Integrating Usb Into Products

Integrating USB into Products Allan Neville Design Continuum Fall – Boston – 2006 Class #343

Abstract In recent years USB has taken the place of traditional serial standards as the main communications protocol between devices. Lately, there has been an increase in the market demand for products to support USB devices like keyboards, printers, and memories; or for products to communicate with a PC through the USB interface. This course will go over the USB standard and how it can be integrated into present and future products such that it can interface many of these devices. It will go over the design implications and some solutions.

Introduction Since its introduction more than a decade ago, the Universal Serial Bus (USB) protocol has become a technology “rock star”. Walk into any electronic store and most likely one out of three devices has a USB port. It started as a substitute to old communications ports on the PC such as RS232 and the parallel port to meet the new demands of the PC market. And since then it has been used in everything from coffee mug warmers to 10GByte hard drives.

Coffee Mug Warmer

Swiss Army Thumb Drive

Illuminated Aquarium

The USB specification defines everything from mechanical connectors and cables, all the way through operation of the system, network layers, and power management. We will review most of these items through the paper and look at some examples.

Brief history The USB standard was officially introduced in 1995 by a consortium form by seven computer and telecommunication industry leaders: Compaq, DEC, IBM, Intel, Microsoft, NEC and Northern Telecom. Since then it has grown to more than 1000 members in the USB implementers forum (USB-IF). In 1996 the USB 1.0 standard was released specifying the main aspects of the USB standard as we know it: support for 12Mbps high speed bus and a low speed bus at 1.5Mbps, the definition of the mechanical connectors, and the specification of the software stack.

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By 1999 due to the popularity of the USB protocol and some problems that had been identified, revision 1.1 was released. This version clarified some ambiguities regarding timing and corrected some problems that existed in the previous revision. But not until 1999, when Microsoft and Apple incorporated the use of USB into their operating systems did USB find “stardom”. In the year 2000, the USB revision 2.0 was released specifying the implementation of a 480 Mbps bus and backward compatibility with revision 1.1. Later in 2001 the USB On-The-Go (OTG) supplement was added. This was done to accommodate the increasing demand of USB handheld devices and the need to have devices talk to each other. Some of the features include smaller connectors and more efficient power management. Last year in May, the Wireless USB standard was released which is a point to point wireless communications link based on the USB network protocol. Presently, there are a few IC companies making parts for the standard but these are in their final testing stages.

Hardware Topology The USB standard specifies a star network configuration where each hub is the center of the star. The network has only one host per system, typically this is the PC but with the introduction of the USB On-The-Go (OTG) standard any device can be the Host. The network can have a maximum of seven levels of tiers; the first one being the host, which is the root hub. The last level would have only devices, which means that there can only be five non-root hubs connected together. Each segment will have a hub at the center; the cable length for each segment can be 5 meters. That means that the maximum distance between the host and the most remote device can be 30 meters (uses six hubs). This could probably be increased by using devices like IOGear’s cable boosters which boost host-device lengths of 75ft for full and low speed. The USB host manages and controls the use of the bus, and it schedules and initiates data transfers. No slave devices can assert signal on the bus until the USB host asks for it. The host and hubs also take care of detecting when a device is attached or removed. Each slave device on the bus is assigned a unique USB address through a process called enumeration which is managed by the host. The maximum number of devices allowed to connect to the bus is 127, this limitation is due to the address bit-field which is 7 bits long.

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The hub provides the connecting ports for the devices and provides power to the bus. It also manages any fault detections and takes corrective actions. As seen above, the data transfer rate is given by the hub that the device is connected to. For example if a high speed device is connected to a full speed hub, the data rate will be limited to the full speed rate. Also, higher speed hubs will support lower speed devices and sustain the communications link to the upstream hub at the high speed rate.

Power Management The USB Bus can provide power to devices that connect to it. It initially provides 100mA. This power can be used by devices during configuration. Once the device is configured, the bus can increase the current capabilities to 500mA. Power distribution is one of the nice features that the USB interface provides. As mentioned before; there are many products in the market that use the USB port for power alone, like lamps, fans, and coffee mug warmers. This kind of appliance usually use less than 100mA since they do not initiate any handshake with the host, nevertheless still it is 0.5W. Most of the USB devices (that communicate with the host) can be classified under three groups: bus-powered, self-powered and hybrid powered. The bus-powered device sinks power from the USB bus. This type of device could be either low power if it consumes less than 100mA or high power if it consumes more than 500mA. USB devices that are bus-powered have to report to the host that it will be drawing a given amount of electrical current from the VBus in units of 100mA load, this is specified in the descriptor.

