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RF modules simplify wireless sensor networks

ADDING WIRELESS CONNECTIVITY TO INDUSTRIAL APPLICATIONS IS RAPIDLY BECOMING ESSENTIAL IN AREAS SUCH AS BUILDING AUTOMATION. AN EVER-GROWING RANGE OF OPTIONS MAKES IT EASIER TO APPROACH WHAT CAN BE A DAUNTING TASK.

Suddenly, wireless is everywhere. With headlines such as “Don’t bother searching for your new killer application—it’s wireless”, analysts universally agree that the drive to dispense with cabling is the semiconductor industry’s primary growth opportunity for the next several years. According to a report that recently appeared in EDN Europe’s sister publication Electronics Weekly, analyst Future Horizons expects the fastest growing markets for ICs between 2007 and 2012 to be ultra-wideband with a 139% compound annual growth rate, followed by ZigBee (132%), RF identification for retail use (87%), and WiMax (63%). Trailing slightly at 46% and 45% are robotics and near-field communications, respectively, with biometrics achieving a possible 34% (Reference 1).

From an industrial designer’s perspective, these predictions signify that industry is busy adopting RF techniques on a wide scale. This might be surprising as even today, many process-control engineers consider the relatively robust wired Ethernet system inadequate for their applications, instead adopting dedicated systems such as those that use the HART (highway-addressable remote transducer) Communication Foundation’s open protocols. The recent emergence of the HART 7 protocol, which embodies WirelessHART, and the development of the ISA100 wireless systems for automation standard by the Instrumentation, Systems, and Automation Society suggest that this process-control-specific trend may ultimately persist, but these initiatives are currently in their infancy—the latest news being that the two organisations are to collaborate and investigate opportunities to incorporate WirelessHART within the ISA100 project.

Meantime, there are multiple applications where wireless links offer benefits that are impossible to ignore—such as connecting sensors to control systems within buildings—that leave engineers who are not proficient in RF design to ponder technology choices. Such networks now enjoy the term WSN (wireless sensor network) to differentiate their relatively low data-rate requirements from those of systems such as wireless Ethernet. Despite its ubiquitous deployment and high speed, wireless Ethernet—or IEEE 802.11—has so far failed to significantly penetrate the general-purpose industrial market. This situation may change as deployment costs and power-per-node in particular continue to fall. Currently, my experience of attempting to integrate the technology shows that costs are relatively high when pre-approved modules are the only solution, as—even assuming that you have the necessary RF expertise and resource base—silicon vendors typically won’t even enter into a dialogue for annual volumes of less than 100,000 devices. You might then take advantage of USB (Reference 2) and buy in readymade Wi-Fi “dongles” from retail brands such as 3Com and Linksys.

ZIGBEE VERSUS PROPRIETARY

Arguably the largest disadvantage of purchasing consumer hardware is the total lack of visibility or control over product revisions—and even availability—leading most designers to specify dedicated industrial products whose lifetimes and support cycles are more predictable. Your off-the-shelf technology choices are then between multiple sub-1-GHz proprietary solutions and the new wave of 2.45-GHz products that promise the benefits that standardisation brings. Thanks to continuing development work by the ZigBee Alliance, ZigBee is rapidly becoming the technology of choice for wireless sensor network applications that demand low-power, low-data-rate communications over short ranges. Low deployment costs are also essential, in terms of both equipment cost and easeof- use. Modules are the most common implementation route, but do-it-yourself options exist that are worth exploring. You can even use the 2.45-GHz band and the same IEEE-802.15.4 physical-layer specification that the ZigBee Alliance’s standards build upon to develop your own protocols. Firstly, it’s worth reviewing the major characteristics of the available radio systems, all of which exploit licenseexempt ISM (industrial, scientific, and medical) bands (see sidebar “ISM bands overcome RF licensing issues”).

Offering a theoretical data rate of 250 kbps, ZigBee’s acknowledgementbased protocol ensures robust data exchanges between monitoring and control devices such as coordinators, routers and end devices. Supporting star, tree, and mesh topologies, the system’s network layer has an address range of 65,535 devices within a single PAN (personal-area-network). In the star model, the coordinator is the network controller that is responsible for initiating and maintaining all communications with end devices—that is, the remote sensing or control devices. Ideally, all such devices run from battery power to dispense with the necessity for power-supply wiring. The key to minimising power consumption is to allow end devices to sleep, only waking up periodically to send short data messages.

