USB Tutorial

Presentation Transcript

Objectives

Review USB functionalities
Be able to read a USB trace
Understand the enumeration process
Highlight that class drivers are part of the USB success

Keyboard

Root Hub

Speaker

Printer

Disk

Few generalities on USB

Tiered start topology
2 types of peripherals:

HUB
Function
Up to 127 peripherals

A master/slave architecture
USB 2.0 spec supercedes USB 1.1 spec

USB 2.0 LS (  USB1.1 at 1.5Mbps)
USB 2.0 FS (  USB 1.1 at 12Mbps)
USB 2.0 HS (480 Mbps)

Power Management

Different device class: bus powered or self powered
Vbus > 4.35V. 100mA, (500mA after negociation), 500uA in suspend mode
Host disables power to ill-behaved peripherals

Host

HUB

The Transaction Protocol is Host Based

Host based token polling

Data from host-to-function and function-to-host
Host handles most of the protocol complexity
Peripheral design is simple and low-cost

Robustness

Handshake to acknowledge data transfer and flow control
Very low raw physical bit error rate (< 10 –10)
CRC protection plus hardware retry option
Data toggle Sequence bits

Bounded transfer characteristics

Data transfer bandwidth and latency prenegociated
Flow control for peripheral buffer management

No asynchronous message/interrupt from the peripheral

Dok

@: 3

Mouse

@: 4

Keyboard

IP phone

@: 6

@: 5

IP phone

@: 5

Audio Driver

MSD Driver

HUB Driver

HID Driver

USB Topology

A unique device address is assigned to each USB device
Physical tiered start network:

Dok

@: 3

HOST

Hub

@: 2

@: 1

Mouse

@: 4

Root Hub

USB cable

Logical network:

Root Hub

@: 1

A driver has no information on the topology of the physical network

HOST

Hub

@: 2

Logical link

Each device sees all traffic generated by the host
A device does not see data sent by another peripheral

USB transfer

A device has several endpoints
Each endpoint is assigned to a logical pipe with the host
Each pipe is characterized by:

Device address
Endpoint number
Transfer type

Transfer type:

SOF

@5EP0

@6EP1

@6EP2

A 1ms USB Frame

Control: configuration/command/status type communication
Bulk: large amounts of data at highly variable times
Isochronous: constant-rate, error tolerant transfers
Interrupt: send or receive data infrequently but with bounded service periods

Disk On Key

Device Address 3

Host

Control Transfer

Ep0

Bulk IN Transfer

Ep1

Bulk OUT Transfer

Ep2

Mouse

Device Address 4

Control Transfer

Ep0

Interrupt IN Transfer

Ep3

IP Phone

Device Address 5

Control Transfer

Ep0

Iso IN Transfer

Ep1

Iso OUT Transfer

Ep2

Hub

Device Address 2

Control Transfer

Ep0

Interrupt IN Transfer

Ep1

Logical Pipe

USB pipes

Device address is affected by the host
Endpoint configuration depends on the device implementation
Time multiplexing of transfer is under host control

Example of USB controller

USB transactions

A transfer is composed of one or several transactions
Example of control transfer (several transactions)

Setup Stage

Data Stage

Status Stage

Example of bulk transfer (one transaction)

Token

Data Transfer

Handshake

USB Transactions (1)

A transaction is made of 3 packets

Token: device address, endpoint number, transfer type
Data : data to be sent
Handshake: acknowledge

Example of bulk transaction:

Token

Data Transfer

Handshake

USB Transactions (2)

Example of bulk OUT transaction:

Endpoint Address

Device Address

PID

Host

Token

Data

Device

Handshake

Control

Bulk

Iso

Int

Handshake pkt

Token pkt

Data pkt

Summary

The host affects an address to the device
The device has several endpoints
To each endpoint corresponds a pipe and a type of transfer
Transfer is made of one or several transactions
Transactions are composed of 3 packets

