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

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 any embedded 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.
 

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.