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A self-powered device gets its power from an independent power supply provided with the device. A hybrid powered device takes power from both the USB bus as well as from its own power supply. As in the case of a bus-powered device, it will have to report how many units of load of electrical current will be drawn from the host via the USB bus. USB hubs can also be self-powered or bus-powered. If the hub is bus-powered the maximum current it can provide to any device downstream is 100mA, so, only low power self-powered devices will work properly. Any high power device that is detected would be able to report to the host to indicate an error. Also, the USB bus allows for a suspend mode. In this mode a device can be put into a low power consumption state without initiating a new configuration process. This is done by the host sending three consecutive no SOF (Start-Of-Frame packets, see below). Once the device detects this the device goes into suspend mode. In this mode the device will not consume more than 500uA. In a special case, if the device can wake-up, the host the current will be limited to 2.5mA. This specification would be indicated in the descriptor of the device. Some groups are working on creating a new standard for Powered USB or USB PlusPower. The USB PlusPower design would provide power voltages up to

24VDC and current capacity of 6A. This is achieved by two additional wire pairs inside the cable and modified connectors that are backward compatible with the standard USB connector.

Mechanical Connectors The standardized connectors and cables defined in the USB specification details the mechanical, electrical/mechanical stress, materials and electrical characteristics of the cables. The cable is made of four 28 AWG conductors, two for data and two for power. The data pair is twisted to increase noise immunity. The cable construction can vary depending on the USB speed designed for. Low speed cables do not require shield and should be attached to the device while full and high speed cables are shielded and are detachable. As mentioned before the cable length is limited to 5 meters. A cable description is listed below: Contact Number

Signal Name

Wire Color

1

Vbus

Red

2

D-

White

3

D+

Green

4

Gnd

Black

Shell

Shield

Drain wire

The USB standard also describes how the connector should be constructed and designed. In USB rev. 1.0, the connector and receptacle type A and B were specified. These connectors were designed such that devices could be differentiated from hosts. All -4-

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of the connectors have longer power and ground pins to ensure that the device has a stable voltage before the data signals are applied. Later in revision 2.0, the connector and receptacle type Mini-B was added to the standard due to the increase of small appliances that were using the USB standard. Recently with the introduction of the OTG supplement, connector and receptacle type Mini A was added, and a receptacle type Mini AB was added. These were added to accommodate the duality of products that could be host and device. The receptacle Mini AB could receive connectors type Mini-A and Mini-B. Also, a pin was added which is used for identification.

Type A

Type B

Type Mini A

Type Mini B

Device detection Once a USB device connects to the USB host data rate speed negotiations are carried out. The current limiter on the host limits the current consumption to 100mA, this should be enough power for a bus powered device to initiate the negotiations. USB Cable Current Limiter

Host Controller

1.5K

Vbus

Vbus

D+

D+

D-

D-

Gnd

Gnd

Device Controller

Once the device is connected, either the D+ or the D- line will be pulled high. This indicates to the host that a device is being connected. If the D+ is high, the device is a full speed, if it is the D-, the device is a low speed device. High Speed negotiation protocol occurs during the Bus Reset phase. A high-speed device is initially detected as a full speed device. Once a device is detected, the host will issue a reset signal by driving both D+ and D- to low (SE0). This resets the USB device to a default USB address of 0. Soon after detecting the reset signal, the high speed device -5-

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will signal the host with a 480 Mbps chirp that lasts from 1 to 7 ms. A high speed host or hub will recognize this signal as a request from a high speed device and respond with a series of chirps. Once the high speed device sees this it will increase its speed to high speed.

Physical Layer signaling The USB standard does not include a clock signal, so the communications are asynchronous. Data on the bus is encoded using Non-Return to Zero Inverted (NRZI) with bit stuffing. This encoding enables the receiver to remain synchronized with the transmitter without the overhead of sending a clock signal or start and stop bits with each byte. Instead of defining logic 0s and 1s as voltages, NRZI defines logic 0 as a voltage change, and logic 1 as a voltage that remains the same. Each logic 1 will result in no change. The bits transmit least-significant-bit (LSB) first. Bit stuffing is required because the receivers synchronize on transitions. If the data is all 0s, there are plenty of transitions. But if the data contains a long string of 1s, the lack of transitions could cause the receiver to get out of sync. If the data has six consecutive 1s, the transmitter stuffs, or inserts, a 0 (represented by a transition) after the sixth 1. This ensures at least one transition for every seven bits. The receiver detects and discards any bit that follows six consecutive 1s. The bit-stuffing overhead for random data is just 0.8%, or one stuff bit per 125 data bits.

