This is the first article from .NET Micro Framework Vehicle series. Today about connecting XBee module from Digi/Maxstream to Tahoe development board. XBee module is very sophisticated device controlled over serial link. It can operate in old fashion AT command mode as well as more advanced API mode. In the “robotic vehicle” case it’s used as transparent point-to-point connection operated via AT commands. There are development boards for XBee with standard 9-pin DSUB which comes with jump-start-kit. Even if these can be easily connected to Tahoe serial port UART1, I’ve decided to go with my own daughterboard connected to UART2.

Tahoe and XBee module 480x300 Figure 1: Tahoe and XBee together

XBee and Tahoe

Tahoe from Device Solutions is development board around the Meridian CPU. It is 5 Volt CPU, but XBee is 3.3 Volt module. In terms of power supply there is no problem because Tahoe has plenty 3.3V power pins on it’s headers. The problem might be in communication via serial port. The 5V signal from Tahoe mightdestroy the 3.3V input port of the XBee module. Many 3.3V devices of today are 5V tolerant, so it should be fine. Unfortunately I didn’t found this information in XBee technical manual. After some research I’ve decide to go with few resistors. Output pin (TX) of the XBee is connected through 1k8 resistor directly to RX pin of the Tahoe. The input pin (RX) of the XBee is connected through voltage divider, where R1 is 1k8 and R2 is 3k3 ohms. It’s possible to use any resistors, which ratio will produce the 3.3 Volts. Even if the Meridian needs 5V power supply, the CPU pins gives 3.3V. Since I was not sure about that, I’ve handled the pins like the output is 5V and used the voltage divider. If you will face the problems in communication, you can remove resistor R2 and replace resistors R1 with wires.

To be sure that module is running, LED is connected to association pin of the XBee module. This will flash, once the module reach the ZigBee channel.

XBee daughterboard for Tahoe 560x240 Figure 2: Daughterboard

Designing the PCB

The PCB is pretty simple and don’t need to much soldering. Ready to print layout and shouldering schema is available in two versions: CAD software DWG file and PDF file. The best way is to go with DWG True View, which is a free to download DWG file viewer from Autodesk.

PCB scheme 430x200 Figure 3: PCB scheme

Parts list:

Daughter board assembled 320x240 Figure 4: Assembled daughter board

Putting all together

Once everything is soldered together, the daughter-board is connected to J6 header (pins 2 to 30) on the Tahoe. See figure 5 for details. For the “robotic vehicle” application, XBee module must be configured for 115200 baud rate. This can be done by issuing following commands to the XBee.

ATBD7   ' set baud rate
OK      ' module response
ATWR    ' store settings
OK      ' module response

Tahoe and XBee detail 640x480 Figure 5: Connection detail