Bright future for ultra-low-power RF
In particular, ultra low power (ULP) wireless applications – using tiny RF transceivers powered by coin cell batteries, waking up to send rapid “bursts” of data and then returning to nanoamp “sleep” states – are set to increase dramatically. For example, according to analysts ABI Research, 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 Bluetooth low energy chips will ship in 2016.
ULP wireless connectivity can be added to any portable electronic product or equipment featuring embedded electronics, from tiny medical and fitness sensors, to cell phones, PCs, machine tools, cars and virtually everything in between. Tiny ULP transceivers can bestow the ability to communicate with thousands of other devices directly or as part of a network – dramatically increasing a product’s usefulness.
Yet, for the majority of engineers, RF design remains a black art. But while RF design is not trivial – with some assistance from the chip supplier and a decent development kit – it’s not beyond the design skills of a competent engineer. So, in this article I’ll lift the veil from ULP wireless technology, describe the chips, and take a look at how and where they’re used.
Inside ULP wireless
ULP wireless technology differs from so-called low power, short-range radios such as Bluetooth technology (now called Classic Bluetooth to differentiate it from the recently released Bluetooth v4.0 which includes ultra low power Bluetooth low energy technology) in that it requires significantly less power to operate. This dramatically increases the opportunity to add a wireless link to even the most compact portable electronic device.
The relatively high power demand of Classic Bluetooth – even for transmission of modest volumes of user data – dictates an almost exclusive use of rechargeable batteries. This power requirement means that Classic Bluetooth is not a good wireless solution for ‘low bandwidth – long lifetime’ applications and it’s typically used for periods of intense activity when frequent battery charging is not too inconvenient.
Classic Bluetooth technology, for example, finds use for wirelessly connecting a mobile phone to a headset or the transfer of stored digital images from a camera to a Bluetooth-enabled printer. Battery life in a Classic Bluetooth-powered wireless device is therefore typically measured in days, or weeks at most. (Note: There are some highly specialized Classic Bluetooth applications that can run on lower capacity primary batteries.)
In comparison, ULP RF transceivers can run from coin cell batteries (such as a CR2032 or CR2025) for periods of months or even years (depending on application duty cycle). These coin cell batteries are compact and inexpensive, but have limited energy capacity, typically in the range of 90 to 240 mAh (compared to, for example, an AA cell which has 10 to 12x that capacity) – assuming a nominal average current drain of just 200 µA.
This modest capacity significantly restricts the active duty cycle of a ULP wireless link. For example, a 220 mAh CR2032 coin cell can sustain a maximum nominal current (or discharge rate) of just 25 µA if it’s to last for at least a year (220 mAh/(24 hr x 365 days)).
ULP silicon radios featuring peak currents of tens of milliamps – for example, current consumption of Nordic Semiconductor’s nRF24LE1 2.4GHz transceiver is 11.1mA (at 0 dBm output power) when transmitting and 13.3 mA (at 2 Mbps) when receiving. If the average current over an extended period is to be restricted to tens of microamps, the duty cycle has to be very low (around 0.25 percent) with the chip quickly reverting to a sleep mode, drawing just nanoamps, for most of the time.
Diverse applications
If a transceiver is asleep for 99.75 percent of the time, it has to work very hard when awake to achieve anything useful. ULP transceivers do this by waking up quickly, sending very short but relatively high-bandwidth “bursts” of data (up to 1 or 2 Mbps), before immediately returning to the low energy consumption sleep state.
As we’ve seen, because they draw on such modest power reserves, ULP RF transceivers are not capable of high duty cycle applications and therefore don’t compete directly with Wi-Fi and Classic Bluetooth applications. However, ULP operation does open up a wide new range of applications that are beyond the capabilities of other wireless technologies.
The sheer diversity of these applications is remarkable. ULP wireless 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.
