Providing single chip solutions for IEEE802.15.4 based systems

low power wireless systems are not a new phenomenon. Radio systems that offer long battery life have been around for a number of years.

However, until the advent of a standard covering this area, these systems were destined to remain proprietary. Proprietary systems have a number of attributes: they are just sufficiently complex to perform the requirements of the application; they require significant design work at the protocol layer for each new implementation; they are specific to the local regulatory regime; and they cannot usually interoperate with other systems.

Hence, taking a proprietary wireless intruder detection system, this can be implemented with well established silicon, but will only operate in a limited geographical region.

It will most likely use a simplistic protocol designed only for that system and will be unable to interoperate with other systems.

All of the successful wireless systems in recent years have been based on standards (GSM, Wireless LAN, Bluetooth, DECT and Mobitex). The existence of standards provides a number of benefits that promote the availability of equipment.

Most importantly, they allow devices to interoperate. Effective interoperability is crucial to success as it enables users to buy with confidence that they will be able to use products in multiple scenarios.

For system developers, the existence of a well defined standard effectively provides access to a free protocol design.

In most cases, the standardised protocol has been developed by a large number of industry experts and therefore incorporates many features that would be expensive for individual developers to include.

Finally, component and IP suppliers can develop devices and software that address the standard, again greatly easing the difficulty of introducing new end products onto the market and providing cost benefits from the associated economies of scale.

An added bonus for the system manufacturers is the availability of multiple sources for components, helping to keep prices low and providing continuity of supply.

The emergence of IEEE802.15.4 and ZigBee will provide the standardisation and impetus needed for growth in the market for low power wireless products.

IEEE802.15.4 and ZigBee

Standardisation of systems for use in low power wireless applications has now been defined by two bodies. Task group 4 of the IEEE802.15 working group on Wireless Personal Area networks has generated a standard for low data rates–IEEE802.15.4.

This defines appropriate PHY and MAC layers for low data rate wireless systems, such as wireless sensor networks. The ZigBee Alliance is a trade association, with membership from across the whole spectrum of stakeholders in low power wireless systems, from OEMs through to chip and software suppliers.

Using IEEE802.15.4 as a transport medium, the Alliance has defined a networking layer, security, application profiles and is putting in place an interoperability and certification scheme.

The IEEE802.15.4 PHY specification defines two bands - the 868MHz (EU) / 902-928MHz (USA) bands with datarates of 20 or 40kbps respectively or the worldwide 2.4-2.5GHZ ISM band with an on air datarate of 250kbps.

The latter band is under consideration here. The standard has been developed with a view to enabling the manufacture of low cost, low power systems.

It uses a half sine pulse filtered O-QPSK modulation scheme, which is mathematically equivalent to GMSK and hence is a constant envelope modulation.

This makes it possible to use non-linear transmitters and receivers with very little performance degradation and consequent benefits in cost and power efficiency.

The 250kbps data is assembled into 4 bit symbols at a rate of 62.5ksymbols/s. Each symbol is then mapped onto one of 16 quasi-orthogonal 32-bit noise-like spreading codes, giving an on air chip rate of 2Mc/s and adding an element of spread spectrum protection.

The choice of PHY characteristics allows the system to meet worldwide local regulatory requirements in the 2.4-2.5GHz band, whilst operating on one of 16 defined channels in the band.

Operation with direct sequence on a fixed channel, rather than using frequency hopping spreading, ensures that there is no need to wake regularly to maintain synchronisation with the frequency hopping sequence as required in Bluetooth.

This ensures that systems can use the minimum of power by remaining fully asleep between infrequent transmissions.

The MAC layer provides a number of sophisticated features that would not normally be available in a proprietary system.

It supports the formation of peer to peer or star networks, enabling many simple systems to be designed without requiring additional networking complexity.

The protocol is fully acknowledged, with retries if required. Each packet also includes a Frame Check Sequence to enable the validity of the data to be verified. These features ensure robustness of data transmission. The use of Clear Channel Assessment (CCA) and collision avoidance techniques through random back-off periods ensures that data is not transmitted unnecessarily, maximising channel availability and reducing power wastage through unwanted retransmissions.

