usb 2.0 architecture overview

8/12/2019 USB 2.0 Architecture Overview

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USB 2.0ARCHITECTURE Overview

KANWAR SAAD BIN LIAQATResearch Assistant - UG


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Main Features & History

Basic Definitions

Device Enumeration Steps

Data Transfer Modes


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HISTORY

USB 1.0 (Jan 1996)

USB 1.1 (Sep 1998)

USB 2.0 (Apr 2000)

USB 3.0 (Nov 2008)


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BASIC DEFINITIONS-- Bus Speeds

Speed Name USB version Data rate

Low Speed USB 1.0 1.5 Mbps

Full Speed USB 1.1 12 Mbps

High Speed USB 2.0 480 Mbps

Super Speed USB 3.0 4.8 Gbps


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BASIC DEFINITIONS-- Bus Topology

Tiered Star Bus Topology


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BASIC DEFINITIONS-- HOST

Master of the bus

Initializes all transactions.

directly with each other.

The only exception is that a

suspended device can initiate a

remote wake-up call.


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BASIC DEFINITIONS-- Cables

Shielded cable containing 4 wires

Two of these D+ and D- form a twisted pair, responsible for

carrying differential data and single ended signals.

Third one is the ground wire as a reference for D+ and D- named

GND.

Fourth one is the 5V supply called VBUS


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BASIC DEFINITIONS-- Cable Length

Max cable length = 5m

Since hub levels are allowed so a device can be at the most 25m

away from the host.

Hence for transferring data through USB the device should be

closer to the host.


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BASIC DEFINITIONS-- Connectors


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BASIC DEFINITIONS-- Line States

Detached : No device plugged in. D+ and D- are low.

Attached : Device is plugged in. Either D+ or D- will go high.

Idle : State before and after a packet is sent. One of the D+ or D-

line remains high and other remains low.


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BASIC DEFINITIONS-- J, K, SE0 and SE1

J : Same polarity as the idle state.

K : Opposite of K.

SE0 : Single Ended zero state is when both lines are pulled low.

SE1 : When both lines are pulled high. There is no such signal as

, ence w enever suc a s gna comes, s ows an error.


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BASIC DEFINITIONS-- Packets

Smallest Element of data transmission.

Each packet contains integral no. of bytes.

Least Significant Bit(LSB) is transmitted first.

The bus is in the idle state, before and after the packet.

ac e = sync a a y es NRZI Encoding is used.

To ensure synchronized clock bit stuffing is used. A zero is inserted

after every 6 consecutive 1s.


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Device Enumeration Steps


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DEVICE ENUMERATION

Step 01 : Plug the device in USB port

Port may be in the host or hub.

Hub provides power to the port.

Device is in the owered state.

Step 02 : Hub detects the device

The hub monitors the voltages of signal lines. If there is change in any of the line voltages it means a device is

connected.


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DEVICE ENUMERATION

Step 04 : Hub detects whether device is low orful l speed.

Hub has both D+ and D- lines pulled down through 1.5k resistors.

A full speed device has a 1.5k pull up resistor on D+ line.

- . . Initially a high speed device is also detected as a full speed device.

Hub detects which line is pulled up by the device. Based on the pulled

up line, the hub knows whether the device is full speed or low speed.


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DEVICE ENUMERATION



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DEVICE ENUMERATION



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DEVICE ENUMERATION

Step 05 : Hub resets the device.

Host learns that a new device is attached.

It requests the hub to reset the device.

. . .for at least 10ms.


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DEVICE ENUMERATION

Step 06 : Host learns if a full speed devicesupports high speed.

During the 10ms reset the device that supports high speed sends a

chirp K signal.

,signal. KJKJKJ.

When device recieves this signal it removes it full speed pull up and

performs rest of its communication at high speed.

All high speed devices must be capable of responding to enumeration

requests at full speed.


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DEVICE ENUMERATION

Step 06 : Host learns if a full speed devicesupports high speed.


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DEVICE ENUMERATION

Step 07 : The hub establishes a signal pathbetween the device and the hub.

The host first verifies that the device has exited the reset state or not

by sending a GET_PORT_STATUS request.

not.

If necessary the host repeats the request until device exits the reset

state.

When the hub removes the reset, the device is in the default state.

The device is ready for control transfers through EP0. The device can communicate with the host using a default address of

00h.


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DEVICE ENUMERATION

Step 08 : The Host learns about the maxpacket size of the default pipe of the device.

The host sends a request to device 0 at EP0. Because the host can

enumerate only one device at a time hence only one device has an

. The device returns a 8 byte device description that contains the max

packet size supported by EP0.


