Embedding USB Lane Hauck Cypress Semiconductor. Embedded Systems Conference, March 2002 Embedding...

Embedding USB

Lane HauckCypress Semiconductor

2Embedded Systems Conference, March 2002Embedding USB

Agenda

Why USB? USB Advantages USB Basics

• With embedded system emphasis An Implementation Roadmap USB On The Go An Embedded Host Example Conclusion


Agenda




Why USB?

No other way to talk to so many things• Input devices

– Mice, keyboards, joysticks

• Memory devices– Disk drives– Memory cards

• Fingerprint scanners It’s standard

• USB Implementers Forum, Inc.• Plugfests


Agenda




USB Advantages

Many standardized, low cost devices are available

Connectors and cables are standard USB devices are compliance-tested to

insure compatibility and uniform performance

Using hubs, multiple USB devices can share one connector

USB devices can tell you about their capabilities


USB Advantages(Cont’d)

USB provides device power USB devices can:

• Be put to sleep• Be awakened• Provide a system wakeup call

USB protocol takes care of low-level details• Flow control• Error checking

You decide how complex to make the firmware


Agenda




USB Basics with embedded system emphasis

Speeds Cables Topology Endpoints Transfer Types Handshakes


USB Basics cont’d

Hubs Power Management Device Classes Host Chips & Drivers Protocol


Speeds

USB 1.0 and 1.1• Low speed is 1.5 Mbits/sec

– Packet size is limited to 8 bytes– Transfer type limited– Don’t want to wait slowly

• Full speed is 12 Mbits/sec USB 2.0

• “Hi-speed” is 480 Mbits/sec USB 1.x retro-named “Classic” USB 12 Mb/s is great for embedded systems


Cables

Shielded 4-wire cables• Vbus = 4.4-5.25 volts• GND• D+ and D-

Reflected-wave signaling• Low power buffers

5 meter limit• Hubs extend effective cable length

Cables are standard, plentiful and cheap


Cables1 foot cable

Dri

ve

rR

ec

eiv

er


Cables2 meter cable

Dri

ve

rR

ec

eiv

er


Cables5 meter cable

Dri

ve

rR

ec

eiv

er


Topologytiered star

PC

Hub

P

P

P

Hub

PPP

P

USB Host

Root Hub

One USB connector can support up to 127 devices


Endpoints

Endpoints are logical channels inside a peripheral device

In hardware they’re addressable buffers (FIFOS)• Every host transfer includes a 4-bit

endpoint number and a direction bit CONTROL endpoint 0 is mandatory All others are up to the design Endpoints are handy for separating

data types


Transfer Typesfour types to suit data

requirements

Control• Mission-critical, highest error protection• Used to interrogate, configure and control

devices Bulk Interrupt Isochronous

BULK is easiest to use and the most flexible


Transfer Typesanatomy of a USB frame

• 12MHz = 1.5MB/s or 1500 bytes/ms (full speed)• Isochronous/Interrupt traffic have guaranteed bandwidth• Control traffic is “best-effort”• Bulk uses what is left

Video Audio Mouse Control Printer Printer

1 msec frame

SO

F...Isochronous Interrupt Control Bulk

SO

F


Handshakes

ACK means “success” NAK means “try again” No response means error Handshakes used for

