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Electronic Design

Hopping On ZigBee

Rabbit Semiconductor is known for its Rabbit line of microcontrollers and modules based on these chips. The modules allow developers to get a jump on the competition by providing a platform with 90% of the hardware design completed. A carrier board is typically needed to provide connectors and possibly some custom peripheral interfaces. Rabbit Semiconductor has a wide range of kits based on the Rabbit modules including the ZigBee/802.15.4 Application Kit reviewed here. This kit is based on MaxStream XBee wireless modules. Just to fill in the blanks, Rabbit Semiconductor is part of Digi International that also owns MaxStream. Digi has its own line of ARM-based products that will show up later in this series of reviews. The XBee modules can operate by themselves and the kit includes battery carriers that can be used with the modules for mobile operation. Of course, the kit includes a Rabbit module too, the RCM3270. The Rabbit module and associated development board can be tied to the XBee module. A final product would likely replace the development board with a custom board that would accept both modules and include any other peripherals necessary for a particular application. The Modules The XBee modules are small, (about one square inch) 20-pin boards that support 802.15.4 and ZigBee. It draws about 45mA when transmitting and has a range of about 100m—typical for a ZigBee transceiver. The XBee Pro has the same footprint but draws about 215ma while pushing the line-of-sight range up to a mile. Both modules draw significantly less power when receiving and they can operate in a powerdown mode that consume microamps. The XBee, like most ZigBee modules, takes a lot of the strain out of working with wireless communication by handling it all internally. The interface to the XBee is a simple serial interface that uses and AT-style command set popularized by modems of the past century. A text command sequence can configure the module. The interface is also used to exchange data with a remote device. Typically, this will be another XBee although it is possible to interact with most 802.15.4 or ZigBee devices as some level. It does not implement the more complex ZigBee profiles that are currently being developed but more complex devices can typically interact with devices like the XBee. The XBee supports the range of ZigBee network configurations including point-to-point, point-to-multipoint, peer-to-peer and mesh networking. Kept within their own network, the XBee modules can provide a sophisticated level of support. Data coming in one of the 16 pins can be delivered to a remote device. The serial interface and power pins make up the rest of the XBee module pins. The modules that come with the kit use chip antennas but versions with a whip antenna or a connector for external antennas are available. The difference is typically the range and the positioning of the antenna for a particular application. The modules run about $20 and work with any application, not just those based on the Rabbit microcontroller. The carrier board has an RS-323 interface, allowing it to be connected directly to a PC. In fact, one reason for three devices is to have one for the Rabbit module, one standalone and one attached to a PC. TTL or CMOS connections to the module are also possible. The Kit The three XBee modules need to be plugged in to the carrier boards along with the feet and wiring in the battery modules. Batteries are included. Jumpers must also be added to the carrier board. These select the serial interface configuration for either the PC or the Rabbit development board. The RS-232 interface drivers are external to the XBee module and an RS-232 interface chip is found on the carrier boards. Plugging the hardware together takes a few minutes after reading the Quick Start Guide. The debugging interface for the RCM2730 Rabbit module is a serial interface so you will need a serial to USB interface cable if your PC lacks the old RS-232 interface. The same is true if you plan on connecting an XBee carrier board to a PC via USB. The software installation consists of a pair of CDs. The first to be installed is the Dynamic C development environment. The second is the ZigBee/802.15.4 application CD. I’ve reviewed Dynamic C before (see Something Red, Something Blue). It is a variant of C with enhancements like co-routines that are handy for real-time process control. Not surprisingly, it is very popular with Rabbit customers and is tuned for the Rabbit modules. The ZigBee CD adds sample applications and documentation, though it would be nice if a link to the documentation was added to the Windows Start menu. The majority of the ZigBee and XBee features and interface are presented via this online documentation. Getting started with the modules is relatively painless. The default rate for the XBee modules is 9600 baud. It is easy to change and the reference guide does a good job of presenting the information about the AT command set and the parameters for the various functions. There is a special application for downloading new firmware into the XBee units and you need to choose 802.15.4 or ZigBee. The Rabbit software does not support the more advanced API interface that the XBee is capable of supporting. In fact, most users will take advantage of the function library for the XBee so they are insulated from even the AT command set. Source code is provided for a range of network configurations. Once set up, most configurations are used in a similar fashion, passing data between nodes. The XBee modules handle most of the heavy lifting, and tools like Daintree Networks’ SNA will only be needed if there are problems with network layout or if other devices need to be put into the same network. Although the functions are different, it is not much more difficult to use the wireless support or the built-in Ethernet connection of the RCM3270. Getting up and running with 802.15.4 or ZigBee using the Rabbit application kit is relatively easy and painless. The use of the module is a slightly more expensive approach than going with a chip on a per-item basis, but eliminating the need to debug a protocol stack and get FCC approval is well worth it. In fact, it can be a bargain unless the quantities move into the tens of thousands. If you want to use 802.15.4 or ZigBee simply as a transport mechanism, then Rabbit Semiconductor’s approach is worth investigating. The XBee is great for a simple standalone device and the Rabbit module provides an easy way to program a device to the network. Related Links Digi International Maxstream Rabbit Semiconductor

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