Transfer

Device
Device address

Handles

Offers

Pipe/Endpoint
Endpoint number
Endpoint type

2+

1+

Transaction

Control transfer

The device is not ready…

The device does not

acknowledge the transaction

The host retries…

The device has

Acknowledged the transaction

@3

@6

@5

@2

@5

SOF

USB scheduling(1)

USB transfers occurs in a millisecond time-frame window
The host sends a Start Of Frame (SOF) Packet at the beginning of each frame
This is the host which starts a transfer with a logical peripheral
Transactions are time multiplexed in one or several frames
This is the host which do the transaction arbitration

SOF

Time

A 1ms USB Frame

Host handles most of the protocol complexity
Peripheral design is simple and low-cost

USB scheduling (2)

The bus allocation depends on the host controller
A constant interval width is allocated to isochronous and interrupt transfer
A minimum interval width is reserved for control transfer
The rest of interval is for bulk transfer

Variable width

Valible width

Fixed width

Controltransfers

Iso + Interrupt transfer

Bulk transfer

A host can refuse a new peripheral which requests large isochronous transfer size
Such peripheral offers several configurations

Bandwidth

Theorically up to 1216 bytes can be transferred through a bulk endpoint in a frame
In practice ~950 bytes

1ms frame

3.3V

1.5 Kohms

Device detection

The host (hub) downstream port integrates 2 pull-downs (15Kohms). The bus state is SE0 (DP = DM = 0V)
When a device is connected, the bus state is idle (DP = 3.3V DM = 0V). DP is forced by the device pull-up (1.5Kohms).
The hub/host detects the port status change and forces a bus reset to the new device (DP = 0V DM = 0V).
Then the host sends requests to the device through the default control endpoint (endpoint number 0). A newly connected device address is 0.

HOST

DP

15 Kohms

DP

DM

15 Kohms

Device

Host

Hub

Device

Connect Cable

Status Change

Addressed

Configured

Attached

Powered

Default

Query Change

Port Enable

Reset Device

Get Device Descriptor via default Pipe and Address

Assign a Unique Address

Read all Configuration Information

Configure and Assign Configuration Value

Enumeration

Enumeration is the Process of Assigning Addresses and Setting Configurations

Enumeration trace

Default Address, default control endpoint

Enter Address State

Enter Configured State

USB Device State

After 3ms of bus inactivitythe device must enter Suspended state and drain less than 500uA

Standard requests

A setup transaction includes an 8-byte formatted data packet

Standard requests are defined in Chapter 9 of the USB spec

Host

Controller

regs

Host software architecture

Linux and WIN CE provide

OHCI/UHCI/EHCI HCD driver
USBD Driver
Main class drivers: Hub, HID, Mass storage, Printer, …

HID

MS

HUB

Etc…

Symbian and RTOS does not provide USB host stack driver
SW IPs providers are able to provide solutions for RTOS

Softconnex, Philog, …
Expensive: (65k$ for a mass storage solution)

It is still possible to build a mini host from our full host:

the SW is only able to drive some kinds of devices

USB Protocol Driver API

Host Controller Driver (HCD)

OHCI-UHCI-EHCI

Attach/DetachProcessing

Endpoint FIFORegisters

Device USB driver components

USB Device driver API

Control Protocol
Standard Requests
Class Requests
Vendor Requests

Endpoint Data
Transfer ProtocolsBulk In/Out
Isochronous In/Out
Interrupt In/Out

PowerManagement

Status&Control

ClockRegisters

Pull-UpResistor

USB ControllerRegisters

Device descriptor

The USB Host stacks uses the descriptor retrieved from the device to find the corresponding driver.
Windows looks for a matching Vendor ID/Product ID in its .inf library
If not found, it will search for a matching class driver
If not found then it will request the user to insert a CD to install the corresponding driver

Device Descriptor

Configuration Descriptor

Interface Descriptor

EndpointDescriptor

USB class drivers

Building on top of the USB specifications, there are Device Class Specifications from the Device Working Group
Matching device class requirements allow use of standard host class drivers provided by Windows or Linux
Each class driver specifies the endpoint configurations required
Existing class drivers: