Embedding USB
Lane Hauck
Cypress Semiconductor
Embedded Systems Conference, March 2002
Embedding USB
1
Agenda
Why USB?
USB Advantages
USB Basics
•With embedded system emphasis
An Implementation Roadmap
USB On The Go
An Embedded Host Example
Conclusion
Embedded Systems Conference, March 2002
Embedding USB
2
Agenda
Why USB?
USB Advantages
USB Basics
•With embedded system emphasis
An Implementation Roadmap
USB On The Go
An Embedded Host Example
Conclusion
Embedded Systems Conference, March 2002
Embedding USB
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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
Embedded Systems Conference, March 2002
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Agenda
Why USB?
USB Advantages
USB Basics
•With embedded system emphasis
An Implementation Roadmap
USB On The Go
An Embedded Host Example
Conclusion
Embedded Systems Conference, March 2002
Embedding USB
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USB Advantages
Many standardized, low cost devicesare available
Connectors and cables are standard
USB devices are compliance-tested toinsure compatibility and uniformperformance
Using hubs, multiple USB devices canshare one connector
USB devices can tell you about theircapabilities
Embedded Systems Conference, March 2002
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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-leveldetails
•Flow control
•Error checking
You decide how complex to make thefirmware
Embedded Systems Conference, March 2002
Embedding USB
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Agenda
Why USB?
USB Advantages
USB Basics
•With embedded system emphasis
An Implementation Roadmap
USB On The Go
An Embedded Host Example
Conclusion
Embedded Systems Conference, March 2002
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USB Basics with embedded system emphasis
Speeds
Cables
Topology
Endpoints
Transfer Types
Handshakes
Embedded Systems Conference, March 2002
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USB Basics cont’d
Hubs
Power Management
Device Classes
Host Chips & Drivers
Protocol
Embedded Systems Conference, March 2002
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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
Embedded Systems Conference, March 2002
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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
Embedded Systems Conference, March 2002
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Cables1 foot cable
Driver
Receiver
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Cables2 meter cable
Driver
Receiver
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Cables5 meter cable
Driver
Receiver
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Topologytiered star
One USB connector can support up to 127devices
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Endpoints
Endpoints are logical channels inside aperipheral device
In hardware they’re addressablebuffers (FIFOS)
•Every host transfer includes a 4-bitendpoint number and a direction bit
CONTROL endpoint 0 is mandatory
All others are up to the design
Endpoints are handy for separatingdata types
Embedded Systems Conference, March 2002
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Transfer Typesfour types to suit data requirements
Control
•Mission-critical, highest error protection
•Used to interrogate, configure and controldevices
Bulk
Interrupt
Isochronous
BULK is easiest to use and the most flexible
Embedded Systems Conference, March 2002
Embedding USB
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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
SOF
...
Isochronous
Interrupt
Control
Bulk
SOF
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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
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Handshakesa CONTROL Transfer
I
N
A
D
D
R
E
N
D
P
C
R
C
5
Token Packet
D
A
T
A
1
Payload
Data
C
R
C
1
6
Data Packet
A
C
K
H/S Pkt
H
P
H
DATA
H
H
P
H
H
P
D
A
T
A
1
O
U
T
A
D
D
R
E
N
D
P
C
R
C
5
Token Packet
C
R
C
1
6
Data Pkt
N
A
K
H/S Pkt
D
A
T
A
1
O
U
T
A
D
D
R
E
N
D
P
C
R
C
5
Token Packet
C
R
C
1
6
Data Pkt
A
C
K
H/S Pkt
(Operation not completed)
(Operation completed)
HANDSHAKE
D
A
T
A
0
8 bytes
Setup
Data
C
R
C
1
6
Data Packet
A
C
K
H/S Pkt
S
E
T
U
P
A
D
D
R
E
N
D
P
C
R
C
5
Token Packet
H
H
P
SETUP
H is Host
P is Peripheral
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Hubs
PC
USB
USB
Device
Device
Hub
USB
USB
USB
USB
PC
USB
USB
Device
Device
Device
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Hubspower
Hub
USB
USB
USB
USB
PC
USB
USB
Device
Device
Device
500
100
100
500
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Hubsmore power
Hub
USB
USB
USB
USB
PC
USB
USB
Device
Device
Device
500
500
Wall Wart
500
500
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Hubsmore devices
Hub
USB
USB
USB
USB
PC
USB
USB
Device
Device
Device
500
500
Wall Wart
500
500
USB
USB
USB
Device
500
Easy way to expand and power devices
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Power Management
USB power consumption is carefullycontrolled
Host determines power needs of all attacheddevices
•Only configures those within power limits
USB devices must enter low power modewhen host signals ‘suspend’
•500 microamps
USB devices can signal a remote wakeup
Built-in mechanism prevents peripherals fromdrawing too much power from your system
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Device Classes
USB-sanctioned method to extend thespecification to device classes
Classes are devices with similarcharacteristics
Huge advantage—the driver is built intothe OS
•Not written and maintained by YOU
Immense simplification of the software effort if theOS supports your device
Embedded Systems Conference, March 2002
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Protocol
USB architects went well beyond theelectrical interface
•Remember IrDa?
