Choosing (and Implementing)
the Correct USB Connectivity
for Your Embedded System
Steve Wetterling, MSEE
Email = octoberbest@servenger.com
Pat Barrett, BSEE, P.E.
Email = pat@barrettengineering.com
USB Basics
from the User’s Point of View
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Easy to use – all configuration is automatic
Fast data transfer
Reliable data transfer
Adaptable to different applications
Wide range of devices and peripherals available
Cheap to buy
Fully supported by Windows™ and Linux
The USB development community keeps
improving it
Where Did USB Come from ?
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A joint-effort by a group of manufacturers (Compaq,
HP, Intel, Lucent, Microsoft, NEC and Philips)
Replaces the aging Parallel Printer Port and
Serial Port PC interfaces.
Supported by the USB Implementers Forum, Inc.
(USB-IF, www.usb.org), in Beaverton
USB-IF Member companies get access to Vendor Ids,
specifications, testing, and use of the USB logos.
Describing USB
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Universal Serial Bus
Serial data transfer @ 1.5M, 12M or 480M bits / sec.
Short distances (16 ft), with Hubs to 96 ft max
Asynchronous (not slaved to a common clock)
Master to Slaves = Host to Peripherals
A 1 to N network, where N = 1 to 127
Peripherals can be powered by Host (500mA limit)
Provides data error detection with retransmission
How USB Works
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A single Host communicates with each/all Peripherals
The Host recognizes a newly attached Peripheral and
“Enumerates” to establish communication
The Host assigns an address to each Peripheral
Transfer speed is established during Enumeration:
Low Speed
Full Speed
High Speed
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1.5 M/ bits per sec
12 M/ bits per sec
480 M/ bits per sec
More How USB Works
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Each Peripheral can contain 1 or more “Endpoints”
Successful Enumeration establishes a logical “Pipe”
between the Host and each Endpoint.
Data transfers between the Host & Peripheral through
established Pipe, at the agreed-upon speed
Pipes are automatically terminated when the peripheral
is disconnected or fails to respond.
More How USB Works
 Four
“Data Transfer types” are available, to fit
specific applications:
Control –
For short messages used during Enumeration,
Identification and Control
Bulk –
Good for moving big files to/from Printers,
scanners, and disk drives
Interrupt –
Used for Mice & Keyboards (small amounts of
data that must be delivered to a PC quickly
and predictably)
Isochronous – For unidirectional streaming of audio & video,
no error correction.
More How USB Works
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Host sends & receives data in “time slices”
called Frames:
For Low Speed & Full Speed – Frames are 1 msec
For High Speed – Microframes are 125 μsec
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Data transfers are organized as “Transactions”
consisting of:
Token Packet (destination ID bytes)
Data Packet (variable length)
And / Or
Handshake Packet (message acknowledgement)
More How USB Works
Different transfer types use Frames differently:
For High Speed:
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Control:
31 64-byte Transactions / microframe
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Bulk:
13 512-byte Transactions / microframe
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Interrupt:
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3 1024-byte Transactions / microframe,
with error correction
Isochronous: 3 1024-byte Transactions / microframe,
without error correction
USB technology continues to evolve
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Basic USB
Includes Low Speed & Full Speed
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High Speed USB
Includes Low Speed, Full Speed & Hi-Speed
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USB On-The-Go
Enables direct Camera to Printers
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Certified Wireless USB
New wireless standard (later than planned)
Implementing a USB Product
Many Choices to Make:
Will this be a Host, a Hub or a Peripheral ?
Implementing USB Host
More Choices:
 General Purpose (full feature set)
Or
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Specific Purpose (application specific feature set)
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Windows based: Use existing Microsoft standards
and PC hardware
Or
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Non-Windows based: Use existing open source
standards and Linux kernels - customize to application.
Implementing USB Hub
Chipsets from Asian sources are now available
The decision is often:
Make? - copy a reference design
Or
Buy? - re-label / re-package existing product
Implementing USB Peripheral
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Determine
a selected sub-set of USB features for your product.
List
the USB functions to include and exclude
 This is an essential step for the project!
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Locate
chipset and software solutions that come close
Ex: FTDI chipsets (www.ftdichip.com) enable rapid USB
Peripheral development on a reasonable schedule and
budget
USB Limitations
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Consider speed, distance & latency
USB is not “peer to peer” (USB On-the-Go is a partial fix)
New products must work with legacy hardware
Complex Protocol
Software is available from hardware & OS vendors
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Not free – Pay fees to USB-IF Inc. for
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Access to official USB committees, website and meetings
USB Vendor ID and Product ID
Compliance te$ting required (to use the USB logo)
USB Limitations
Not a good choice for real time controls:
 Software in the Host controller cannot predict or
control the exact time to make something happen
(Synchronous USB attempts to address this)
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Peripherals cannot demand service
No Host-to-All-Peripherals “Broadcast” capability
...Better choices would be CAN BUS, Field Bus, etc.
Overcoming USB Limitations
However, really interesting USB based products are
being created every day.
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Using USB-controlled stepper motors, actuators,
valves, sensors, displays and digital radio links.
Example: Googling “USB motor control” produces 119 hits
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Using Windows™ or embedded Linux for the control
software.
Using new FTDI “Vinculum” chipset to simplify the
implementation of the USB Host function.
References
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USB COMPLETE, Third Edition, Jan Axelson, ©2005
Microchip Technology (www.microchip.com):
Drill down to Design Center > Connectivity > USB for Devices,
Application Notes, Development Tools, and free Firmware.
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FTDI Inc (www.ftdichip.com)
for their Serial-Data-over-USB products and new USB Host
chipset (the FTDI U.S. design center is in Hillsboro)
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Cypress Semiconductor
offers a broad line of 8-bit processor-based USB interface devices
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ARM9-based processors with USB interfaces
are available from Cirrus Logic, Atmel, NXP and other vendors.
More References
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Microsoft:
Drill down to Windows Hardware Developer Central > USB
Architecture plus their extensive .NET software products.
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The Open Source Initiative (www.opensource.org)
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Linux and variants
Providers now support many USB standards
Many are active in Oregon, including:
Linux.com
Open Source Development Labs
Even More References
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HIDmaker USB
software development tools from
TraceSystems (www.tracesystemsinc.com)
“HID” means “Human Interface Device” - the most
simple implementation for USB peripherals.
What We Do
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New Product Definition and Development:
30 years in the New Product Development business.
Have completed successful development projects
resulting in more than $1 Billion in revenue.
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Circuit Board design, prototype, test,
and introduction to manufacturing (locally or Asia)
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Problem Solving
Project Management
Who We Are
Steve Wetterling, MSEE
steve_wetterling@ieee.org
503-860-5594
Pat Barrett, P.E.
pbarrett@ieee.org
503-245-0325
Email specific to this presentation = OctoberBest@servenger.com