Nuts and Volts - February 2009

THE DESIGN CYCLE

■ SCHEMATIC 2. Maybe if I wait a bit longer they'll roll the capacitors and ESD diodes into the CP2103, as well.

With the Eval Kit, we have all of the necessary hardware to make a USB-to-RS-232 connection. However, not one bit of data will transfer without the help of the Virtual COM Port (VCP) driver. The Kit comes with a CD-ROM which contains the VCP drivers, the CP2103 datasheet, a User's Guide, application notes, the USBXpress Development Kit application, and some example software. The USBXpress Development Kit is intended for those of you that wish to roll your own CP2103 based application using Visual C++ or Visual Basic. Some very useful CP2103 configuration utility programs are also part of the USBXpress Development Kit package.

I loaded up the Windows VCP driver without incident. Since our goal is to replace an old-fashioned piece of RS-232 hardware with a new fangled piece of tiny USB hardware in an embedded environment, I decided that a good initial test of the CP2103's capabilities would be to interface a WLAN Phoenix's RS-232 port to the Eval Kit. The Phoenix/CP2103 lashup can be seen in Photo 2. For those of you that are unfamiliar with the Phoenix, it is a PIC18LF6722-based Wi-Fi device that was featured in a previous edition of Design Cycle.

The Phoenix will transmit a menu via its RS-232 port at power-up. If the CP2103 is really a candidate to be a transparent replacement for the SP3232, I should be able to fire up a Tera Term Pro terminal emulation session on my laptop and receive the Phoenix's power-up menu message from its RS-232 port by way of this new USB interface. With that, I proceeded to the Windows Device Manager to force the CP2103's VCP driver to lock into COM1 of my laptop. I then invoked a Tera Term Pro session on the laptop and set the terminal emulator for COM1 operation at 57600 bps, which matches that of the Phoenix's serial port. I powered up the Phoenix and behold! The menu you see in Screenshot 1 appeared in

the laptop's Tera Term Pro emulator window. I've seen enough. Let's lay the groundwork towards building up some project hardware that incorporates the CP2103.

DESIGNING WITH THE CP2103

If you take a count of the actual electronic components in Schematic 2, you can total them all on the fingers of one hand. Let's see if we can figure out what those five components are doing for us. All of the CP2103 pins on the left-hand side of the schematic are dedicated to the RS-232 interface with the exception of the CP2103's GPIO pins, which we're going to ignore for now. The datasheet tells us that all of the modem control signal pins can be left disconnected if not used. Only the TXD and RXD signal pins are excluded from optional disconnection. So, let's begin with pin 6 of the CP2103 and work our way down the right-hand side of Schematic 2.