These applications have one thing in common that plays to the strength of ULP wireless technology – they’re based on compact sensors and peripherals with small batteries. These devices send small quantities of data (typically a few bits) infrequently (i.e. once every few seconds to a few times per second at most). Despite this commonality, applications as diverse as a wireless PC peripheral (for example, a wireless mouse), a bike computer and associated performance sensors (such as a speed & distance monitor), an RF remote control, and a medical sensor (such as a heart rate monitor) demand very different engineering solutions.
In simple terms, wireless connectivity requires a radio (the transceiver), a protocol (the software code or “stack” that controls how the radio communicates) and an application processor (with its own code, that supervises the specific application, such as a heart rate monitor). But how these elements are implemented affects the efficiency, size and cost of the wireless system.
To illustrate the point, let’s consider two examples employing different approaches: a wireless mouse and a bike computer. A wireless mouse is a relatively simple (but certainly not trivial), high-volume ULP RF application. Wireless mice manufacturers demand a compact, efficient and inexpensive connectivity solution. In other words, they want their wireless mouse to be sleek, feature long battery life and retail at a price that large numbers of consumers can afford.
This best alternative for this application is a system-on-chip (SoC) comprising radio, a factory-supplied protocol and application processor on a single slice of silicon. The high volumes offset the vendor’s higher non-recurring engineering (NRE) costs from developing a SoC. In addition, the vendor can optimize the hardware and software performance to meet the demands of the target application.
The key advantage for the customer (the mouse maker) is that they don’t have to spend development time and dollars selecting and purchasing an external processor (and associated development kit) and then generating the code to run the application. The transceiver vendor has already done the work as part of the SoC design. (Although, if desired, the customer can still develop their own protocol using development and evaluation kits supplied by the transceiver vendor.)
Nordic, for example, supplies its nRF24LE1 SoC to the desktop peripherals market. The nRF24LE1 comprises a Nordic nRF24L01+ 2.4 GHz ULP transceiver, Gazell™ software protocol stack in flash or one time programmable (OTP) memory and an enhanced 8-bit microcontroller. This single chip device measures just 5 by 5mm – allowing it to fit into even the smallest of wireless mice designs.
An nRF24LU1+, another SoC that integrates a Nordic nRF24L01+ transceiver, USB 2.0 compliant device controller, flash (or OTP) memory an 8-bit microcontroller and, that plugs into the USB port of the “host” PC to complete the wireless link. The nRF24LU1+ 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.)
Single-Chip-Connectivity
A SoC has many advantages for high-volume applications. But there are some drawbacks; for example, the high level of integration required for a SoC increases the part’s size and therefore its cost. As described above, wireless SoCs typically include a microcontroller, but many applications already employ such a device to run other functions which could also be used to control the wireless application.
Moreover, some design engineers prefer to choose their own microprocessor – because, for example, they have lots of expertise of working with a particular device – rather than being stuck with the one the comes with the transceiver. In these cases it would be more convenient (and cheaper) to buy a transceiver without an onboard microprocessor.
For example, consider a wireless bike computer. Professional and amateur cyclists alike use these handlebar-mounted devices to monitor performance sensors such as heart rate monitors, speed & distance pods, cadence monitors and crank power meters. The bike computer is a sophisticated device that has its own processor that can also be used to supervise the wireless function so there is no need for the wireless chip to integrate an embedded processor. (See figure 2.)
Nordic and its design partner ANT Wireless of Cochrane, Canada, have significant experience in providing wireless connectivity for the cycling sector (in fact, many of the riders in the 2010 Tour de France used wireless performance sensors linked to bike computers powered by Nordic chips and ANT software).
The chip used by the wireless sensors and bike computers preferred by the professionals is Nordic’s nRF24AP2. This device features a 2.4 GHz ULP transceiver, ANT wireless protocol and high-quality microcontroller/processor interface in a single chip. There is no application processor on the chip – saving cost, reducing power consumption and shrinking chip size. In use, the nRF24AP2 looks after the wireless connectivity and links seamlessly to the application processor in the bike computer that supervises the wireless application. Nordic refers to this approach as “Single-Chip-Connectivity” as it precisely describes the functionality offered.
