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Aug
19
Written by:
Gary Holbrook
8/19/2008 4:38 PM
Now that I seemed to have squared away some nagging problems with my USB development board, I was ready for a new and exciting challenge: wireless communication. In case you have never heard of XBee, you really should hop over to www.digi.com and do some reading.
After not nearly enough research, I decided to purchase a pair of XBee  pro modules. These little buggers are quite small, so they don't use standard headers. This turned out to be a blessing in disguise because they also expect 3.3v for power, and communication. The board I wanted to use to talk to the XBee module was powered by USB at 5V. I knew that something would have to be done about the mismatch in voltage, so I did some digging about.
A company called SelmaWare created the perfect board for interfacing with an  XBee, at a reasonable price. The AppBee-Sip cost me about $12 for two bare boards, shipped. The parts required to populate the boards cost almost nothing from DigiKey. After getting my modules and soldering the parts to the board, I powered one up and tried to talk to it.
Nothing. Nada. Not even flashing lights. After re-examining the boards, I realized that I had soldered the LEDs in backwards. Sure, I could have checked the schematic...but why, when I could simply guess wildly?
After fixing the LEDS, I tried again. The lights blinked when I typed. I tried the escape code "+++" and got ABSOLUTELY NO RESPONSE! By now I was pretty irate. I took the parts with me on vacation (different people unwind differently, ok?), and took another look at them. After some consideration, I tried forcing my USB board to 9600 BPS. Nothing. Finally I realized that the Pro version of the module is pretty power hungry. Toss in the level shifter, LEDS, and regulator...it might simply be that USB wasn't providing enough power.
I tried a battery pack. Still no response, so I tried my other USB\TTL USART board. Voila! It worked! As it turned out, one of my TTL\USB boards is not able to receive data, and I needed to use an external battery. The very next night I had the two boards alive and talking to each other wirelessly.
It might be interesting to build myself an AT90USB board at 3.3v, and then plug the XBee board right in without the extra hoolabalu. It might work out, but I'll need to review the spec sheets first. In the meantime, I'm pretty impressed with how well these modules worked. Next I'll try testing their range and see how close I can get to that "1 mile" range.
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