The fundamental element of communications on the USB bus is the packet. A packet is made of three parts: the start, the information, and the end of the packet. The start of a packet will be the transition from the idle state (J) into an active state (K). In low speed the D+ line is low and the D- is high when the bus is idle, and in full speed the D+ line is high and the D- is low. The active state is defined the opposite way. For high speed the J and K states are defined as the full speed bus specification but the idle state is when both D+ and D- are low. At the beginning of the packet there is sequence of transitions which is called the SYNC sequence. This sequence is made of a chirp pattern of 32-bits where each bit is a J or K state, as described above. The SYNC chirp pattern is KJKJKJK...KJKJKJKJKK. Some of these bits will be used to synchronize the clocks of the host, hubs, and devices. The packet information varies from 1 byte up to 1024 bytes. This section of the packet is formed by the Packet Identifier (PID) and the payload. The PID is the first byte of the

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packet, the first 4 bits of the byte are to identify the packet type while the other four are the complement of the first four. This is used for error checking. The last part of the packet is end-of-packet identifier (EOP). The EOP is indicated by having both D+ and D- low for two bit times. In a high speed bus, the EOP is indicated with 40 bit-times without a transition.

Packets As previously mentioned, the packet is the basic component of the communications standard. There are four types of packets: token packets which are used to set-up the communications link and describe the direction and use of future packets; data packets are used to carry data; handshake packets which are used as controls to maintain the reliability and integrity of the link; and last is the special packets which are mainly used with high-speed devices. Below is a small diagram that describes the most common packets. The SOF (start of frame) packet is used to indicate the beginning of a frame or microframe (see below). It has 11 bits dedicated to the frame number and a 5 bit CRC. The IN, OUT and SETUP packets are token packets that are used to setup the data transfer between the host and the device. They contain a 7 bit device address, a 4 bit endpoint address, and 5 bit CRC. The IN packet is used to setup data transfers from the device to the host, the OUT packet is used for the transfer of data from the host to the device, and the SETUP packet is a high priority OUT packet indicating the device that it must accept it. The DATA packets are used for data transfer, and this can have a payload that varies from 0-1024 bytes. This type of packet has a 16 bit CRC for error detection. Also, when transmitting data the software can toggle between the DATA0 and DATA1 token, this helps to identify if packets have been lost. The handshake packets are used by the data receiver to indicate the quality or the reception. ACK indicates that the data was received without error. NAK is used by the device to indicate that it is busy, STALL is used when a control request failed, and NYET indicates that is not ready to receive more data.

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Frames All USB communication links are broken into frames. Each frame is transmitted every 1 ms. A high-speed host will also add a microframe every 125us to minimize the buffer requirements for high speed devices at the cost of increasing the system complexity. The first packet of each frame is the Start-Of-Frame packet (SOF).

Transfers A predefined sequence of packets is called a transfer. The USB standard specifies four different transfers to move different types of data, these are: Control, Isochronous, interrupt, and Bulk.

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The control transfer type is non-periodic, it is used mostly for commands and status operations. All the devices must support control transfers over the default endpoint (Endpoint 0). Control transfers require both and IN and OUT pipe. Isochronous transfers have a fixed number of bytes per frame, the bandwidth is guaranteed. The maximum number of bytes is 1024 per frame for full speed and 1024 per microframe for high speed. This transfer type is typically used for streaming data like audio. It is error tolerant and it is only supported by full and high speed devices. The interrupt type is periodic and the latency between transactions is guaranteed. The maximum packet size is 8 bytes for low, 64 for full and 1024 for high speed bus. The bulk transfer type is not periodic and it is used mostly to transfer large amounts of data. The host controller ensures that bulk transfers are eventually completed but it does not guaranteed its bandwidth. It is only supported by full and high speed devices.

