Wireless Systems Design

Chip Set Widens ZigBee's Web

This Chip Set Comes Complete With A 900-MHz/2.4-GHz Controller, A Low-Power 868-MHz/902-To-928-MHz 802.15.4 Radio, And Both 802.15.4 And ZigBee Protocol Stacks.

Right now, a multitude of wireless-connectivity standards exists. Each of these standards promises to fill a unique void in the path toward total wireless-network connectivity. Many experts agree that getting to this connected reality is possible. Yet they debate how and when the industry can arrive at that future. Many stumbling blocks stand in the way, such as cost. To encourage widespread proliferation and entry into mass markets, the bill-of-materials (BoM) cost for each wireless standard must be low. In addition, every standard must replace proprietary options. It also has to be accepted by the markets that it serves.

One standard that works to confront these challenges is ZigBee. This low-data-rate, ultra-low-power, networked wireless standard is based on the IEEE standard 802.15.4. It is ideal for control and monitoring applications like home lighting, security, and industrial controls. If it's successful, this standard may someday drive the demand for a single remote control. This one device would control everything in a home ranging from the television or light switches to the clocks and coffee machine. With its new Z-Link chip set, Atmel Corp. hopes to bring this networked-home vision closer to the present day (FIG. 1).

This complete plug-and-play 802.15.4 and ZigBee solution stands out for many reasons. Most importantly, all of the necessary interfaces are already built into it. As a result, it requires no customer involvement in the 802.15.4 or ZigBee firmware stacks. Consequently, designers are free to focus on their areas of core competency: developing application firmware while getting a fully differentiated, cost-effective, ZigBee-based end product to market ahead of the competition.

The initial Atmel Z-Link chip set is comprised of four pieces: the low-power AT86RF210 900-MHz transceiver, AT86ZL3201 controller, 802.15.4 firmware stack (MAC and PHY layers), and ZigBee firmware stack (networking, security, and profile layers). It isn't the first ZigBee-related product to come to market. Yet it does represent a significant move forward in the adoption and proliferation of the standard. It is the only 802.15.4 radio with a ZigBee-specific microcontroller and protocol-stack solution that's designed to meet the standard's special memory, security, and peripheral requirements. In doing so, Z-Link offers designers the necessary performance. It also carries the low-cost price tag that makes ZigBee so appealing in the first place.

At the heart of the AT86ZL3201 controller is Atmel's proven AVR processor, which is fully supported by standard AVR development and debug tools. Such support allows the designer to generate the necessary application code with existing AVR tools. At the same time, it grants the Z-Link controller lower cost, higher performance, and greater flexibility. To date, the Z-Link controller is the only dedicated ZigBee-ready, full-function device (FFD) for the 900-MHz and 2.4-GHz 802.15.4 frequency bands (FIG. 2).

The controller features 32-KB Flash, 8-KB SRAM, and 8 KB of ROM. In addition, it boasts a four-channel, 10-b ADC with bandgap reference. The pre-loaded and fully verified 802.15.4 protocol stack is found in the controller ROM. Written in AVR assembly code, it requires just half the memory of a typical 802.15.4 stack. Thanks to its inclusion, the designer doesn't have to do anything to obtain 802.15.4 support. No other controller on the market claims to have this capability.

Through buffered transmit (Tx) and receive (Rx) serial ports, the Z-Link controller allows a more efficient interface with the Z-Link transceiver. The chip also houses a watchdog timer, four 16-b timer/counters, and 32 GPIOs that are configured as four 8-b ports. In addition, the controller includes an interrupt controller, UART, and two SPI ports. One SPI port controls the radio interface. The remaining SPI port and UART can then be used for the application itself. In contrast, off-the-shelf 8-b microcontrollers generally have 8-b timer/counters and just one SPI port. Such a configuration is not adequate to accommodate emerging 802.15.4/ZigBee applications.

The Z-Link controller stands out for its built-in security capabilities including a 128-b AES encryption engine and a hardware-based random-number generator. Remember: Most general-purpose microcontrollers do not offer hardware encryption. AES algorithms must therefore be executed in software—a fact that can negatively impact the system through increased processor load and additional program and data-storage requirements. Increased system power drain also can become problematic.

Another defining characteristic of the Z-Link solution is the appropriate memory mix in the AT86ZL3201 microcontroller (see table). The Z-Link microcontroller embeds 8 KB of ROM, which is utilized for the low-cost storage of optimized assembly code for the 802.15.4 stack. To evolve the ZigBee stack and application code, 32-KB Flash is provided. Competitive solutions require a 64-K or 128-K Flash off-the-shelf microcontroller to run the 802.15.4 stack, ZigBee stack, and application firmware. For the user, this translates into a significantly higher cost than the price associated with the AT86ZL3201.

The Z-Link controller also offers 8 KB of SRAM. This memory can more than accommodate full-function ZigBee devices. Such devices are expected to require somewhere between 5 and 8 KB of data storage for the 802.15.4 and ZigBee stacks and applications. Other off-the-shelf, 8-b microcontrollers typically have only 2 or 4 KB of SRAM.

The IEEE 802.15.4- and ZigBee-compliant, Z-Link AT86RF210 transceiver acts as a complement to the Z-Link controller. It can support 10 channels at 40 kbps in the 915-MHz ISM band. In the European 868-MHz band, it can support one channel at 20 kbps. Using BPSK, the transceiver's air interface is DSSS (including spreading and despreading). With a range of up to 100 m for +3-dBm output power, this chip can accommodate up to 65,000 nodes in either star, cluster, or mesh networks. This range is nearly double the range of a 2.4-GHz transceiver for the same output power. Channel access is possible via CSMA-CA using contention-based and contention-free access modes.

Additional characteristics of the transceiver include a receive current of 14.5 mA, 1-µA power consumption in sleep mode, and an SPI port control. The transceiver also boasts a crystal-stabilized Fractional-N synthesizer, internal voltage regulation, and battery monitoring. They enable high performance at low power. At 1.8 V, the transmit power is just 4 mW (6 dBm) minimum.

Compared to competing 900-MHz 802.15.4 radios, the Z-Link transceiver promises to consume 25% less current (in receive mode). While the transceiver operates from 1.8 to 3.6 V, its 1.8-V power supply extends battery life beyond what would normally be expected from 802.15.4 radios with 2.0-V or higher power supplies. In addition, the transceiver flaunts a receive sensitivity of −95 dBm. It measures just 7 × 7 mm2.

The transceiver's PHY includes receiver energy detection, link-quality indication, and clear-channel assessment. Its MAC allows network association and disassociation. It also has an optional superframe structure with beacons for time synchronization and a GTS mechanism for high-priority communications.

Users can choose to utilize the Z-Link chip set and firmware stacks out-of-the-box as a full-featured, plug-and-play ZigBee-compliant solution. Or, if they aren't yet ready to commit to ZigBee, they can utilize their own proprietary firmware stacks for the baseband and network topology.

Now available, the Z-Link chip set is priced at $6.75 in quantities of 100,000 for the radio, FFD controller, and 802.15.4 and ZigBee protocol stacks. Development boards also are available. A 2.4-GHz ZigBee-ready transceiver will be available in the first half of 2005. The AT86ZL3201 controller comes in either a 64-ball LFBGA or 64-lead TQFP package. The AT86RF210 transceiver is available in a 48-pin QFN package.

Atmel Corp.
2325 Orchard Parkway, San Jose, CA 95131; (408) 441-0311, www.atmel.com.

TAGS: Components
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