USB ed

Table of Contents

Theory ed

Definitions ed

Device
The physical thing (mostly).
Identified by a vendor-id and a product-id, each 2 bytes, e.g. 0x256c / 0x006e.
Can also be identified by its physical connection path, e.g. 1/7/2 for bus#1, port#7, sub-port#2.

Function
A logical device.
A webcam might contain a camera and a microphone. Both will be presented as separate devices (functions).
Let's just keep calling them devices...

Endpoint
A communication point.
Identified by an address number 0x00-0x0f for output (device -> host) and 0x80-0x8f for input (host -> device).
Communication type an be either control, isochronous, interrupt or bulk.

Interface
A collection of endpoints in one device. Each device can have several interfaces.

Configuration
A device can only be in one configuration state at a time.
Identified by a 1-byte value.
Defines power consumption etc. And also interfaces.

Alternate setting
Each interface (within a configuration) can be in one of several alternate settings, each defining a set of endpoints.

Graph ed

[device]
  vendor-id: ...
  product-id: ...
  class: ...
  sub-class: ...
  [configuration]
    value: 1
    power: ...
    [interface]
      number: 0
      [alt setting]
        number: 0
        [endpoint]
          address: 0x81
          attributes: control/iso/bulk/interrupt
        [endpoint]
          address: 0x82
      [alt setting]
        number: 1
        ...
    [interface]
      number: 1
      ...
  [configuration]
    value: 2
    ...

Communication ed

All communication is initiated by the host!

Control transfer ed

This is usually sent to the "default" endpoint 0.

The host sends an 8-byte packet:

1b  type
1b  request
2b  value
2b  index
2b  length
For "out" transfer (host -> device), length defines the length of the following payload. For "in" (device -> host), it defines the maximum expected answer payload size.

For example, "SET-CONFIGURATION" is

0x00  0x09  #CONFIG  0  0
"SET-INTERFACE" to set the alternate setting
0x01  0x11  #ALT  #INTERFACE  0
"GET-DESCRIPTOR-DEVICE"
0x80  0x06  (0x00 0x01)  0  18
"GET-DESCRIPTOR-CONFIG"
0x80  0x06  (#CONFIG 0x02)  0  59..

Categories: Computer, Programmieren