Endpoints All packets are sent to and received by the device via endpoints. Endpoints are buffers where a device either puts data to go out or gets data that has arrived. Up to sixteen endpoints can reside within a device. Each endpoint has a direction and an address. The direction is from the host perspective: OUT is going into the device, and IN is coming out of the device. Endpoint 0 is always the control endpoint and it contains both input and output endpoints. Once an endpoint is identified and defined, a pipe is established between the host and the endpoint. The pipe is only removed once the device is removed from the USB bus. Pipes are established for an interrupt and isochronous endpoints to ensure there is sufficient bandwidth for the data transmissions.

Enumerations Once the device is attached, enumeration starts. During enumeration, the host requests a number of data structures called descriptors from the device. These descriptors contain information about the number and type of communication channels, or endpoints, that the USB device desires to use, as well as information about any device class. Enumeration occurs on the default endpoint, which is endpoint 0, also known as the control endpoint. The host also assigns a unique 7-bit address to the device, directing communications to a particular device.

Descriptors Descriptors are data structures that are provided by the device to the host during enumeration. It describes the USB device’s attributes. It is typically stored in an EEPROM in the device’s circuit. There are two types of descriptors; the standard descriptor that pertains to all USB devices and the class descriptor that are associated with each particular class. Among the standard descriptors, is the Device descriptor, which contains general information about the USB device, such as Product ID, Vendor ID, and such. The Configuration descriptor provides information regarding specific device configuration. The Interface descriptor tells the host how many endpoints a device feature uses, and it -9-

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also declares the class identity. The last type of descriptor is the Endpoint descriptor, which describes the properties of an endpoint. These properties are namely whether the endpoint is IN or OUT, etc. Each endpoint has its own descriptor.

Classes The growth of applications for the USB bus has motivated the USB organization to specify certain standard for products that have similar characteristics, these groups are called classes. The creation of these specifications allows developers to create generic device drivers that could be used with different devices within the same class. A single device can belong to multiple classes. Below we can see a list of the most common classes and examples. Class Name Audio Class Mass Storage Class Human Interface Device (HID) Class Imaging Class IrDA Class Printer Class

Sample of Application MP3 players, speakers Flash drives, external CD-ROMs Mice, keyboards, joysticks Cameras, video recorders IrDA interface Printers

USB On-the Go The On-The-Go specification is given as a supplement to add-on to the main USB2.0 specifications. Therefore the essence of USB being a Host-centric system has not changed. What is new is that a device can have dual capability of a Host and Device on the same USB connector. However, they can only be used as a Host or Device one at a time and not concurrently. As mentioned before a set of smaller connectors were defined to accommodate the smaller size of the devices. The minimum power consumption is reduced to 8mA since many of these devices can be battery powered and are limited in resources. This supplement also adds specifications for the protocol that handles the dynamic switching between device and host, and defines session request protocols (SRP) that allow the host to turn on or off bus-powered devices.

Wireless USB The Wireless USB specification was added in 2005 to meet the increasing necessity of wireless devices. This specification uses Ultra Wide Band (UWB) as the physical platform for communications. The standard specifies throughput rates of up to 480Mbps (at 2 meters) and ranges of up to 10 meters. This protocol is design as a host spoke architecture and allows up to 127 devices to attach to it. It has built in security features and power management features for the radio. As in previous standards it should be compatible with previous versions of USB standards and for this purpose it defines entities that bridge the wired and wireless domains.

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Devices that meet this standard are still in their infant stages. Some companies like NEC and Wisair have some products available that would allow Wireless USB communications.

Compliance The USB organization has created a compliance program to ensure that devices meet the standard specifications. Though no law mandates that any device must pass these tests, doing so ensures that the user’s experience with your product will be as troublefree as possible. The compliance program's two criteria are checklists and compliance testing. The checklists contain questions relating to a product and its behavior. Separate checklists exist for vendors of peripherals, hubs, systems with USB hosts, and cables. The checklist covers mechanical design, device states and signals, and operating voltages and power consumption. The checklists are available from USB-IF's website.

Basic-Speed Version

Hi-Speed Version

On-The-Go BasicSpeed Version

On-The-Go Hi-Speed Version

Wireless USB

For thorough testing of a product under a variety of conditions, USB-IF members can enroll a device in the Compliance Program for a small membership fee. When a device meets the compliance program's criteria, USB-IF considers it to have reasonable measures of acceptability and adds it to its Integrators List of compliant devices. Usage of the USB logo requires a product to be compliant as demonstrated by passing the USB-IF Compliance Testing Program. There are a set of registered USB logos which are not interchangeable, see above. On receiving a signed license agreement and payment, USB-IF authorizes the device to display the USB logo. In addition to successfully completing the USB-IF Compliance Program and having their product included under their company name on the Integrators List, companies must execute the USB-IF Trademark License Agreement to be eligible to use the logo in conjunction with the product.