• CONTROL• BULK• INTERRUPT

You don’t need to implement flow control


Handshakesa CONTROL Transfer

IN

ADDR

ENDP

CRC5

Token Packet

DATA1

PayloadData

CRC16

Data Packet

ACK

H/S Pkt

H P H

DA

TA

H H P H H P

DATA1

OUT

ADDR

ENDP

CRC5

Token Packet

CRC16

Data Pkt

NAK

H/S Pkt

DATA1

OUT

ADDR

ENDP

CRC5

Token Packet

CRC16

Data Pkt

ACK

H/S Pkt

(Operation not completed) (Operation completed)HA

ND

SH

AK

EDATA0

8 bytesSetupData

CRC16

Data Packet

ACK

H/S Pkt

SETUP

ADDR

ENDP

CRC5

Token Packet

H H P

SE

TU

PH is Host

P is Peripheral


Hubs

PCUSB USB

Device DeviceHub

USB USB USB USB

PCUSB USB

Device

Device Device


Hubspower

HubUSB USB USB USB

PCUSB USB

Device

Device Device

500

100 100

500


Hubsmore power

HubUSB USB USB USB

PCUSB USB

Device

Device Device

500 500

Wall Wart

500 500


Hubsmore devices

HubUSB USB USB USB

PCUSB USB

Device

Device Device

500 500

Wall Wart

500 500

USB USB USB

Device

500

Easy way to expand and power devices


Power Management

USB power consumption is carefully controlled

Host determines power needs of all attached devices• Only configures those within power limits

USB devices must enter low power mode when host signals ‘suspend’• 500 microamps

USB devices can signal a remote wakeup Built-in mechanism prevents peripherals from

drawing too much power from your system


Device Classes

USB-sanctioned method to extend the specification to device classes

Classes are devices with similar characteristics

Huge advantage—the driver is built into the OS• Not written and maintained by YOU

Immense simplification of the software effort if the OS supports your device


Protocol

USB architects went well beyond the electrical interface• Remember IrDa?

Protocol layer insures interoperability Compliance testing (plugfests) enforce the

protocol USB protocol is extensible

• Device classes• Fully custom ‘vendor’ designs

Device standardization makes the embedded software task far easier than writing custom drivers


Protocol

A large part of the embedded system design is understanding the protocol

How much do you need to support?• How many devices• How many device types• How many hub layers

Embedded USB is by necessity a subset of full PC-based USB

Efforts are underway to simplify the software task (more about this)


Protocolthe eight bytes in a SETUP packet

Byte Field Meaning

0 bmRequestType Request type

1 bRequest The actual request

2 wValueL Varies by request

3 wValueH

4 wIndexL Varies by request

5 wIndexH

6 wLengthL Number of data bytes

7 wLengthH


ProtocolChapter 9 Requests

Standard Class VendorbRequest

CustomProgramming

00 GET_STATUS

0C

STALL

CLR_FEATURE

SET_FEATURE

SET_ADDRESS

GET_DESCRIPT0R

SET_DESCRIPT0R

GET_C0NFIG

SET_CONFIG

GET_INTERFACE

SET_INTERFACE

SYNC_FRAME

01

03

05

06

07

08

09

0A

0B

STALLother

X 0 1 X X X X XX 0 0 X X X X X X 1 1 X X X X XX 1 0 X X X X X

03

DEVICE

CONFIGURATION

STRING

01

02

STALLother

wValueH

bmRequestType


Protocoladded HID Class Requests

Standard Class VendorbRequest

CustomProgramming

00 GET_STATUS

0C

STALL

CLR_FEATURE

SET_FEATURE

SET_ADDRESS

GET_DESCRIPT0R

SET_DESCRIPT0R

GET_C0NFIG

SET_CONFIG

GET_INTERFACE

SET_INTERFACE

SYNC_FRAME

01

03

05

06

07

08

09

0A

0B

STALLother

bRequest

X 0 1 X X X X XX 0 0 X X X X X X 1 1 X X X X XX 1 0 X X X X X

03

DEVICE

CONFIGURATION

STRING

01

02

STALLother

wValueH

0B

GET_REPORT

GET_IDLE

GET_PROTOCOL

SET_REPORT

SET_IDLE

SET_PROTOCOL

01

02

03

09

0A

STALLother

23

HID

REPORT

PHYSICAL

21

22

bmRequestType


Host Chips & Drivers

PC’s use OCHI/UHCI host controllers• Usually part of a ‘Southbridge’ chip• Driver software is part of the OS

– Class drivers– USBD.sys– HAL: OHCI.sys or UHCI.sys

Embedded systems use different, proprietary host controllers• Hardware abstraction layer is custom