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DEVICE ENUMERATION

Step 09 : The Host assigns an address.

The host controller assigns a unique address to the device.

The device reads the request and ACKs it.

.

Step 10 : The Host learns about devices abil ities.

The host sends get_descriptor request to the device at new address.

The device replies with the device_descriptor. Device_descriptor contains max packet size of EP0, the no. of

configurations the device supports and other basic information about

the device.


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DEVICE ENUMERATION

Step 11 : The host assigns and loads a device driver.

After host learns as much as it can of the device, it searches for its

device driver so that it can communicate with the device.

match the correct device driver.

If there is no match then host matches any class, sub-class or protocol

values.

Whatever matches, the host the loads its driver for further

communication.


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DEVICE ENUMERATION

Step 12 : The device driver selects a deviceconfiguration.

The device can support multiple configuration.

The host then asks from the user for what configuration to load or it

. When the configuration is loaded, the device is configured and now it

is ready to use.


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HUB ENUMERATION

Hubs are also USB devices.

.

If the hub has devices attached, the host also enumerates each of these

after the hub informs the host of their presence.


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DEVICE REMOVAL

The host constantly polls the hub.

,occurred.

The host sends a GET_PORT_STATUS request to find out what event

has occurred.

The host then removes that device from its list and the address of that

device is assigned to another newly attached device.


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DATA TRANSFER TYPES

Bulk Transfer

Interrupt Transfer

Isochronous Transfer

Control Transfer


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1. BULK TRANSFER

Useful for transferring data when time isnt critical.

. Has lowest priority.

Examples : sending data from PC to a printer, transferring data from a

flash disk e.t.c

When the bus is idle, they are the fastest mode of error free transfer.


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1. BULK TRANSFER

Availability :

Only full speed and high speed devices can do bulk transfer.

All devices do not need to support bulk transfers, only a certain device

. . . .

Data Size :

Max packet sizes of 8, 16, 32, or 64 bytes.

If the data to be transferred do not complete in one packet then the

host completes the transfer using multiple transactions.


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1. BULK TRANSFER

Speed :

Host guarantees that the data transfer completes eventually.

Dont reserve any transfer for the data hence if bus is busy it may take

. When bus is idle, it is the fastest mode of transfer since it has the

lowest protocol overhead.

Detecting and handling errors:

Use handshake packets such as ACK, NACK e.t.c for error free

transmission of data.

If a device doesnt send handshake packet then host retries twice.

Use data toggle bits to ensure no data is lost.


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1. BULK TRANSFER

Example transfer :


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2. INTERRUPT TRANSFER

When a data has to be transferred within a specific amount of time.

,keyboard.

Limited bandwidth available in low and full speed but it becomes

400x in high speed.

It does not generates interrupt as its name implies rather it also works

the same way that all other transfers do i.e through polling.


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Availability :

All versions of usb support this type of transfer.

All devices do not need to support Interrupt transfers, only a certain

. . . .

Data Size :

Low speed : 1 8 bytes

Full speed : 1 64 bytes

High speed : 1 1024 bytes




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Speed :

No guaranteed transfer rate.

Just ensures that no more latency than specified.

.


Use handshake packets such as ACK, NACK e.t.c for error free

transmission of data.

If a device doesnt send handshake packet then host retries twice.



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Example transfer :


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3. ISOCHRONOUS TRANSFER

Used for streaming and real time transfers.

Ensures a constant data rate.

. Examples: encoded voice played in real time.

Ensures that a large amount of data gets through quickly on a busy

bus.

Completion time is predictable.


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Availability :

Only full and high spped devices support this type of transfer.

All devices do not need to support Isochronous transfers, only a

. . . .

Data Size :

Full speed : 0 1023 data bytes

High speed : 1024 bytes packet size

Amount of data in each frame doesnt have to be same.




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Example transfer :


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4. CONTROL TRANSFER

Two uses.

1. Carry the requests for USB specifications.

. . Interactive transfer. Include IN and OUT transactions.


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4. CONTROL TRANSFER

Availability :

Every device must support control transfer over default pipe EP0.

A device may have additional pipes configured for control purposes.

.

Data Size :

Low speed : 8 bytes

Full speed : 8, 16, 32, or 64 bytes

High speed : 64 bytes




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4. CONTROL TRANSFER

Speed :

Host ensures that all control transfers get through the bus as quickly as

possible.

,speed.


Two retries if no handshake packet is received.

After that host stops all transactions and informs the software that

there is a problem at the control port which must be rectified.


C


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4. CONTROL TRANSFER

Example transfer :


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Questions?

Thank you !

usb 2.0 architecture overview

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