Flexible platform is the key to scaleable ZigBee
01 June 2007
An industry-standard microcontroller family can provide a scaleable option with maximum performance and maximum flexibility for ZigBee systems
Initially, single chip devices, with RF integrated onto the same chip as the microcontroller, might offer a tempting degree of simplicity, but this often comes at the expense of flexibility and performance. At the start of a design it is not always possible to judge the amount of memory required to accommodate all the functionality required in the final ZigBee system, nor to anticipate every peripheral function that will be required.
Stacking up data
The image shows the standard ZigBee stack, which comprises an application profile, network layer and MAC (media access controller) software. The stack can be on a single chip device, available in different formats from a number of suppliers. However, these can have limitations, such as limited on-chip memory, inflexible peripheral options and often use low performance legacy microcontroller cores. The application profile, MAC and network stack can, by themselves, take up to 128kByte of memory, meaning that a separate host controller would be required for real-world applications, thus negating the single-chip-advantage.
An initial two-chip approach provides for greater design flexibility with the complete software stack residing on an industrystandard and scaleable MCU platform with a separate 802.15.4-compliant RF device, completing the ZigBee solution.
The Renesas ZigBee platform is based on the M16C family of microcontrollers, which offers programme memory ranging from 24kByte flash on an M16C/Tiny, up to the M32C family with as much as 1MByte of on-chip flash. The M16C’s low power standby and sleep modes is especially attractive in ZigBee applications. While some microcontrollers appear to have a low-power specification, operating current is actually less important on the whole than drawing a low current in standby or sleep mode since the ZigBee duty cycle is very short. The performance of the processor is also very important to minimise time spent awake, as is the time to achieve stabilisation of the oscillator and hence the recovery.
The Renesas ZigBee demonstration kit offers a simple introduction to designing a ZigBee system. The user can see the link quality between nodes and can experiment with techniques such as forcing nodes to communicate through adjacent nodes. In addition to the low-power, high performance M16C microcontrollers, the kit includes IEEE 802.15.4 standard-based radios (either a TI Chipcon 2.4GHz or ZMD 900MHz device), battery packs, LCD, temperature and light sensors, a potentiometer, switches and LEDs.
At the next level, an evaluation kit adds free software to a demonstration kit, and includes the ZigBee Stack as a fixed firmware file (un-modifiable binary); API access mapped via a jump table to the stack; a sample project in the HEW integrated development environment; IEEE address files; and documentation. With this, the designer can develop up to 16kByte of application programme on top of the stack and produce a prototype.
The company’s ZigBee stack is fullycertified as a ZigBee-compliant platform and incorporates enhancements that address specific problems commonly associated with ZigBee applications. These include providing load/store of network and application data, to facilitate recovery from power loss and
allowing reuse of address after a device leaves the network and is not likely to return, in order to keep the network capacity ‘real’. Other issues are to provide a low power API for RF, to save power automatically and set threshold values for the join process, to avoid connecting with bad links.
Key advantages of this ‘platform’ approach to ZigBee is that the M16C will work with third party RF building blocks from partners such as TI Chipcon, Freescale, UBEC and ZMD, or equally with the customer’s own RF design.
ZigBee offers a huge market potential: according to OnWorld there could be 104million nodes installed worldwide in 2008 growing to 465 million by 2010, by which time it could account for as much as 10 per cent of the worldwide TAM for MCUs.
Companies such as Renesas are developing integrated ZigBee solutions as well as 2.4GHz/900Mhz devices of its own to use with existing M16C-based solutions this will provide for even more choice for
Compliance with the ZigBee open standard is crucial for interoperability with other ZigBee system components. It is important to understand that a ZigBee device by itself cannot achieve compliance, only the complete system. However a design platform (device with stack) can be certified compliant and it is then a simple matter for any system designed on that platform to achieve ZigBee certification.
There are three levels of certification. ZigBee Compliant Platform (ZCP) is interoperable in terms of the application layer (APL), and provides certification for platform sellers; Manufacturer-Specific Profile (MSP) is based on ZCPs and includes a profile that is not specified by the ZigBee Alliance. Finally, ZigBee Certified Product, such as a ZigBee light sensor using the home automation profile, is interoperable in terms of the standard application profile, and is the appropriate level of certification for OEMs.
What is ZigBee and what can it do?
Like Bluetooth, ZigBee is a short-range networking standard that can operate at 2.4GHz. Unlike Bluetooth, however, the IEEE 802.15.4 ZigBee standard is aimed at linking control systems for automating homes and offices, control and communication applications for healthcare and industrial systems, and for data transfer between sensors and detectors of various types. ZigBee is specified with a low operating power to enable battery lifetimes of around two years, and therefore a low maintenance overhead. Crucially there is also a second frequency band defined around 900MHz, which offers greater range and superior penetration through walls and floors. It is estimated that one-third of the market will opt for this frequency band rather than the higher one. The 2.4GHz band is a commonly used worldwide, the lower frequency band is assigned to 868MHz in Europe and 915MHz in the USA. Typical range is 10m to 250m, and data rates from 20kbit/sec for 868MHz to 250kbit/ sec for devices in the 2.4GHz band.
ZigBee–enabled devices can be connected in a variety of different configurations, depending on the complexity of the application and the type of devices to be connected. Installation of new nodes is automatic or semi-automatic, and it is possible to add or remove devices up to a total of 65,000 nodes in up to 100 co-located networks.
The standard is promoted and regulated by the ZigBee Alliance (www.zigbee.org).
ANDY HARDING is manager, new segment and business development , Renesas
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