Solutions When starting a USB design, just as with any other project, one should ask multiple questions that will help the designer to select the right parts and architecture for the design: • Is this design considered a host, a device, or both? • Is the instrument going to interact with other devices or a PC? • Are we communicating at low, full, or high speed? • Is it considered a high power or low power device? • Is the architecture going to be based on stand alone design or multiple processors? - 11 -

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If multiple processors are to be used, how do they interface? To which devices is the design going to interface? HID class? Mass-storage? Etc? Should there be a USB hub? Do we need the USB compliance or could we live without it?

As you proceed in your hardware design and look for solutions, you will encounter an unlimited set of possibilities either for a host design or peripheral. The most simple of these options is to purchase an external USB conversion adapter that interface the communications interface present in your design. To do this, there are products from companies like B&B or Belkin that convert USB to RS-232, or USB to Ethernet. Also there are companies that already provide solutions with USB interfaces to DAQs and motion controllers. If the requirement is more stringent in cost and size, these adapters can be implemented with a set of ICs, and the circuits can be integrated with your present design. Some manufactures such as TI, FTDI, and Kawasaki provide solutions to convert USB signals to Ethernet or RS-232. Other companies such as Philips, Micronas, Cypress, Atmel, and Genesis provide solutions for specific designs including Flash Drive Interfaces, Audio Codecs, and Smart Media. If your solution requires a specific low-cost controller, companies such as Microchips, Atmel, and Cypress provide 8-bit ARM controllers with built-in device interfaces, host controllers, or on-the-go controllers. These solutions usually integrate the physical layer and some level of the error detection. For more sophisticated designs, Freescale, Intel, TI, and NEC provide higher end controllers with 32-bit cores or DSPs that allow you to interface devices that are more complex and require more intensive computing power. The implementation of a USB interface does not only have implications on the hardware but also has implications on the software. If the design for a USB peripheral, then you need to be sure that the software complies with the USB protocol handshakes and class description. Usually the main USB software effort involves providing a descriptor to the USB host and respond to the host instructions. If the software being developed is for a host, the implications are larger since the host is a more complex entity. The software in the host must interface to the host controller (if it is another IC), the USB protocol, and then the drivers for the device which may have to comply with one of the pre-defined classes given by the USB organization. To assist the software development effort and minimize the risk of meeting deadlines, one can look at commercial software that is already available, especially when developing a USB host. One option is to buy software stacks from companies such as SoftConnex (owned by Transdimension) and Jungo. Depending on the company, they usually provide a stack that has been rigorously tested and can interface to either custom OSes or commercial ones. Also depending on their license agreements and business strategies, they provide different packages that include USB protocol only, USB class stacks, source code, or onetime expense.

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Another option is to use a commercial OS such as WindowsCE, Mentor’s Nucleus, Linux (commercial or not), etc. This option provides an OS and in most instances includes drivers that are available and tested. Most of these OSes are already ported to selected microcontrollers. In some instances, the only task left is to create the host controller interface since the host in embedded system designs is implemented with a circuit that it is not common for PCs.

Examples Example 1: A USB Peripheral/Host A biomedical instrument had the following requirements with regard to the communications interface: • • • •

Interface to USB printer Interface to USB keyboard Interface to USB Flash drive Interface to PC USB port

One special note is that the peripherals and the PC interface are mutually exclusive. All of these features could have been implemented with cheaper and simpler technologies such as serial and parallel interfaces, but a primary attractive feature was the standardization and availability of products with USB. The design had included a 32-bit microcontroller to do the main functions of the instrument and the user interface. So at the beginning of the design, we looked at options of microcontrollers that had USB features built in. Most of them only had USB device interfaces, and the few that had host and device controllers were not available or were more expensive than our final solution. So we selected a standard 32-bit controller and looked for a peripheral IC that would meet our specifications. To meet the above specification, we looked at multiple options and ICs, mainly from Cypress and Phillips. We decided on the Cypress EZ-Host solution (CY7C67300) because it had four channels which allowed us to interface all the peripherals at the same time. It also had a few options to interface the main 32-bit microcontroller and our experience with their technical support has been great. The diagram below shows a high level design, it only includes the flash drive interface, the other two interfaces are also attached to the EZ-Host. We connected the Cypress part and memory mapped it into the microcontroller memory space. Since we were connecting to off-the-shelf peripherals, we had to be cautious with the design of the power supply and power management of the USB bus. We had to mitigate in-rush currents and limited the power consumption such that it did not drain our power resources.