Protocol layer insures interoperability
Compliance testing (plugfests) enforce theprotocol
USB protocol is extensible
•Device classes
•Fully custom ‘vendor’ designs
Device standardization makes the embeddedsoftware task far easier than writing custom drivers
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Protocol
A large part of the embedded systemdesign 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 asubset of full PC-based USB
Efforts are underway to simplify the softwaretask (more about this)
Embedded Systems Conference, March 2002
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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
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ProtocolChapter 9 Requests
Standard
Class
Vendor
bRequest
Custom
Programming
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
STALL
other
X
0
1
X
X
X
X
X
X
0
0
X
X
X
X
X
X
1
1
X
X
X
X
X
X
1
0
X
X
X
X
X
03
DEVICE
CONFIGURATION
STRING
01
02
STALL
other
wValueH
bmRequestType
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Protocoladded HID Class Requests
Standard
Class
Vendor
bRequest
Custom
Programming
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
STALL
other
bRequest
X
0
1
X
X
X
X
X
X
0
0
X
X
X
X
X
X
1
1
X
X
X
X
X
X
1
0
X
X
X
X
X
03
DEVICE
CONFIGURATION
STRING
01
02
STALL
other
wValueH
0B
GET_REPORT
GET_IDLE
GET_PROTOCOL
SET_REPORT
SET_IDLE
SET_PROTOCOL
01
02
03
09
0A
STALL
other
23
HID
REPORT
PHYSICAL
21
22
bmRequestType
Embedded Systems Conference, March 2002
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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
Embedded Systems Conference, March 2002
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Host Chips & Drivers
PC
Simple Embedded
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Host Chips & DriversPC implementation
Apps or class drivers create IORequest Packets (IRP)
USB Driver
•Processes IRP’s
•Manages CONTROL pipe
•Division of duties between USBDand 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 transactionlist into packets
Embedded Systems Conference, March 2002
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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 deviceusing “pipes”
Most of the HCD/HC complexity isdue to the flexible nature of USB
•127 devices
•Devices appear and disappear atrandom
•Bandwidth must be fairly sharedbetween devices
Embedded Systems Conference, March 2002
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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
Simple Embedded
Embedded Systems Conference, March 2002
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Host Chips & DriversRTOS provides a ‘middle’ solution
•Examples
–WinCE
–Linux
–VxWorks
•Host Controllervendor mayprovide a smalldriver to makethe chip looklike somethingthe RTOSsupports
Embedded Systems Conference, March 2002
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Agenda
Why USB?
USB Advantages
USB Basics
•With embedded system emphasis
An Implementation Roadmap
USB On The Go
An Embedded Host Example
Conclusion
Embedded Systems Conference, March 2002
Embedding USB
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An implementationroadmap
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
Embedded Systems Conference, March 2002
Embedding USB
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Agenda
Why USB?
USB Advantages
USB Basics
•With embedded system emphasis
An Implementation Roadmap
USB On The Go
An Embedded Host Example
Conclusion
Embedded Systems Conference, March 2002
Embedding USB
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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 theexisting 900 million USB devices
•Forecast: 1 billion devices by 2003
Embedded Systems Conference, March 2002
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USB On The Gomarketing foil
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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)
Embedded Systems Conference, March 2002
Embedding USB
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USB On The Gothe best news may be on the software side
Impossible for a small battery operateddevice 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 DeviceWorking Group
Moves complexity from your application code anddrivers into the Operating System
Embedded Systems Conference, March 2002
Embedding USB
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Agenda
Why USB?
USB Advantages
USB Basics
•With embedded system emphasis
An Implementation Roadmap
USB On The Go
An Embedded Host Example
Conclusion
Embedded Systems Conference, March 2002
Embedding USB
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USB peripheral dev tools
Full-speed USBDevelopment Board
Test USB
USB
PC
Develop--Debug
Serial
Embedded Systems Conference, March 2002
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Simple embedded host
Full-speed USBDevelopment Board
Add-on boardcontains USB hostplus 4-port hub
PC
USB
Serial
Develop--Debug
Test HostTransfers
Embedded Systems Conference, March 2002
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Simple embedded host
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Simple embedded host
Embedded Systems Conference, March 2002
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Simple embedded host
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Simple embedded host
Example:
BTC Keyboard + 2-port Hub withembedded 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
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Simple embedded host
(live example)
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Conclusion
Embedded host chips are here now
Software is the main effort
Point solution is easy
More complex solution requires anRTOS with USB support
•Third-party choices
•Some classes are supported
USB OTG aims to provide simplifiedand standardized host drivers