Usb

UNIVERSAL SERIAL BUS

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Overview  Introduction  Components  Protocol  Icons  Developments  USB

OTG 2

What Is USB? 1.    5. 6.

Invented & standardized – 1995 U – universal S – serial B – bus 127 Devices ? Data transmission using packets.

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Why Use USB?      6. 7. 8. 

Single port for many devices. Plug and play. Automatic detection of device. No need to restart the system. Speed flexibility. Low speed Full speed High speed Robust – error detection capability. 4

COMPONENTS  Host

controller (m) and device controller (s)  Hub is optional  Five “basic” components Cables

and Connectors

Power Electrical

Signaling

Protocol Device

Framework

Standard

commands Standard descriptions 5

Connectors 3

“standard” plugs (not including OTG mini-A) ‘A’

plug leads upstream to host ‘B’ or ‘mini-B’ plug goes to device “captive”cable -- device end of cable uses a “vendor-specific connect method”: hard-wired or special plug

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Standard Cables 5

m maximum length (shorter for low-speed) 26

ns maximum delay between ‘A’ plug and ‘B’ plug 125 mV maximum voltage drop 4 conductors plus shield D+

and D- (twisted) data pair VBUS (power) Ground

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Electrical Signaling (1)  Three

Signaling rates

High-Speed

== 480 Mbps Full-Speed == 12 Mbps Low-Speed == 1.5 Mbps  Basic

signals

Idle Reset

(only full- and low-speed; long enough SE0) Suspend (more than 3 ms of continuous idle) Resume

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Electrical Signaling (2)  Data

moved via ~Differential Signaling  Full- and Low-Speed – USB 1 specification  High-Speed - USB 2 specification  NRZI with bit-stuffing baseband transmission

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Protocol - Transactions  Packets

always part of a transaction  Transactions always target a Device Endpoint (source or sink)  Transit via a Pipe that connects Host & Device  Transactions have 1 to 3 components Host

“talks”, Device “answers” Token packet (always present and from host) Data packet (direction as specified by Token) 

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Endpoint  Endpoint

: source or sink of data  Also a interface between hardware and software.  Every device will many pairs of buffer  One

is used for IN transaction  Other is used for OUT transaction  All

devices should have ENDP = 0  Each buffer is of 8 bit length. 12

Pipes  Logical

connection b/t host & endpoints  Parameters  Bandwidth  Type

of transfer  Packet size  Buffer size etc

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Packet Id  Basically

4 bits  But a byte is used for PID  Every packet should have PID field Pid Pid P P P P P P 0 id2 id3 id4 id5 id6 id7 1 nibble is complement of lower  Higher nibble  Advantage 

Self check (i.e., CRC is not used to check the PID) 14

Token packets Setup , IN, OUT , SOF SYNC

PID

ADDR

ENDP

CRC5

EOP

SYNC – START OF PACKET (8) PID – PACKET ID (8) ADDR – ADDRESS(7) ENDP – ENDPOINT(4) CRC5 – CHECKSUM(5) X^5 + X^2 + 1 EOP – END OF PACKET(D+ , D- both zero) 15

Token packets  Setup  Device

configuration  Device descriptors etc  In  Host

ready to receive data

 Out  Host

ready to send data

 SOF  Start

of frame 16

Data packets  Data

0 , Data 1 ?  3 to 1026 bytes – High speed USB  11 bytes – Low speed USB PID

DATA

EOP

SYNC CRC1  DATA – PAYLOAD 6  CRC16 – CHECKSUM (16) X^16 + X^15 + X^2 + 1. 17

CRC o

o

WHY CRC5 – Token packets , CRC16 – Data packets ?  size of packets Polynomial 16 ? All 4 errors can be detected Single bit error Two bit error Odd error Burst error 18

Handshake packets  ACK

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, NAK , STALL  ACK – Positive acknowledgement  NAK – Negative acknowledgement  STALL – The device finds its in a state that it requires intervention from the host. Transmission is in opposite direction of that of data packets. SYNC PID EOP

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Descriptors  Information

to the host about device  Type of device , Manufacturer , Version , number of endpoints and configuration example:  Device descriptor: Vendor id ,USB  

compatibility Configuration descriptor: Power , interface type Endpoint descriptor: To determine bandwidth for the 20 bus

Working

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SERIAL INTERFACE ENGINE  Part

of both the host's and the device's physical layer FUNCTIONS  SERIALIZATION

AND DESERIALIZATION  NRZI ENCODING AND DECODING  BITSTUFFING AND DESTUFFING  CHECKSUM GENERATION AND ERROR CHECKING  PID , EOP , SOP DETECTOR 22

IN TRANSACTION  ORIGINAL DATA + CHECKSUM  PARALLEL TO SERIAL  BIT STUFFING  NRZI ENCODING OUT TRANSACTION  NRZI DECODING  BIT DESTUFFING  SERIAL TO PARALLEL  CRC CHECK 23

ICONS

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USB OTG

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What is USB OTG ?  Supplement

to USB 2 specification  Gives dual role capability to increasingly intelligent platforms , on the same port  Result is “point-point” USB from users perspective.  Supports 12 Mbps , 1.5 Mbps 480 Mbps (optional) 26

What changes does USB OTG require?  Reduced

connector size.  Power consumption is less.  Adds protocol for dynamic switching between host and device (HNP).  Session request protocol 

USB power can switched on/off on discretion of device A

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OTG Dual-Role Device  has

a single mini-AB receptacle

Is

A-device (host) when mini-A plug inserted Is B-device (device) when mini-B plug inserted  operates

at Full-speed  may operate at High-Speed  may support one or more hubs (as an A-device) 28

COMPARISON USB OTG product -Must source 8mA

Vbus min on Vbus -May or may not be able to which has load more drivers mechanism to drivers. -PCI not present

USB Host in PC -Sources 500mA on

-Supports device drivers. -Always has load more 29

-Standardized PCI

For more information  Visit  www.usb.org  www.usb.org/developers/onthego  http://www.howstuffworks.com/

 http://www.trenz-electronic.de/down/dwdten.ht

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