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The next task was to choose a software development path. If this instrument were a peripheral only, the software path would have been much simpler since a peripheral only has to support a limited number of functions to be USB compatible, mainly provide a descriptor and do some handshaking. But since this instrument also had host functionalities, the software implications were large and became critical in the short timeline we had for its implementation. We evaluated off-the-shelf USB stacks, commercial OSes, and custom OSes. The solution we came about was to use a commercial version of embedded Linux. This reduced the development effort of building the USB drivers, it also gave us the flexibility to have drivers for a variety of USB devices (printer, keyboard, and flash drive) that already exist, and it had a lower cost than some of the other software stacks available. The only missing piece was the software interface required by the host controller, but fortunately this was also provided by Cypress with their development kit for the EZ-Host controller IC. Then it just became a system integration effort. Since this design other products are available in the market that might ease the design process. Among these, there is a controller designed by GHI Electronics called USBWiz that can interface USB devices and interface a secondary processor through a serial port.

Example 2: Software Power Switch A biomedical instrument had several USB devices (a camera and a motion controller) all connected through a USB hub, built in. The design of the instrument required a software-controlled power switch and limited the number of cables between the PC and the instrument to one. Also, the power switch meant that it had to be powered from a second source since the main power supplies were not supposed to be ON.

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Our solution was to integrate the power switch to the USB bus. The design was based on a Microchip PIC18F2455 which has a USB interface. The PIC was the interface to the USB port and it controlled a solid state relay. This relay then switched the main AC power lines that were the source for the main power supplies. We presented the device as a HID interface such that the software development effort on the PC was not a huge task.

Example 3: USB/RS232 Adapter In another instance, a design came about that required a product update. The present product was instrumentation equipment that had a RS-232 interface to the PC. The RS232 interface was part of a proprietary circuit that was not well-documented, and the owner did not want to make much of the schematic available. For this application we used a similar Microchip part to the one above. The reason this option was selected was were previous knowledge of the vendor USB hardware, low cost, and flexibility to add new features, such as more digital I/O lines and analog input.

Other solutions we could have used were RS-232 adapters from Belkin or B&B, but these were too big for the space constraints the design had, ICs from FTDI and TI were also available that required no software development but the limitation was the digital I/O.

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Example 4: USB Hub Design In this application we had a need to connect four RS-232 devices into a highly integrated device that had a built in PC. Unfortunately the PC did not have any RS-232 ports. The only option was to convert all the RS-232 lines to USB and put all off them through a USB hub, such that we could connect it to the USB port of the PC. For the RS-232/USB adapter we used a similar solution to the one mentioned above. For the USB hub, we used a simple TI part number TUSB2046, the reason we used this part was its simple integration, low cost, and that it was USB 1.1 compatible.

Other options were available from Cypress and Philips, especially components that were USB 2.0 compatible.

Summary This paper has shown you a glimpse of the USB standard and how can it be used in your future projects. The USB protocol has features that appeal to both product developers and users, including bus powered devices, auto-detection, self configuration, expandability, and speed. Of course, all of this simplicity is at the cost of a more complex and expensive hardware and software design than the older serial and parallel interfaces it replaces. The interface is flexible enough to use for common peripheral types like drives and keyboards, as well as custom, application-specific designs like medical instruments, instrumentation equipment, or coffee mug warmers with PID controls.

Reference Documents • • •

Universal Serial Bus Specification, Revision 2.0 USB Design by Example: A Practical Guide to Building I/O Devices, Second Edition by John Hyde USB Complete by Jan Axelson

Web Sites •

www.beyondlogic.org/index.html#USB – Has a great USB reference and links to a bunch of vendors

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www.usb.org – Has all the standards and a few presentations that have useful information www.everythingusb.com – Has a lot of products and new applications for USB www.usbman.com – Has a lot of developer tips and other useful stuff

About the author Allan Neville is a Sr. Electrical Engineer with Design Continuum, Inc. He has over ten years of experience developing embedded systems for medical and consumer products. He has worked on a variety of projects designing and developing analog and digital circuits, RF circuits, motion control systems, and PC interfaces. He also has vast knowledge of firmware and software development. Allan can be reached at [email protected]

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