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.

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.

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 some of these issues in this article and look at some example im[lementations.

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.

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.

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 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.

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.

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.

Doing USB designs - some design guidelines
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?
* 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.

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.

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 in Figure 1 below shows a high level design, it only includes the flash drive interface, the other two interfaces are also attached to the EZ-Host.

Figure 1

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.

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 up with 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.

As shown in Figure 2, below, 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.

Figure 2

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 (Figure 3, below). The RS- 232 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.

Figure 3

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.

Example 4: USB Hub Design
In this application (Figure 4, below) lwe 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.

Figure 4

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
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.

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. 

References and resources
1) Universal Serial Bus Specification, Revision 2.0
2) USB Design by Example: A Practical Guide to Building I/O Devices, Second Edition by John Hyde
3) USB Complete by Jan Axelson http://www.penram.com/PC-Communication/USB-Complete.asp
4) www.beyondlogic.org/index.html#USB " Has a great USB reference and links to a bunch of vendors.
5) www.usb.org " Has all the standards and a few presentations that have useful information
6) www.everythingusb.com " Has a lot of products and new applications for USB
7) www.usbman.com " Has a lot of developer tips and other useful stuff

This article is excerpted from a paper of the same name presented at the Embedded Systems Conference Boston 2006.