Host Chips & Drivers

Application

Class Driver

USB Driver

Host Controller Driver

Host Controller

IRP

Transaction List

Application

Host Controller

PC Simple Embedded


Host Chips & DriversPC implementation

Apps or class drivers create IO Request Packets (IRP)

USB Driver• Processes IRP’s• Manages CONTROL pipe• Division of duties between USBD

and HCD is not defined Host Controller Driver (HCD)

• Provides abstraction– Of host controller registers– Of data transfer specifics

• HCD creates a transaction list– Schedule of transfers

Host Controller turns transaction list into packets

Application

Class Driver

USB Driver


Host Controller

IRP

Transaction List


Host Chips & Driverswhy the PC architecture is

complex

App needs no knowledge of• Which host controller is installed• USB signaling details• Bus utilization details• Transaction lists

App talks to buffers in your device using “pipes”

Most of the HCD/HC complexity is due to the flexible nature of USB

• 127 devices• Devices appear and disappear at

random• Bandwidth must be fairly shared

between devices

Application

Class Driver

USB Driver


Host Controller

IRP

Transaction List


Host Chips & Driverssimplest embedded hardware

Looks simple The hardware is simple But software is “low-level”

• Detect USB device attachment• Detect speed• Enumerate devices• Suspend and resume devices• Manage error recovery• Generate USB resets• Conduct transfers

Application

Host Controller

Simple Embedded


Host Chips & DriversRTOS provides a ‘middle’ solution

• Examples– WinCE– Linux– VxWorks

Application

Class Driver

Host Controller

RTOS with aUSB

"Stack"

• Host Controller vendor may provide a small driver to make the chip look like something the RTOS supports


Agenda




An implementation roadmap

Pick a host controller For simple interface (a few simple devices)

• Write custom firmware• Use chip supplier tools

– Compilers, debuggers– Sample code

For more general interface• Use an RTOS with USB support• Chip supplier usually provides hardware drivers• Check the supported device class list


Agenda




USB On The Gogoals

Connect two peripherals together• PC is not required (but still supported)

Allow peripherals to awaken each other• Session Request Protocol (SRP)

Allow devices to exchange the host role• Host Negotiation Protocol (HNP)

New OTG devices can tap into the existing 900 million USB devices• Forecast: 1 billion devices by 2003


USB On The Gomarketing foil


USB On The Gohardware details

Defines a new connector and cable• “Mini-AB” receptacle• Mini-A to Mini-B cable• Cable establishes the default host

– A-Device is the default host– Host turns on VBUS

Dual-role USB devices:• Sometimes a peripheral, sometimes a host• Must use the new AB connector• Provide limited host capability• Supply 8 mA (min) on VBUS• Operate at full speed (high speed optional)


USB On The Gothe best news may be on the

software side

Impossible for a small battery operated device to support everything a PC does

Solution: a Targeted Device List• Simplifies and standardizes device classes• Example: OTG printer class

This effort is underway in the OTG Device Working Group

Moves complexity from your application code and drivers into the Operating System


Agenda




USB peripheral dev tools

Full-speed USB Development Board

Test USB

USBPCDevelop--Debug

Serial


Simple embedded host

Full-speed USB Development Board

Add-on board contains USB host plus 4-port hub

PC USB

Serial

Develop--Debug

Test Host Transfers









Example: BTC Keyboard + 2-port Hub with

embedded Fingerprint Scanner– Addr1 = 2 port Hub– Addr2 = Fingerprint Scanner– Addr3 = Keyboard

Fuji Digital Still Camera– Addr4 = Mass Storage Class

(MSC)

Mitsumi Mouse– Addr5 = HID Class– Data being read through EP1



(live example)


Conclusion

Embedded host chips are here now Software is the main effort Point solution is easy More complex solution requires an

RTOS with USB support• Third-party choices• Some classes are supported

USB OTG aims to provide simplified and standardized host drivers

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Embedding USB Lane Hauck Cypress Semiconductor. Embedded Systems Conference, March 2002 Embedding...

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