Nuts and Volts - February 2009

THEDESIGNCYCLE ADVANCED TECHNIQUES FOR DESIGN ENGINEERS

■ BY FRED EADY

A REASONABLE REPLACEMENT FOR RS-232

I'VE FINALLY BEEN PUSHED OVER THE EDGE. I'm going to include USB interfaces on subsequent Design Cycle projects that require serial communications with a personal computer. This has been a long time coming as I have been purposely steering clear of anything USB in our Design Cycle projects. My gun-shy attitude towards USB is rooted in my past experiences with attempting to understand and apply USB technology to my embedded designs. I recall trying to understand all of the tables and states that the USB protocol required for proper operation, and the tremendous coding effort that, at the time, was necessary to build a USB interface. All of that stuff turned me off to using USB in my projects.

I've been peeking through the fence at stand-alone USB ICs that are capable of replacing the venerable RS-232 interface ICs I've grown to love over the years. The new generation of USB ICs claim to be able to fall into the place of legacy RS-232 drivers with nothing more than a printed circuit board (PCB) change. That's fine on the hardware side. However, what really has to be done on the firmware side to make the transition? Is the transition to the USB firmware interface driver a "no-brainer," as well?

THE SILICON LABORATORIES CP2103

The majority of the microcontroller projects I do for Design Cycle include an RS-232 interface in the design.

The advantages of implementing a microcontroller RS-232 interface are many. For instance, a microcontroller equipped with an internal UART can easily transfer data between itself and another microcontroller, or itself and a PC application. A microcontroller RS-232 interface is also an easy way to provide visual indicators for human consumption. A good example of a microcontroller application using its RS-232 interface to convey information to a human via a PC terminal emulator application is the WLAN Phoenix. Not only does the Phoenix microcontroller application display IP and MAC addresses, the embedded RS-232 interface allows the personal PC emulator application to pass human-generated information from the PC to the microcontroller.

According to the CP2103 datasheet, using it to replace the RS-232

■ PHOTO 1. Most of the Evaluation Kit printed circuit board is dedicated to allowing the user access to the CP2103's hardware resources. The SP3243EU RS-232 converter IC is included to allow easy connection to a device equipped with a standard RS-232 interface. The USB side of the Evaluation Kit is intended to connect to a personal computer.