The demand for interoperability
A proprietary wireless connectivity solution (i.e. one that uses technology belonging to a single company) will always outperform an interoperable technology such as ZigBee or Bluetooth. Why? Because the manufacturer is able to optimize the protocol without the encumbrance of the additional overhead required for assured interoperability. The benefit is a more efficient solution with lower power consumption and reduced cost. The drawback is that lack of interoperability.
A proprietary wireless connectivity solution’s lack of interoperability with devices from other chip manufacturers is a problem for OEMs that require a technology that’s guaranteed to seamlessly connect with wireless chips in other companies’ products (for example, the bike computer in the example above linking to sensors made by other firms). Such standardized interoperability is typically underwritten by a formal alliance of commercial companies such as the ZigBee Alliance, standards bodies such as IEEE, or trade associations such as the Bluetooth SIG. Products must be tested to the relevant specification in order to qualify for interoperability certification to a particular standard.
Although enhancements to standards can take a long time to emerge, and testing to gain certification takes time and adds expense for product developers the advantages are significant. Interoperable solutions tend to stimulate market growth because equipment manufacturers gain confidence that the technology will be available for many years; there is a multiple source chip-supplier market, increasing competition and driving down prices, and quality is assured because chip makers have to pass a regulated certification process.
The ANT+ technology described above is one example of an interoperable ULP wireless technology. It is supervised by an alliance of over 220 companies and has been adopted as a de facto standard by manufacturers such as Garmin and Trek in the cycling sector. And, recently, in addition to Nordic Semiconductor, another semiconductor company has started to offer ANT chips. However, the most successful interoperable short range RF solution (in terms of shipment volumes) is still Bluetooth wireless technology.
Extending Bluetooth
The Bluetooth SIG has extended its Bluetooth technology with a version that can operate from coin cell batteries. So-called Bluetooth low energy has been designed to allow sensors and peripherals to communicate with each other and devices such as the next generation of mobile phones. In December 2009, Bluetooth low energy was adopted as part of Bluetooth Core Specification Version 4.0. Nordic Semiconductor has played a significant role in the development of the specification, donating its extensive ULP wireless design heritage to the technology.
Semiconductor vendors are now shipping Bluetooth low energy chips. Nordic, for example, recently announced the first in its µBlue™ Series of Bluetooth low energy chips. The first product in the µBlue family is the nRF8001 – 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 – 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 3a and b.)
The watch is one of the first commercial products to employ Bluetooth low energy and includes features such as time correction from smartphone to watch, incoming call, email and SMS alert notifications from smartphone to watch and a finder function that enables users to locate a misplaced phone.
The Bluetooth SIG’s stated intention is to follow up the publication of Bluetooth Version 4.0 with the release of Profiles for Bluetooth low energy technology including Personal User Interface Devices (PUID) (such as watches), Remote Control, Proximity Alarm, Battery Status and Heart Rate. Other health and fitness monitoring profiles such as blood-glucose and -pressure, cycle cadence and cycle crank power will follow. (See Figure 4.)
The low cost and low maintenance (because batteries require only infrequent changes) of Bluetooth low energy sensors will encourage widespread use in public places. One key application could be indoor location (where there is no GPS signal) whereby sensors around a large public building (such as an airport or rail station) constantly broadcast information about their location. A Bluetooth low energy equipped cell phone passing within range could then display that information to its owner. Sensors could transmit other information such as flight times and gates, location of amenities, or special offers from nearby shops. (See figure 5.)
Bluetooth v4.0 chips are also becoming available. Devices such as cell phones should start to incorporate these devices in the second half of 2011. Once that happens, the full potential of this exciting new technology will start to be realised.
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.”
About the Author
Jay Tyzzer is a U.S.-based senior applications engineer with Nordic Semiconductor. The company is a leader in ULP wireless connectivity in the 2.4 GHz Industrial, Scientific and Medical (ISM) band. In 2010, the company shipped over seventy million transceivers to leading consumer electronics manufacturers. For more information on Nordic Semiconductor’s nRF24LE1, nRF24AP2 and Bluetooth low energy wireless technology products please visit www.nordicsemi.com.
If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :
