April 2005, Issue 177

Simple USB Data Acquisition

CROSSING THE BRIDGE

After the code is running on the LPC2138, the raw temperature data exits the MCB2130 board’s serial port and meets the USB-to-UART bridge board, which contains the CP2101 USB-to-UART bridge controller and an RS-232 transceiver. The board’s power also comes from the USB port, once again eliminating the need for an ugly wall wart power supply. I used the board to convert RS-232 serial data from the MCB2130 board to compatible USB data for the PC.

The CP2101 is highly integrated and requires no components other than a USB connector. It includes a USB 2.0 full-speed function controller, USB transceiver, oscillator, EEPROM, and asynchronous serial data bus (UART) with full modem control signals. The device’s packaging is an unbelievably compact 5 mm × 5 mm MLP-28. I’ve soldered many surface-mount components under the microscope on prototype boards, but this device was by far the trickiest, especially because it doesn’t have external leads! (If you plan on soldering by hand, make your PCB footprint pads a bit longer to allow for better solder flow with a fine-tipped iron.)

Looking toward the software end of things, the nice part about using this device is that special software isn’t needed for the RS-232-to-USB conversion. This allows USB communication to become totally transparent for the LPC2138 and its UART. Just connect the LPC2138 UART pins to the CP2101, and it will take care of the rest.

You’ve probably guessed that there must be some software intervention for CP2101 data to get to the PC over USB. Yes, there is. It’s via a virtual COM port driver installed on the PC side. These drivers make your USB port seem like another COM port on your PC’s operating system (thus the virtual COM port name). Silicon Labs provides the virtual COM port drivers with its development board for Windows, MAC, and Linux.

The interesting thing about the drivers is that existing PC applications, like terminal emulators, will work with them. You can have a terminal session over USB or use existing applications that use COM ports to talk over USB. The Visual Basic PC application does this.