Tree networks add routers that employ a hierarchical routing strategy to extend the network’s range, with a single coordinator again being responsible for network initialisation. In the full-mesh topology that attracts most developers’ attention, each node connects with every other node to provide maximum availability and redundancy. The coordinator is the system’s host, but ZigBee’s design allows nodes to link automatically to form a network, with the protocol seamlessly handling routing, acknowledgements, and retries. This requires a protocol stack that essentially comprises the IEEE-802.15.4 physical layer and its medium-accesscontrol layer, the ZigBee network layer and its application-support sub-layer, and the ZigBee-device object-management plane, with the user’s application at the top level (Figure 1). In addition, a security plane works alongside the network and application-support sub-layers to support activities such as data encryption and security-key management. The result is a structure that follows the ISO/IEC 7498 standard for open-system interconnection, building layer upon layer to abstract the user’s application from the underlying hardware and services.

Interestingly, industry insiders estimate that this message-exchange mechanism is approximately five times less complex than that of Bluetooth, which is a major factor in ZigBee’s cost-reduction strategy. Estimates vary, but you can budget from about 32 kbytes for an end-device’s firmware to 60 kbytes or more for a router or coordinator. Taking the communications overhead into account, the effective data throughput may be only 125 kbps. The range between nodes is about 70m, with each node being able to act as a repeater that extends the system’s reach within the ability of the system to re-route traffic in the event of a node’s failure. Typically, coordinators and routers use ac-line power for reliability.

You can freely download the ZigBee specification from the ZigBee Alliance’s Web site, where you will also find the new ZigBee Pro stack profile that improves scalability and security, particularly for larger networks. With two feature sets to choose from, the Alliance is now concentrating on developing application profiles, such as the home-automation-application profile, which is the first it has released. Usefully, the site also maintains a list of suppliers of ZigBee-compliant equipment that includes hardware, software, and modules for integration within your equipment. A partial list of module vendors includes Digi, Ember, Jennic, MaxStream, MeshNetics, One RF Technology, Radiocrafts, Radio- Pulse, and Telegesis.

According to Peder Martin Evjen, Radiocrafts’ managing director, “ZigBee has yet to show its full potential, partly because of the availability of a stable and reliable stack, and partly because of the lack of application profiles. However, both of these issues are now solved or being solved.” He says that the stacks are maturing, pointing to Texas Instruments’ (TI) object-code Z-Stack for the network layers and C-format application examples that are free to use with TI’s chips, and hence with his company’s range of ZigBee modules. Evjen notes that the ZigBee Alliance has now ratified the first profiles—the home-automationapplication profile and the commercial- building-automation-application profile: “The first is important for any real-life product, and the second is very important for customers looking for full cooperability.” He particularly acknowledges the success of IEEE- 802.15.4 as a platform for many proprietary solutions, as well as in forming the basis for other network standards, such as 6LoWPAN. Currently at the draft stage, 6LoWPAN refers to the effort to standardise Internet-protocol-version-6 communications over low-power wireless PANs, specifically for industrial use (Reference 3).

SILICON GIANTS TARGET ZIGBEE

Unsurprisingly, semiconductor vendors such as Freescale and TI are targeting the low-power radio sector—and especially ZigBee. Helping to promote chip sales, TI’s Z-Stack is free to use with devices such as the company’s CC2420 ZigBee transceiver, CC2430/2431 SoC (System-on-Chip) devices, and MSP430 platform. Its ZigBee-development-kit support ranges from the CC2431ZDK for $1,999, which includes multiple demonstration boards and a 30-day evaluation version of IAR’s Embedded Workbench for the 8051 core, to the eZ430-RF2500 wireless development tool that packs the CC2500 radio alongside a MSP430F2274 processor within a USB dongle for just $49. The mid-March delivery estimate made testing this low-cost product impossible, but it’s worth investigation by anyone wishing to exploit the ultra-low-power MSP430. Furthermore, TI also offers its proprietary SimpliciTI network protocol for free for the MSP430, plus a selection of its transceivers. Suiting networks of up to 256 devices and with resource-usage estimates of under 4 kbytes of flash and less than 512 bytes of RAM per node, you can compile small applications at no cost using the evaluation version of IAR’s MSP430 compiler.

