usb tutorial[1]

7/27/2019 USB Tutorial[1]

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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


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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

HUB

Root Hub

Keyboard

Speaker

Disk

Printer


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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


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USB Topology A unique device address is assigned to each USB device

Physical tiered start network:

Logical network:

Each device sees all traffic generated by the host

A device does not see data sent by another peripheral

HOST

Root Hub @: 1

Hub @: 2

Dok @: 3

Mouse @: 4

IP phone @: 5

Logical link

HOSTHub @: 2

Dok @: 3

Mouse @: 4

IP phone @: 5

Root Hub @: 1

USB cable Keyboard

@: 6

Keyboard @: 6

HID Driver

MSD Driver

HUB Driver

Audio Driver

A driver has no

information on the

topology of the

physical network


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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:

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

Type Di rec tion Packets per fram e M ax Packet Size

Control Bidir Several 64 Bytes

Bulk Unidir Several 64 BytesIsochronous Unidir One 1024 Bytes

Interrupt Unidir One max 64 Bytes

A 1ms USB Frame

SOF @6EP1@5EP0 @6EP2


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USB pipes Device address is affected by the host

Endpoint configuration depends on the device implementation

Time multiplexing of transfer is under host control

Hub

Mouse

Disk On KeyControl TransferBulk IN TransferBulk OUT Transfer

Ep 0Ep 1Ep 2

Device Address 3

Control Transfer

Iso IN TransferIso OUT Transfer

Control TransferInterrupt IN Transfer Ep0

Ep 3

Device Address 4

IP Phon eEp 0Ep 1Ep 2

Device Address 5

Device Address 2

Host

Ep 0Ep 1

Control Transfer

Interrupt IN Transfer

Logical Pipe


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Example of USB controller


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USB transactions A transfer is composed of one or several transactions

Example of control transfer (several transactions)

Example of bulk transfer (one transaction)

Setup StageData Stage

Status Stage


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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


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USB Transactions (2) Example of bulk OUT transaction:

Device Address

Endpoint Address

Host

Device

PID

Token

Data

Handshake

Token Data Transfer Handshake


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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

Pipe/Endpoint

Endpoint number

Endpoint type

DeviceDevice address

Offers

Transfer

Control Bulk Iso Int

2+ 1+ 1+ 1+

Transaction

Token pkt

Data pkt

Handshake pkt

Handles


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Control transfer

The device is not ready

The device does not

acknowledge the transaction

The host retries

The device hasAcknowledged the transaction


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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

Time

A 1ms USB Frame

SOF SOF SOF SOF@3@5 @5 @5@6 @6 @6 @2

-Host handles most of the protocol complexity

-Peripheral design is simple and low-cost


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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

Controltransfers

Iso + Interrupt transfer Bulk transfer

Fixed width Variable widthValible width

-A host can refuse a new peripheral which requestslarge isochronous transfer size

-Such peripheral offers several configurations


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Theorically up to 1216 bytes can be transferred through a bulk

endpoint in a frame

In practice ~950 bytes

Bandwidth

1ms frame


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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 busreset 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.

DM

DP

15 Kohms

15 Kohms

3.3V

1.5 Kohms

DP

DM

HOST

Device


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Enumeration

Host Hub Device

Attached

Powered

Default

Addressed

Configured

Connect CableStatus Change

Query Change

Port Enable

Reset Device

Get Device Descriptor via default Pipe and Address

Assign a Unique AddressRead all Configuration Information

Configure and Assign Configuration Value

Enumeration is the Process of Assigning Addresses and Setting

Configurations


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Enumeration traceDefault Ad dress, default con trol endpo int

Enter Ad dress State

Enter Conf igu red State


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USB Device State

After 3ms of bu s inact iv i tythe device mus t enter

Suspended state and d rain lessthan 500uA


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Standard requests A setup transaction includes an 8-byte formatted data packet

Standard requests are defined in Chapter 9 of the USB spec


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Host software architecture

Host

Controller

regs

Host Controller Driver (HCD)

OHCI-UHCI-EHCI

USB Protocol Driver API

MS HUB EtcHID

Linux and WIN CE provide

OHCI/UHCI/EHCI HCD driver

USBD Driver

Main class drivers: Hub, HID, Mass storage, Printer,

Symbian and RTOS does notprovide USB host stack driver

SW IPs providers are able toprovide solutions for RTOS

Softconnex, Philog,

Expensive: (65k$ for a mass

storage solution)It is still possible to build a minihost from our full host:

the SW is only able to drivesome kinds of devices


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Device USB driver components

USB Device driver API

PowerManagement Attach/DetachProcessing

Status&Control

Control ProtocolStandard RequestsClass Requests

Vendor Requests

Endpoint DataTransfer ProtocolsBulk In/Out

Isochronous In/Out

Interrupt In/Out

Pull-UpResistorClockRegisters USB ControllerRegistersEndpoint FIFO

Registers


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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 Configuration Descriptor

Interface Descriptor Interface Descriptor Interface Descriptor

EndpointDescriptor

EndpointDescriptor

EndpointDescriptor

EndpointDescriptor

EndpointDescriptor

EndpointDescriptor


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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:

- Audio class (speakers, )

- HID (keyboard, mouse, )

- Mass Storage (disk on key)

- Printer class

- Smart Card CCID

- Communication Data Class

-


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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


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What is happening?

Transact ion NACKed by the device


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What is the physical topology?

How many device connected?

Which device answers?

Where is connected the bus analyser?


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Answer

HUB

Device Nb 3

Device Nb 2

New Device

Bus Analyzer

USB Wire

D

ownstreamPorts

UpstreamPort

The bus analyzer logs all downstream packets from the hostand only upstream packets from the new device

usb tutorial[1]

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