device can be configured
If bandwidth not available
Other configurations checked
If every configuration exceeds bandwidth
Device not configured
Configuration Value Assigned
Once configuration selected
Host software configures device
Done by assigning configuration value that corresponds to
Chosen configuration
Configuration value obtained from
Selected configuration descriptor
Device can now
Be accessed by client software
Consume the max amount of current specified in its configuration
Client Software Notification
Once device successfully configured
USB system must locate appropriate
Class driver or drivers
Designated to access device
USB class drivers must be notified that
Device has been installed
Must be provided with information regarding
Characteristics
Capabilities of device
Exact procedure USB system software uses to identify
USB device driver etc
Operating system dependent
USB Device Configuration
Prior to configuring a device
Hub to which device connected
Must have already been configured and had power applied to ports
Next hub and configuration software must detect
Connected device
To convey device information back to host
Device implementers must create device descriptors
These reflect
Characteristics
Behaviour of device
Standard descriptors include
Describes number of configurations supported by device
Configuration Descriptors
Specifies one or more interfaces
Defines certain attributes associated with this configuration
Defines number of endpoints related to interface
Defines certain attributes associated with interface
Specify attributes associated with given endpoint
Information needed by host software
To determine how endpoint should be accessed
Optional descriptors
Consist of UNICODE string that provides human readable information
That can be displayed
Class-Specific Descriptors
Given device may require additional descriptors
As defined by particular device class spec
Device Classes
Important aspect of device configuration
Determine which class particular device belongs
Device’s class definition
Provides information used by host and client software
To determine how a device is
Controlled
Accessed
Host software uses information
Identify corresponding device driver
Class driver uses information
Can use information to determine specific characteristics of device
Device classes defined in individual specifications
At present following device classes are defined
HID Device Class - Human Interface Devices
Communication Devcie Class
Monitor Device Class
Mass Storage Device Class
Audio Device Class
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