Freescale’s latest transceiver is the MC13202, which adds a transmit/ receive antenna switch to its first generation MC13192. Both use SPI ports for host connectivity. Particularly suitable for use with HCS08-family microcontrollers that run Freescale’s protocol stacks, these 2.45-GHz devices provide IEEE-802.15.4 physical- layer functionality within a 32-pin QFN. Alternatively, the MC1321x system-in-package family places an HCS08 alongside a transceiver and an antenna switch to create a complete radio modem of 1 mW power output. The 71-pin LGA devices measure 9x9x0.9 mm and offer 16, 32, and 60 kbytes of flash with 1, 2, or 4 kbytes of RAM. Cyril Zarader, wireless- connectivity marketing manager at Freescale’s Toulouse (France) operation, says that for ZigBee-specific use, 32 kbytes suffices for most end devices while coordinators and routers require a 60-kbyte version. The top-specification MC13213 is available now for around $3.9 (10,000).

Protocol support for any of these devices comprises Freescale’s SMAC (simple medium-access-control), IEEE- 802.15.4 MAC layer, and ZigBee-2006 compliant BeeStack. Written for the HCS08 and CodeWarrior environment but adaptable to other targets, SMAC is a free-to-download ANSI-C sourcecode package that you can use to develop proprietary applications. The IEEE-802.15.4 package is freely available in executable format for HCS08 hardware. The BeeStack provides full ZigBee capabilities that you can freely evaluate for 90 days, after which time a one-time fee of $995 applies.

Development support includes the 1321XNSK-BDM BeeKit, whose major components comprise a network-coordinator board and two sensor boards, all of which use the MC13213. The coordinator board features an LCD screen, buzzer, four LEDs and user switches, reset switch, and USB- and RS232-interface ports for connection to a host PC (Figure 2). Four headers access the processor’s GPIO, while its BDM (background-debug-mode) port permits programming and debugging via the CodeWarrior environment and the USB Multilink BDM interface from P&E Micro that accompany the kit. Each sensor board carries a USB interface, a BDM port, and GPIO ports, with the switch/LED arrangement mirroring that of the coordinator; the sensor hardware is Freescale’s tripleaxis MMA7260Q acceleration sensor. Each board comes in a plastic box with battery trays for mobile operation and arrives pre-programmed with a simple binary-count demonstration program. You also get power supplies for each board, batteries, cabling, and two CDs.

The 1321X development-kit CD contains documentation, example programs, and utility software that support the SMAC and IEEE-802.15.4 protocols. Examples include the acceleration-sensor demo that you can flash into the two sensor boards using a conventional USB port or the BDM port. The BeeKit CD installs the wireless-connectivity- toolkit application that supports SMAC, IEEE-802.15.4 and the ZigBee protocols. This application employs a top-down development model with the solution being a container for one or more projects that may combine userapplication code with pre-built code templates. Currently, templates exist only for the ZigBee home-automation applications, but Zarader says that Freescale will roll out further profiles as the Alliance ratifies them. The 1321XNSK-BDM BeeKit is available now for $549.

SIMPLEST CAN BE BEST

Using IEEE-802.15.4 radio systems without the ZigBee stack might seem attractive as it circumvents the obligation to pay the ZigBee Alliance its obligatory and not-inconsiderable royalty fees, but integrating radios and developing complete applications can be a daunting task. Don’t be tempted to underestimate the ZigBee learning curve either—there are the protocols and profiles to understand and—if you are keen to embed your own subsystems— the development toolchain that you’ll need to use. As a result, many engineers turn to pre-packaged proprietary solutions from vendors such as Low Power Radio Solutions (LPRS), One RF Technology, Radiocrafts, Radiometrix, RF Digital, and RF Solutions that typically use the lower ISMband frequencies. The characteristics and capabilities of their products differ widely, yet make RF links immediately accessible to non-specialist designers.

