June 06, 2009

start:Embedded RF: The perfect storm

By Phil Ling

The electronics industry has caught the scent of new opportunities, empowered by wireless technology. The continued demand for wireless solutions is now being fed by much more than the cellular sector; the plethora of frequencies and protocols in use is evidence of that.

But it comes at a time when there is an equal and, to a degree, opposing force, in the form of demand for ultra low power (ULP) devices, something wireless transceivers have battled against since their inception. With signal quality a direct result of transmission power, can the two ever be conjoined to create the ideal solution?

Adding to the maelstrom is the emerging trend for turning background radiation in to reusable power, or energy harvesting.

Ostensibly, creating devices that require no external power supply or batteries couples perfectly with wireless telemetry. It would allow devices to be placed in the most remote of locations without ever being truly out of contact.

Creating the right conditions

Another feature in its favor is that energy harvesting typically only returns enough power to run ULP devices. Together, energy harvesting, low power wireless transceivers and the ongoing developments in ULP CMOS could be combining to create the right conditions for the perfect storm, technologically speaking.

Instantiating RF circuits in a standard CMOS process has, for some time now, been a prerequisite for success; in order to make their production cost effective they must use the most cost efficient manufacturing platform. While this imposes some performance barriers, the industry has rallied in overcoming them, which has promoted the widespread adoption of wireless technologies.

In reality, the performance barriers " while being eroded all the time " only limit the technology in terms of range and frequency, two parameters that match the trend for license free technologies coming under the remit of the Industrial, Scientific and Medical bands (ISM).

Prerequisites for ULP are a microcontroller/microprocessor that has been developed with ultra low power operation in mind. Here, Texas Instruments has shown a commitment to this application area through its MSP430 architecture, which when operating at a peak of 25MHz consumes just 160A/MHz.

With TI's acquisition of Chipcon and its low power transceiver technology in early 2006, the company has been developing low power two-chip wireless solutions for some time. It demonstrates TI's commitment to wireless technologies across all wavebands.

To see a bigger version of this graphic click here.

High integration reduces complexity, lets designers do more with less.

The MSP430 also has its own low power protocol stack, proprietary to TI but available as open source code, called SimpliciTI, which provides a star network topology that can be extended to mesh networking, according to the company.

Shaking it up

Target applications for devices created using the CC430 platform include industrial monitoring, personal area networking and automotive metering infrastructure.

In particular, TI seems to be targeting those applications that are making best use of energy harvesting technology. Speaking as a customer, Roy Freeland, CEO of Perpetuum, said: "Sensing applications are limitless, power supplies are not; solutions like the CC430 platform that combine low power and high functionality with the know-how to take the mystery our of RF design help bridge this gap, to help usher in a new age of energy solutions."

Perpetuum specializes in microgenerators; a technology based on a highly optimized magnetic circuit coupled to a mechanical resonator, which transforms kinetic energy (harvested through the vibration of, typically, large machines) into electrical current.

It has recently introduced a wireless sensor node assessment kit, using TI technology and comprising four energy harvester-powered wireless sensor nodes, which send vibration and temperature data to a receiver connected to a laptop. The sensor nodes run an industry standard IEPE accelerometer with integrated temperature sensor on a flying lead. Each node transmits the data gathered using an IEEE 802.15.4 compliant physical interface, over a distance of up to 100m. Vibration spectra and temperature trends are displayed on the laptop, allowing basic level alarms similar to the ISO10816-3 standard to be set.

"These products will enable OEMs and end users to accelerate the wireless revolution," said Freeland: "It makes it possible for plant managers to quickly achieve the improvement in operational performance from increased reliability and lower costs that is so important to remaining competitive."

Svein Vetti, business development manager for short range devices in Europe for TI, and an ex-Chipcon employee, explained that the CC1101 transceiver targets a standard 0.18m CMOS process, which makes it relatively easy to integrate with the MCU. Clearly, implementing an MCU " even the MSP430 " in a smaller geometry could deliver performance improvements, but Vetti explained that the market for these devices doesn't need high performance application processors, at least not yet.

While the process was likely chosen to get the required performance out of the RF transceiver rather than the MCU, TI does also have RF transceivers operating at the popular 2.4GHz range and Vetti claims the CC430 platform also has the potential to extended performance:

"We are planning several more devices in the family," he confirmed, although he wasn't able to state at what frequency those devices will operate. VPower in the CC430 is a key parameter and with two functional elements to consider, the MCU and the RF transceiver, power management becomes even more important.

According to Vetti; how that power is distributed across the device depends on how it is used: "It is determined by the duty cycle of the RF, if you are transmitting all the time this is a bad idea if you want to save power. If you only transmit twice a day, the RF will be the smallest part."

The CC430 platform is, therefore, targeting applications that exhibit this kind of use pattern; they remain in sleep mode for the majority of the time, wake up infrequently and transmit a burst of data, and then return to sleep mode. With these applications comes the need for some obvious characteristics, such as a very short wake-up time, however more commonly required in today's applications " particularly for wireless devices " is the need for secure transmission, which implies some form of encryption.

The CC430 platform, therefore, also includes a hardwired AES encryption block (see figure 1). The core itself also features a hardware multiplier and DMA channels, and represents the lowest power MCU TI could implement in this platform.

As a result, Vetti believes it is particularly well suited to energy harvesting applications. The next big challenge for Vetti and his team is delivering higher performance and to that end he is currently involved in the development of a new RF core, which will target higher performance and offer greater range: "There should be a high performance RF core that is suitable for integration available in the near future."

Moving forward in the sub-1GHz range, Vetti sees a continued need for separate transceivers, but in most other cases demand will be for SoCs, where more complex protocols and a greater need for security will likely dictate it.

Shine on

Creating applications that use only harvested power is perhaps a daunting prospect and one that may require more than just an application note. Because of that, TI has recently introduced a development kit that demonstrates how solar energy can convert ambient light in to power, to run a wireless node.

The eZ430-RF2500 isn't based on the new CC430 platform, but uses the same MSP430 MCU, with a separate transceiver from the same family; in this case the CC2500, which operates at 2.4GHz. The kit also features a thin-film battery, called EnerChip, from Cymbet Corp., a rechargeable battery in the form of a surface mount integrated circuit incorporating battery management logic.

TI claims it is taking the mystery out of RF design, with RF reference designs, SmartRF Studio software, an RF packet sniffer, and design notes.

Third Party software development providers are also contributing, in the form of IAR, while TI's own Code Composer Essentials IDE is also now available with MSP430 specific collateral.

With any number of applications for wireless networks that could exist without maintenance " even to the point of requiring a separate power supply " is likely to spawn a new phase in data gathering and its distribution. It is also likely to raise interesting issues, such as how much value to put on that data, and how to pay for it if the data is gathered from resources owned by others.

Another interesting aspect likely to arise in the near future is the business model for energy harvesting. In the 17th century residents of England were subject to the notorious 'window tax', which levied a fee based on the number of windows in a property. While it was always intended as a tax on the homeowner's income, based on their apparent wealth (the more windows, the bigger the house and therefore the bigger the income), it was dubbed a tax on light and air. Is it unthinkable to assume something similar could, some day, be imposed on devices that operate purely from harvested energy?

This story appeared in the Janjuary-February 2009 print edition of Embedded Systems EuropeEuropean residents who wish to receive regular copies of EE Times Europe, subscribe here.

You can download a digital edition of this Embedded Systems Europe print edition here.