Bright future for ultra-low-power RF

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+ 2.4GHz 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.)


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

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.)


Figure 2: Nordic’s proprietary ULP transceiver, the nRF24AP2, dominates wireless connectivity in the cycling sector. (Courtesy: Suunto)

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 2.4GHz 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.