Representing one of the simplestavailable solutions that also offers verylow- power-consumption potential, the TX2A and RX2A transmitter and receiver modules from Radiometrix support data rates of up to 64 kbps with an in-building range of up to 75m. These devices come in single-in-line, through-hole-mount packages with outlines that measure 32x12x3.8mm and 48x17.5x4.5mm, respectively. Versions are available that run from 2.2 to 16V-dc supplies with a normal operating frequency of 433.92 MHz that the European specification ERCREC 70-03 limits to a maximum duty cycle of 10%—it is your responsibility to ensure that the application does not exceed this limit. When low, the transmitter’s enable pin shuts the device’s internal regulator down to reduce consumption to less than 1µA; +10 dBm of RF transmit power is available within 1.5 msec of taking this pin high, at which time the module consumes 11 mA. Serial data with 0-to-2.5V levels generate a frequency-modulated signal with a nominal 27 kHz shift from a logic zero to one.

The RX2A receiver uses a single down-conversion superheterodyne architecture with a surface-acoustic-wave front-end filter that provides over 50 dB of image rejection. It requires about 10 mA when receiving, powering up from an externally switched supply to output valid data within about 10 msec. Sensitivity is better than 100 dBm for a 1-ppm bit-error-ratio, and an RSSI (received-signal-strength-indicator) pin is available to check reception levels. Two pins provide a buffered, filtered analogue output from the receiver’s demodulator and a digital data stream. This digital data is logically true with respect to the signal that feeds the transmitter. You can thus consider the TX2A/RX2A as RF logic gates that may require additional coding to support data exchanges of any complexity. One of the most straightforward yet reliable techniques is Manchester encoding—in support of its ISM-band products, Maxim publishes a useful application note (Reference 4). The RX2A and TX2A modules are available for about €40/pair.

Also available in a single-in-line, screened-can format, easy-Radio devices from LPRS include the ER400/900 series of transmitter, receiver, and transceiver modules. For instance, the ER400TRS- 02 transceiver is a half-duplex wireless modem that interfaces with a host microcontroller using TTL/CMOSlevel RS232 protocols. The hardware interface includes an analogue RSSI output that ranges from 0V at maximum signal strength of -65 dBm to 1V at the -115-dBm minimum; sensitivity for a 0.1% bit-error-ratio is typically -102 dBm. An internal 3.3V regulator allows the device to operate from supplies in the range of 2.5 to 5.5V, requiring 25 mA for transmission and 19.5 mA for reception at the 5V level. When idling between data exchanges, consumption falls to around 5 mA. A sleep-mode command reduces consumption to about 120 µA, requiring the host to toggle the device’s RTS pin to re-enable the link. If the module has powered up beforehand at the current frequency assignment, it is ready within 13 msec; recalibration to a new setting takes 75 msec.

The PIC 16F688 microcontroller within the ER400TRS-02 buffers up to 180 bytes of data and supports baud rates from 300 to 115,200 at its digital I/O pins. You can also set any arbitrary value within this range. Removing the overhead for the protocol and its Manchester encoding, the air interface operates at the equivalent of 9,600 baud with current-generation modules that use the CC1000 silicon radio from Chipcon (now part of TI). Other programmable features include channelfrequency selection, RF output-power settings up to a 10-mW maximum, and a repeater mode that extends the network’s range beyond the typical 250m line-of-sight. Because most applications in the 433-to-434-MHz band use a 433.92-MHz spot frequency, the ability to select from 10 channels within this range overcomes potential interference or network co-existence problems. An optional 16-bit identifier confines reception to easy-Radio modules that share this address, and you can encrypt data using a 16-bit seed.

David Schmider, technical director at LPRS, notes that the air-interface firmware occupies about 2 kbytes of hand-crafted assembly code that, as attempts to port to C have shown, increase to the 8-to-10-kbyte level when using the high-level language option: “Most programmers find writing air-interface code to be very challenging and prefer to concentrate on their real application’s needs—which is exactly what easy-Radio allows them to do”. Handling the easy-Radio’s repeater functions is a common task of interest, when the programmer can opt to listen and re-transmit on the same or different frequencies. In the first instance, the sending module receives its transmission after a short delay, which it can then regard as an acknowledgment; alternatively, using the frequency-swap capability allows modules to relay messages without application software having to handle the one-to-many identical message exchanges that may follow.