Short packets allow data to be transferred in gaps between other potentially interfering transmissions. Powerful data encryption is provided using AES encryption with 128 bit key length. This is particularly important to ensure data integrity and resistance to hacking in commercial applications.

The optional use of a superframe structure allows sophisticated power saving techniques to be used. Frame beacons transmitted at regular intervals indicate that messages are available for sleeping devices. Hence the sleeping device can synchronise to the beacon interval and only need wake up during the beacon itself to establish if there is a message to be received during the next superframe period.

The superframe facility can also establish contention free periods to give guaranteed time slots for critical data. The ZigBee Alliance has defined a protocol stack which sits above IEEE802.15.4 to provide networking, security and applications profiles for end products.

Equally importantly, they are defining the tests and procedures required to establish qualification of end products and to ensure that they are interoperable. Interoperability is a key feature which will ensure the success of the standard.

To this end, a regular pattern of interoperability trials are being performed, together with a well defined conformance testing procedure. On completion of the trials and conformance testing, end products can then be certified as ZigBee compliant and will be permitted to display a ZigBee logo.

The main features of the ZigBee protocol stack itself include the provision of self configuring mesh networking, system security and device application profiles.

Mesh networking is particularly suited for low data rate, low duty cycle systems as it enables easy deployment of systems provided that there are sufficient nodes capable of mesh routing within range of each other.

The ZigBee security architecture uses the AES encryption provided by the MAC layer, but further defines Key types and Key setup and maintenance procedures. A trust centre is used for each network to control and manage the distribution of keys.

Authentication prevents unauthorised devices from joining a network whilst encryption secures the data transmitted.

Low Power Wireless System Requirements

The major requirements of low power wireless applications are for long battery life, low implementation cost and robust and secure transfer of low rate data.

Since the bulk of systems have very low duty cycle indeed–typically less than 0.1%, the most important contributor to battery life is the standby current–under 5uA is required. The actual transmit and receive power is less critical as it is a small contributor to the energy drawn from the battery.

Low system cost is achieved both by the availability of low cost chips, and the minimisation of external components contributing to the BOM cost.

Key factors here are the minimisation of external RF components, the ability to use a simple printed antenna, use of just one crystal and the ability to use small, low cost circuit boards. Many of the applications which use low power wireless to communicate utilise small, cheap and simple microcontrollers. For example, a thermostat may need timing, control and measurement algorithms running on an internal microcontroller.

Effectively, the systems are microcontroller applications which happen to require wireless connectivity in preference to any other form of connectivity. Hence, the focus of interest for chip suppliers is very much in the performance and suitability of the microcontroller for the application rather than in the radio system.

A minimum standard for the radio performance is defined by the standards, but in practice, the performance is dictated by limits readily attainable on chip and is hence similar between alternative implementations.

In order to add IEEE802.15.4 / ZigBee wireless connectivity, at least three approaches can be adopted: the existing microcontroller can be expanded in memory capacity and sometimes processing power in order to run the ZigBee protocol stack as well as the existing application; the existing microcontroller can be replaced with one which is already proven to run the protocol stacks; or an additional microcontroller can be added to the system with the sole purpose of running the ZigBee protocol stacks.

Many of the applications are by their nature quite simple, however, the choice of application controller may well be down to different criteria to those mandated by the protocol, for example specific peripherals. The other issues revolve around the provision of a proven software development kit to enable engineers to write code using C and compile onto the target microcontroller together with the protocol stack software and the availability of a microcontroller with sufficient processing power to handle both the simple endpoint devices and the more power hungry routing mesh nodes.

There are currently two approaches to silicon chips in the marketplace–a radio transceiver coupled with a standard microcontroller or a single chip approach–the wireless microcontroller. At first sight, the separate microcontroller and radio approach offers a high degree of flexibility to choose an appropriate microcontroller for the application.

However, in practice, the protocol stacks have only been ported to and tested with a limited number of controllers. Since the stacks are usually large in comparison with the applications, the engineering effort required to port the stacks and test the resulting solution is very significant.

There are several issues which reduce the attractiveness of the fragmented radio transceiver/microcontroller approach com. Board size and complexity is increased due to the use of two separate chips and the additional wiring required between them.

