USB simplified - adding USB connectivity to applications with legacy serial connections

by Pedro Pachuca , TechOnline India - February 09, 2011

USB is becoming the interface of choice for enabling connectivity to new applications with its ease of use, plug-and-play functionality and increased robustness.

Universal serial bus (USB) has become an enormous success in industrial and commercial applications as it continues to replace many legacy serial connections (i.e. RS-232, 485). USB is becoming the interface of

choice for enabling connectivity to new applications with its ease of use, plug-and-play functionality and increased robustness.

However, for embedded solution designers, a USB implementation requires expert knowledge of the USB protocol, exhaustive software development and significant design time. In addition, USB-based microcontrollers

(MCUs) may not offer the right peripheral set required for an application, resulting in time-to-market delays, increased design complexity and added cost.

This is, however, a relatively painless and economical alternative:using fixed-function USB bridge chips to add USB connectivity to anyembedded MCU-based system that uses serial communications, as shown in figure 1. This bridge-chip approach requires no USB expertise or firmware development, thus enabling designers to focus their time and resources to innovate and differentiate their applications.



  Figure 1: Using USB-to-UART bridge chips to add USB to legacy serial designs


USB has achieved its primary goal of simplifying the way consumers control peripherals and transfer data. With more than three billion USB-enabled devices shipped into the market, USB is not only the fastest growing interface in consumer applications but has also achieved significant growth in industrial markets. However, USB’s ease-of-use, plug-and-play functionality and robustness do not come for free for embedded solutions designers. Developers are often forced to spend a significant amount of time learning about the USB protocol, covering everything from its fundamentals to more sophisticated implementations.

Even after overcoming this learning curve, designers face another major challenge when they are forced to develop a USB software stack. This is not only time consuming but may also require specialized debugging
tools, which can further increase overall development cost. Although there are commercially-available USB software stacks, they represent an extra cost, and significant time is still required to learn about implementation details. Moreover, the challenge can be even more complex when the final product needs to maintain compatibility with multiple operating systems or operate in an environment where operating systems are constantly being updated.

Finally, another factor that needs to be considered when implementing USB is that of increased hardware design cost due to the USB-based MCU requiring external components, such as crystal oscillators and termination resistors, to provide the USB functionality. This is typically the case with many USB-based microcontrollers.

“Wouldn’t it be a major step forward in USB evolution if all the benefits that end users enjoy (ease of use, plug and play functionality and robustness) were also available to designers?”

Pursuing a simplified and economical USB implementation for designers should be the cornerstone for new USB IC solutions. This approach requires a highly-integrated solution that simplifies hardware design and reduces cost by eliminating external components. It also requires the elimination of USB software development to enable USB ease-of-use and maximize design time. Finally, to eliminate the need for driver installation, solutions should come with fully-tested, royalty-free drivers that are compatible with most operating systems and capable of supporting the USB-defined classes natively supported by most operating systems.

Solutions that can provide all of the above benefits and can be implemented in the system using standard interfaces, such as RS-232, RS-485 or I²C, are available on the market today. These solutions represent a major step forward in supporting the continued penetration of USB in the marketplace.

The CP21xx USB bridges from Silicon Laboratories are examples of such solutions. They provide an array of benefits to embedded developers and can help minimize design complexity and reduce implementation costs.

The CP21xx USB bridge family is a highly-integrated solution that enables USB connectivity to be added to virtually any microcontroller-based solution. These devices provide a bridge to the USB world through the use of the standard UART or I²C interface common on most microcontrollers. In order to simplify the design and reduce cost, the CP21xx family integrates the hardware and software necessary to interface with USB and serial interfaces. In addition, CP21xx products use a novel clock recovery system that reduces costs by
eliminating the need for an external crystal oscillator.


Fully-compliant USB-HID bridges

To further simplify and enhance plug-and-play capabilities for designers, innovative fully-compliant USB-HID bridges are also available. Typical USB-HID to UART configurations are illustrated in figure 2. These unique bridge devices fully support the USB-HID class that is natively supported by most operating systems, thereby eliminating the need for driver installation. The USB-HID class predefines functions that allow hardware manufacturers to design a product to USB-HID class specifications and expect it to work with any software that also meets these specifications. USB-HID class drivers are included in most modern operating systems allowing faster deployment of devices and easier installation by end users. Examples of HID-USB devices include computer keyboards and mice, but the HID-USB class is sufficiently flexible to accommodate many different kinds of USB designs.


 Fig 2: HID USB-to-UART device


To keep a solution small and ergonomically efficient, portable applications require that PCB space be minimized. In addition, some legacy applications may require keeping the same physical dimensions
while upgrading their application to include USB connectivity. The CP21xx USB bridge chips overcome this constraint by using QFN space-saving packages that reduce the PCB footprint and enable the designer to squeeze a solution into a space smaller than an RS-232 connector.

These small-footprint, economical USB bridges benefit a wide range of embedded applications, such as portable medical and industrial connectivity applications, that require easy yet robust implementations of USB to be able to communicate with modern systems or computers. By means of a simple connection to a standard serial interface, such as RS-232, RS-485 or I²C, USB bridges enable designers to add USB connectivity to these applications without the painful efforts involved in redesigning an entire system.

In conclusion, USB bridges are an easy and economical approach to adding USB connectivity to microcontroller-based solutions. Their high integration reduces cost, and their simplicity eliminates the need for
USB firmware development, which, in turn, enables designers to focus their time and resources on innovating and differentiating their applications. USB-HID based solutions further simplify the implementation of USB by eliminating the need to install drivers, thereby providing seamless compatibility with most operating systems. In
combination, this makes USB easy not only for consumers but for designers as well.

About the Author:

Pedro Pachuca manages Silicon Labs’ global microcontroller (MCU) interface product business. Mr. Pachuca joined Silicon Labs in early 2010. Previously, he was a product marketing manager at Freescale Semiconductor where he developed MCU business strategies to penetrate new global markets and managed a business with an annual run rate in excess of $250 million. Mr. Pachuca holds a BSSE degree from the Instituto Politecnico Nacional at Mexico City.

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