LPRS’ Web site offers evaluation kits that comprise a pair of batterypowered base boards to accommodate your choice of ER400/900 modules, together with a pair of quarter-wave whip antennae that Schmider asserts provide the simplest yet best solution. Space-constrained applications may benefit from using a PCB-trace antenna or a surface-mount device—such as a member of the SP series from Linx Technologies—but are almost certain to suffer range restrictions. The boards interface with host PCs via USB interfaces that function as RS232 converters. The evaluation software—which will shortly support Vista—provides access to the devices’ command sets and can establish a bidirectional link that echoes user data. This makes it easy to assess the impact of configuration changes on range (Figure 3). Schmider estimates that reducing RF power output from maximum to minimum reduces transmit-mode current consumption by about 15 mA: “Also, if you only use the minimum power level, you are not confined to the 10%-duty-cycle requirement for the frequency band and can transmit up to 100% of the time—although taking latencies into account, 100% is not achievable”.

With the receiving/echoing board on the fourth floor of a brick building and a laptop connecting to the transmitter/receiver, communications remained stable throughout the building, beginning to fail at around 50m into the street through two brick walls; by contrast, a Wi-Fi network struggles to maintain stability through three floors. For more demanding applications, LPRS offers long-range modules that use narrowband radios. Schmider says that forthcoming 03-version easy- Radio devices will use the CC1020 silicon radio that offers narrowband operation while maintaining compatibility with today’s products. The evaluation kit for the ER400/900 series without any modules costs around €75; the ER400TRS-02 transceiver costs around €22.50 in small quantities.

STAR/TREE MADE EASY

With its RF-Solved products, fabless designer Cyan paves a different path to embedded RF development that exploits the company’s eCOG1kG microcontroller to build star or treetopology wireless links. The 3.3V device’s 128-pin TQFP package houses a 0-to-25-MHz, 16-bit Harvard-architecture core, 64 kbytes of flash, and 4 kbytes of SRAM together with a variety of digital and mixed-signal peripherals. Differentiating hardware features include a memory-management unit, a power-saving code cache, and external host and memory interfaces. As with Cyan’s other microcontrollers, the freely available CyanIDE development environment interfaces with the chip that you’re working on via a USB dongle that links the host PC to the chip’s eICE emulation port. As well as offering a full ANSI-C compiler, CyanIDE’s features include automatic peripheral configuration and user-selectable pin allocation. The interface usefully includes comprehensive documentation that is available from a separate browser window.

The RF-Solved evaluation kit comprises a pair of identical base boards and modules on carrier PCBs, the USB programming dongle, batteries, cabling, and a CD. Each base board accommodates the power supply, a pair of DB-9 serial connectors, and the eICE connector. The carrier PCB accommodates the eCOG1kG controller, the MICRF610 half-duplex transceiver from Micrel, a neat chip antenna from RainSun, and a micro-SMA connector that allows you to evaluate external antennae (Figure 4). The MICRF610 is an 868-to-870-MHz device that comes in a screened-can module measuring just 14.1x11.5x3.0 mm. Capable of data rates of up to 15.2 kbps, its 16 pins include the antenna connection, a three-wire programming interface, a bidirectional serial interface and clock lines, and ground and power pins for its 2.0-to-2.5V supply. Maximum RF output power is +8.5 dBm: at this level, the transmitter consumes 26 mA; with all functions enabled, the receiver consumes 13.6 mA. Receiver sensitivity for a bit-error-ratio of under 10^-3 is datarate- dependent and ranges from -105 to -111 dBm.

After installing software and connecting one module to a PC via RS232, the quick-start guide describes how to connect modules using a demo program that cycles through a simple communications sequence. You then learn how to control the modules using a terminal program that allows you to explore their AT-style command set, establish the master and configure slaves, and control a LED on the slave board. The LED toggle command does exactly that—change the LED’s state—so to use the facility for a quick range test, use a DOS batch script or a terminal program that can repeatedly send a string from a text file.

