Ultra low power (ULP) wireless applications are set to increase dramatically. According to analysts ABI Research, for example, the wireless sensor network (WSN) chips market grew by 300 percent in 2010. And the same company forecasts that no less than 467 million healthcare and personal fitness devices using ULP chips will ship in 2016.
But what are these chips and how do they operate? Let’s take a closer look.
Low duty cycle, low power consumption
Unlike other wireless technologies like Wi-Fi and Bluetooth wireless technology, ULP transceivers are designed to run from batteries of modest capacity such as coin cells (for example, the CR2032 or CR2025 type).
Typical applications are based on compact sensors, for example a heart rate monitor (HRM). To track pulse and display the information on a wristwatch only requires the transceiver in the HRM to send small quantities of data (typically a few bits) infrequently (i.e. once every few seconds to a few times per second at most). Duty cycles of 0.25 percent are common with the transceiver spending much of the time in a low energy consumption sleep state.
The duty cycle is minimised because the transceiver wakes up quickly, sends a relatively high-bandwidth “burst” of data (at a rate of up to 1 or 2 Mbps), before immediately returning to sleep.
The combination of very low duty cycle and the incredible efficiency of today’s silicon radios is the secret behind the technology’s ULP performance.
During the short period of activity, the transceivers operate at peak currents of just tens of milliamps. Nordic Semiconductor’s nRF24LE1 2.4GHz transceiver, for example, features a peak transmission current of 11.1mA (at 0dBm output power) and peak receive current of 13.3mA (at 2Mbps). Thereafter the chip returns to sleep state, drawing just a nanoamps. Because transmit/receive time is so short, the average current consumption over the long term is just tens of microamps.
Diverse applications
The sheer diversity of potential applications for ULP wireless technology is remarkable. It has already made inroads into the sports, health, entertainment, PC peripherals, remote control, gaming, mobile phone accessories, home automation and industrial control sectors, and will spread to many others in the coming years.
In simple terms, wireless connectivity requires a radio, software code to control communication (“protocol”) and an application processor (with its own code, that supervises the specific application). How these elements are implemented affects the efficiency, size and cost of the wireless system.
Until recently, the sector was dominated by proprietary solutions (i.e. one that uses technology owned by a single company) primarily because the semiconductor vendors were able to optimise the silicon and protocol without the encumbrance of the additional overhead required for the assured interoperability typical of a standards-based solution
The result was a more efficient solution with lower power consumption and reduced cost – the two critical factors for companies searching for the best ULP wireless solution. So-called “ultra-low power” wireless technologies such as ZigBee, while offering impressive performance, couldn’t compete with the power consumption of the best of the proprietary offerings.
Figure 1: The nRF24LU1+ allows PC peripheral manufacturers to make tiny USB dongles with a physical profile that hardly extends beyond the USB port of the host computer
Nordic, for example, has had good success supplying its proprietary nRF24LE1 to the wireless desktop peripherals market (mice and keyboards). The nRF24LE1 comprises a 2.4GHz ULP transceiver, Gazell™ protocol in flash or one time programmable (OTP) memory and an enhanced 8-bit microcontroller all on a single chip. This chip measures just 5 by 5mm - allowing it to fit into even the smallest of wireless mice designs.
The other end of the link is completed by an nRF24LU1+ which allows PC peripheral manufacturers to make tiny USB dongles whose physical profile hardly extends beyond the USB port of the host computer. (See figure 1.)
The demand for interoperability
As ULP wireless has diversified into more applications, the lack of interoperability has started to become a problem for some OEMs. The success of Bluetooth wireless technology has demonstrated the huge benefits of establishing a wireless “ecosystem” where products from different manufacturers can seamlessly connect.
Nordic and its design partner ANT Wireless of Cochrane, Canada, have addressed the interoperability demand to a certain extent with a combination of Nordic’s radio expertise and ANT’s ANT+ software and managed network product.
Running on Nordic’s nRF24AP2 – integrating a 2.4GHz ULP transceiver, ANT wireless protocol and high-quality microcontroller/processor interface in a single chip - ANT+ technology is supervised by an alliance of nearly 400 companies and has been adopted as a de facto standard by manufacturers such as Garmin and Trek in the cycling sector. (In fact, many of the riders in the 2011 Tour de France used wireless performance sensors linked to bike computers powered by Nordic chips and ANT software.)
And, recently, in addition to Nordic Semiconductor, another semiconductor company has started to offer ANT chips offering customers a multi-vendor environment.
However, the Holy Grail for interoperable ULP wireless technology promoters is to emulate the success that Bluetooth wireless technology has had in the low power RF sector.
Extending Bluetooth
That ambition is now set to become reality because the custodian of Bluetooth wireless technology, the Bluetooth Special Interest Group (SIG), has now ratified a version that can operate from coin cell batteries. So-called Bluetooth low energy (a hallmark feature of the latest release of Bluetooth wireless technology, Version 4.0) has been designed to allow sensors and other peripherals devices to communicate with each other and products such as the next generation of mobile phones.
Figures 2a and b: Nordic’s µBlue nRF8001 is the company’s Bluetooth low energy chip and has been selected by Casio for its G-SHOCK Bluetooth low energy watch
Semiconductor vendors are now shipping Bluetooth low energy chips. For its part, Nordic – which played a significant role in the development of the specification, donating its extensive ULP wireless design heritage to the technology – has released the first in its µBlue™ Series of Bluetooth low energy chips.
The nRF8001 is a complete Bluetooth low energy solution in a 32-pin 5 by 5mm QFN package incorporating a fully embedded radio, link controller, and host subsystem. The chip is suitable for watches, sensors and remote controls among other applications. Casio’s recently released G-SHOCK Bluetooth Low Energy Watch uses this chip. (See Figures 2a and b.)
The Bluetooth SIG’s stated intention is to release Profiles for Bluetooth low energy, including Personal User Interface Devices (PUID) (such as watches), Remote Control, Proximity Alarm, Battery Status and Heart Rate, in the next several months. Other health and fitness monitoring profiles such as blood-glucose and -pressure, cycle cadence and cycle crank power will follow.
Bluetooth v4.0 chips are also becoming available. Devices such as mobile phones should start to incorporate these chips - as a replacement for the current generation of Bluetooth wireless technology -in the second half of 2011. Once that happens, the full potential of this exciting new technology will start to be realised because the ULP wireless-powered devices will be able to link directly with the huge Bluetooth ecosystem.
As Nordic Semiconductor’s CEO, Svenn-Tore Larsen, puts it: “Once designers have an inexpensive way to add an interoperable wireless link to anything that’s battery powered, even devices with the smallest batteries, the application potential is vast. Designers will come up with thousands of ways to use that link.”
Further information: Kjartan Furset is based in Hong Kong and is a Senior Applications Engineer with Nordic Semiconductor. The company is a leader in ULP wireless connectivity in the 2.4GHz Industrial, Scientific and Medical (ISM) band. For more information on Nordic products please visit www.nordicsemi.com.
Bright future for ultra low power RF Electronics News
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