In order to save pins on both devices, the interface between the microcontroller and the radio is performed using a serial interface. However, this interface can become clogged when the device is acting as a routing node with high levels of traffic.

Clock management becomes much harder with a 2 chip solution. The radio chip requires it’s own crystal oscillator in order to realise good phase noise performance on its internal local oscillator.

This often runs at 16MHz in current devices. Unless the microcontroller has its own separate high speed clock (using a further crystal), it will need to wake up the transceiver over the serial interface before it can commence its own high speed operation, taking many low speed clock cycles to do so.

Many of the microcontrollers suitable for this application are restricted in the low power standby modes which can be used to time sleep periods and as a result use many tens of microamps for this crucial low power period, compromising the system battery life.

A Single Chip IEEE802.15.4 / ZigBee Solution

Many of the issues raised have been resolved by the availability of a single chip device which integrates the microcontroller, analogue and digital peripherals, memory, baseband controller and radio system, implemented on an 0.18um RF CMOS process.

The implementation of a single chip solution at this geometry allows advanced techniques to be used in the system design without incurring cost penalties. For example, the difference in die size between a basic 8051 based microcontroller and an advanced 32-bit RISC is minimal in the context of the whole chip.

This means that the microcontroller can be optimised to suit the application in terms of power consumption and processing power. Furthermore, memory is relatively low cost on the process, so there is less differentiation between devices with little memory and devices with a significant amount.

This differs from the discrete microcontroller where the device is often realised on coarser geometry process, which makes for large differences in price between devices with different memory sizes.

Correspondingly, the incremental cost of adding the memory to accommodate a ZigBee stack on a discrete microcontroller is significant. For these reasons, a 32-bit RISC architecture has been chosen for the microcontroller.

This offers excellent power efficiency whilst having the processing power to handle both protocol stack and application. Because the microcontroller can be closely coupled with MAC baseband circuitry, it has been possible to optimise the partitioning of the MAC functionality between hardware and software. For example, all packet formatting, internal timers for beacons, CCA (Clear Channel Assessment) back off and retries, Guaranteed time slots, AES encryption and automatic acknowledgment handling in both transmit and receive are all handled in hardware. This means that the interrupts given to the microcontroller are greatly reduced during operation, allowing the microcontroller to spend more time asleep, reducing system power consumption or alternatively more time responding to the application.

The example below illustrates a typical scenario for sending a packet with minimal interrupts between the baseband and the microcontroller. Note that interrupts to the microcontroller are only required to initiate the transmission and on reception of a valid acknowledgement, enabling the microcontroller to sleep for the whole period, reducing power consumption. Furthermore, the parallel interface between the microcontroller and the baseband controller ensures that no data transfer bottlenecks occur, even for very high traffic through the device.

Apart from minimising the microcontroller activity, the use of the hardware MAC reduces the power consumption by performing functions efficiently compared to a microcontroller implementation. The radio system is designed to minimise the external component count and RF matching.

It includes both the transmit-receive switching and RF matching to present a resistive 200 ohm interface at the chip boundary. This ensures that the device can be used either with a printed antenna, or directly into a discrete balun to interface to 50 ohm single ended systems.

The radio system itself is a low IF, image rejection receiver with a fully integrated bandpass IF filter which feeds the IF gain stages. The IF output feeds an analogue to digital converter which provides the digitised signal to the modem.

The transmitter is a complex IQ modulator and PA driving the inbuilt T/R switch. A coherent modem has been implemented to robustly recover the modulation and from the receiver and to generate the transmitted O-QPSK signal. A range of analogue and digital peripherals are included to allow the device to be used in a wide range of applications. These include 4 port, 12 bit ADC, 2 channel DAC, 2 comparators, 2 counter-timers, UART, SPI port and general purpose IO. With this functionality, most sensors or control systems can be directly connected to the chip, enabling true single chip systems to be constructed. The functionality of the single chip device is illustrated in figure 4.

References:

[1]: Gutiérrez, Callaway and Barrett, “Low-Rate Wireless Personal Area Networks” IEEE Press,

Nov 2003

[2]: ZigBee Alliance; http://www.zigbee.org

13-Nov-2006