Sean Cochrane, Cyan’s field-application- engineering manager, explains that for a quick range test, it is important to open and close radio channels between sending commands: “Having established a connection to the slave, the master reverts to command mode if it does not receive an acknowledgement, thus closing the connection.” In a real application, the host monitors the RS232 channel for an OK or ERROR response before issuing the next command. He agrees that built-in automatic retry functions can create unforeseen difficulties, from compromising battery life to locking the network. A line-of-sight range test showed a result of some 30 to 35m using the tiny chip antennae, which, Cochrane suggests, may improve if you use a traditional whip antenna for the master; fitting whip antennae for master and slave improves line-of-sight range to several hundred meters and greatly improves penetration within buildings.

The underlying Cy-Net radio protocol is a development of MicrelNet, a license- and royalty-free stack that supports Micrel’s range of FSK (frequencyshift- keying) RF modules and ICs. While MicrelNet’s addressing structure suits basic star/tree topologies, Cy-Net adds a simple method for tree routing that—unlike ZigBee’s mesh—does not require routing tables within each node. A background synchronisation task negotiates the route to the next point within a tree. Cochrane says Cyan’s model particularly suits local telemetry applications when small amounts of data traverse short distances along a tree: “For any application where n nodes connect to a central point, star/tree has a lot to offer.” He notes that his company’s protocols permit considerable flexibility in, for instance, optimising power consumption: “Having full control of both ends is crucial to achieving this.”

Cochrane is keen to point out that the radio stack is only part of the system: “Typically, customers need to take data from interfaces such as Ethernet, GPRS, and USB.” Accordingly, Cyan offers reference designs such as LCD drivers, together with a range of library functions that programmers can freely exploit to control the core and peripherals. Cochrane notes that it is possible to run macros such as a micro-IP stack or the company’s USB library in code space that is separate from the radio stack. He says that Cyan is developing self-forming, self-healing abilities using what is effectively another state table. With its introduction on track for the second quarter of 2008, this forthcoming version of Cy-Net will require about another 20% of overhead but will still avoid routing tables. Other developments include a port to FreeRTOS, which will include reference-design application code. The Cyan RF-Solved evaluation kit is available now for $295; the budgetary price for the eCOG1kG is $4/(10,000).

NARROWBAND MAXIMISES RANGE

Complementing its ZigBee-compliant and 2.45-GHz general-purpose modules, Radiocrafts offers a range of sub-1-GHz devices that notably includes the RC12x0 family of narrowband transceivers. The company’s Evjen explains that for users, the trade-offs between wideband and narrowband communications within the sub-1-GHz band comprise data rate, line-of-sight range, and propagation within buildings—wideband suiting up to 19.2 kbps at short range, while narrowband carries around 4.8 kbps with excellent long-range line-of-sight potential. Furthermore, narrowband transmissions can penetrate several floors of a concrete-and-steel building over distances of more than 100m.

Evjen questions many users’ perceptions regarding overcrowding within the 434-MHz band, noting that devices such as remote garage-door openers employ very wideband modulation that doesn’t interfere with narrowband communications. He says that the 868-MHz band is also relatively interference-free, as common applications, such as alarm systems, can only broadcast in sub-bands at duty cycles of 1% or even 0.1%. By contrast, Evjen observes that the 2.45-GHz band is relatively crowded, with the many devices that operate at these frequencies potentially transmitting 100% of the time. DSSS (direct-sequence spread-spectrum) modulation is a common technique for overcoming interference within this band.

The RC1240 is the 434-MHz version of Radiocrafts’ narrowband modules that embed its RC232 protocol to provide radio-modem functionality using an RS232-like interface. Available in a 12.7x25.4x3.5-mm surface-mount module, the device communicates on one of 69 user-selectable channels, each of which has 25 kHz bandwidth. A 2.7V internal voltage regulator accommodates operation from 2.8-to-5.5V supplies, consuming 20.2 mA when receiving and 26 mA when transmitting. A sleep mode disables transmission and reception to reduce power consumption to 900 A, while taking the ON/OFF pin low turns the device off. Wake-up from the sleep state takes 55 msec; reloading configuration data from internal memory following power-up incurs a 160-msec delay. Points to watch include ensuring external logic’s compatibility with the device’s 2.7V-level outputs; all inputs accommodate 2.7 to 5.5V.

The improvement in selectivity that narrowband operation brings, together with the +8 dBm of RF power that is available for the 434-MHz band, permits the RC1240 to communicate at 4.8 kbps over a line-of-sight as great as 2 to 4 km—for yet more range, the RC1280HP reaches 5 to 6 km using +27 dBm or 500 mW of output power. As a former senior member of Chipcon’s applicationengineering staff, it is natural that Evjen’s modules employ silicon radios from TI’s product portfolio. Preferring to encourage engineers to regard his company’s products as components rather than concentrating on their construction, Evjen reveals that the internal component count of present-generation modules typically numbers around 60, including a MSP430-series microcontroller to handle the air interface, and a critically important temperature- compensated crystal oscillator.

The company’s latest-generation products are taking advantage of SoC radios.

The carry-case that houses the RC12x0-series evaluation kits contains a pair of module-specific PCBs, offline power supplies, quarter-wave monopole whip antennae, SMA-to-BNC adapters for connecting alternative antennae, and RS232 serial cables to connect to host PCs. A single instruction sheet refers you to the Web site to retrieve the latest documentation. Alternatively, follow the quick-start instructions and start sending characters from a terminal program such as HyperTerminal within literally minutes of opening the box. In this mode, each module listens for messages, with each terminal window displaying the text that you send from the other. Download the RC232-CCT (configuration and communication tool) software that includes a terminal program capable of transferring binary and hex strings, which allows you to access the modules’ configuration registers (Figure 5). You can, for instance, set packet lengths of 1 to 128 bytes in nonvolatile configuration memory.

Useful for range testing, the software allows you to check reception strength using the module’s RSSI command, and repeatedly send an arbitrary text string every second. Tests here show that communications appear rock-solid at 125m from a transmitter on the fourth floor, dropping out sporadically after this point but still getting through at 200m. In line-of-sight terms, the test signal travelled through three substantial brick houses in a terrace, then past a telephone exchange and two more houses to arrive in a busy urban street with passing traffic that may generate reflections. The RC1240 development kit is available for around €200, with the module costing around €21 (1000).

YOU NEED MORE?

Apart from the constraints that limit coverage of a vast marketplace, the pace of radio development broadens your choices almost daily. Familiar names with products that you may wish to consider include Atmel, Microchip, and Rabbit Semiconductor—all of whom offer RF-link evaluation kits that use their respective proprietary microcontroller families. Rabbit’s wireless- networking development kits are available for Wi-Fi, IEEE-802.15.4, and ZigBee applications; also, if you are a PIC fan, it is worth knowing that Microchip offers its own radio-stack C-sources for free and that MicrelNet uses the Microchip platform.

Announcing its $99 RZ Raven kit as we closed for press, Atmel’s 2.45-GHzband product consists of a USB stick that serves as a PC-to-wireless network gateway or as the hardware component of a wireless-protocol analyser. The two accompanying Raven boards each carry an LCD, a joystick, and a microphone and speaker for use as wireless nodes. According to the press release, the firmware includes low-level drivers and application stacks that enable simple point-to-point communication, IEEE-802.15.4-compliant solutions, and more complex applications based on ZigBee and 6LoWPAN. The company’s Web site makes available source-code updates together with libraries and documentation for the applications. PC software includes a wireless-network demonstration and management suite, and—most notably—a wireless-protocol analyser and over-the-air upgrade facility.

CONTACT DETAILS

You can reach Contributing Technical Editor David Marsh at forncett@btinternet.com.

REFERENCES

Manners, David, “Wireless is top market for chips, says analyst,” Electronics Weekly, February 7 2008, www.electronicsweekly.com. Marsh, David, “Mastering USB’s software complexities,” EDN Europe, May 2007, pg 24, www.edn-europe. com/article.asp?articleid=1457. IPv6 over Low Power WPAN (6lowpan), www.ietf.org/html.charters/ 6lowpan-charter.html. “Manchester data encoding for radio communications,” AN 3435, Maxim Integrated Products 2004, www. maxim-ic.com.