Energy-efficient wireless protocols for wearables
The design of wearable electronics presents developers with a complex problem. Not only do these computer systems integrated into clothing and everyday objects have to be small, they must also be extremely energy efficient in order to guarantee a high degree of mobility.
This also applies to the wireless components: Low-energy wireless protocols for the data transfer such as Bluetooth 4.0, ANT and near-field communications (NFC) keep the power consumption low, thereby extending the operating life.
Sports shoes with built-in sensors, which transmit running speed, calorie consumption and workout duration to the iPod, jackets with integrated mobile telephones and ‘smart’ watches which allow the wearer to take photos from their wrist – wearable electronics are truly on trend.
According to forecasts from ABI Research, the turnover for wearable applications will exceed USD 6 billion by 2018. In the current year, up to 90 million smart wearables will be sold. Areas of application include the sports and fitness sector, medical and health technology and activity tracking.
In the entertainment sector, smart watches, such as those already available from Samsung, Sony and Qualcomm Toq, offer huge growth potential. More than 90% of these wearable applications use wireless technology to connect with smartphones, PCs and other devices.
In addition to the range, speed and data transfer rate, developers should, above all, pay attention to the power consumption during the data transmission when choosing the wireless protocol. If this is too high, the capacity of a battery will only last for a short time, thereby significantly limiting the operating life.
Bluetooth Low Energy (BLE)
Such energy-efficient wireless operation is facilitated by Bluetooth Version 4.0. Here the core specifications have been expanded by the power-saving component Bluetooth Low Energy technology (BLE). This has been specially designed for small data volumes such as control data, switching commands or sensor values and operates in the 2.4GHz band.
Due to a wider modulation, the low-energy mode guarantees a higher range than the ‘conventional’ Bluetooth with significantly lower power consumption – both at 0 dBm and at 4dBm transmitting power. The BLE chips require so little energy that simple 3 V button cells are sufficient for their power supply.
Often, operation in whisper mode is sufficient for the applications. In this mode, the current consumption, e.g. with the current market-leading SoC nRF51822 from Nordic Semiconductor, is just 5.5 mA during active transmission.
The high-speed data transfer is also an advantage: In order to transmit data over a distance of up to 100 m, the chips require a connection time of just 5 ms. This further reduces the energy consumption. If no transmissions are taking place, the chips enter rest or sleep mode, then waking at regular intervals for so-called signal bursts.
The BLE transceivers are available with very small dimensions. For example, Rutronik supplies the powerful and extremely flexible multiprotocol SoC nRF51822 from Nordic in 3.5×3.8 mm 64 ball wafer-level chip-scale packages.
It is suitable for wearable applications such as smartwatches, glasses, shoes, wristbands and garments. The nRF51822 requires a single energy source and enables the designer to choose between a linear single-chip voltage regulator (1.8-3.6V), a direct mode (1.8 V) and a DC/DC buck-boost converter (2.1 3.6 V).
This converter can be dynamically controlled during operation, peak current ratings are below 10 mA (3 V). The nRF51822 is available with flash memory capacities of 256 kB and 128 kB. The free Bluetooth 4.1 Low Energy protocol for operation in the role of peripheral communication, as is usual with wearables, is called the S110 at Nordic and takes up around 80 kB of this memory as binary.
Customers can use the remaining approximately 40 kB or 170 kB of flash memory for their own application. They are executed by the integrated ARM Cortex M0 with 16kB RAM. Thanks to 31 integrated GPIOs, which are individually assigned to different pins, as well as PWM, ADC and other features, an additional microcontroller is superfluous. This saves space, costs and energy.
In addition to the technical parameters, market acceptance plays a key role when choosing a wireless technology. Bluetooth is clearly ahead of the pack here. In the consumer market in particular, mobile phones, smartphones and tablets have long been Bluetooth Smart Ready, i.e. they are equipped with a dual transceiver supporting both conventional Bluetooth and BLE.
However, since February 2014, products fitted with Bluetooth must have their own Declaration of Compliance, which must be paid regardless of the actual qualification process (QDID). Adopter member companies must pay $8,000 for each Declaration ID for a product. Following successful classification in the ‘Innovation Incentive Programme’, smaller companies and start-ups pay only $2,500.
In order to make wearables as cost-effective as possible and to realise them without specific HF equipment and expertise, certified wireless modules with integrated protocols are already available at Rutronik. Bluetooth 4.0 dual mode modules with integrated expansion by ZigBee Pro or ZigBee IP and wi-fi as well as AccessPoint and WiFi-Direct are also available here.
ANT and ANT+
An alternative to Bluetooth is offered by the proprietary wireless network standards ANT and ANT+ for ultra low-power applications (ULP). As with BLE, the protocol operates in the licence-free 2.4 GHz frequency band and achieves an even lower energy level than BLE. At 1 MHz, the frequency channel is only half as wide as with BLE. Accordingly, the data modulation requires less energy.
In addition, a frequency channel is divided into many communication channels partitioned by time, so that a total of up to 64,000 wireless subscribers can be integrated into one ANT network. In contrast to ZigBee, no differentiation is made between different infrastructure functions, enabling any ANT sensor to function as a repeater and router at the same time.
There is no central coordinator, without which the network would break down. As the sensors remain in sleep mode for the majority of the time, the overall power consumption is very low in principle.
Only around 15 kB in size, the ANT was developed to cost-effectively connect sensors with analysis devices over short distances (up to around 50m). Applications with ANT wireless technology are therefore frequently used in healthcare, telemedicine, in the sport and fitness sectors and in patient monitoring.
This also makes the protocol of interest for wearables aimed precisely at these areas of application. In contrast to Bluetooth, ANT is particularly characterised by its almost unlimited network options. ANT networks can adopt different topologies, such as peer-to-peer networks or meshed networks.
Here Dynastream has launched the N5 range of ANT modules which have very small dimensions and can therefore be used with even greater flexibility in applications. The N5 range is based on the Nordic nRF51422. In principle, this chip is identical in design to the nRF51822 we have already mentioned, but in addition it can also use the ANT protocol. ANT licence fees have already been paid with the acquisition of the chip or module, so this technology is practically free for the device manufacturer.
In addition to various ANT chips and modules, Rutronik has expanded the portfolio to include pre-compiled USB sticks which can even be printed according to customer specifications. This means that older laptops can also be integrated into an ANT network. ANT is already integrated into modern smartphones and tablets, such as the Sony Xperia range or Samsung’s S4 and Galaxy.
Only users of the Apple iPhone are currently required to insert ANT dongles available for free. This means that the ANT infrastructure is progressing in the computer world at a similar level to BLE.
The basic ANT protocol can be expanded by a standardised utilisation profile called ANT+. This enables appropriately equipped wearables to network with other ANT+ devices in their vicinity and exchange sensor data with each other. This open access to data enables athletes to obtain an overall analysis of their fitness, for example, as different devices such as pedometers and heart rate monitors can work together. ANT+ is already in use in a number of devices.
For example, Samsung uses the protocol as a native feature for its flagship product Samsung Galaxy S5 and for the health app Samsung S Health 3.0. The close connection with the smartphone manufacturer could prove to be a genuine competitive advantage, leading to further distribution of the technology. ANT is a genuine alternative to Bluetooth technology, particularly for small and medium-sized providers.
Using two protocols
It is not necessary for a developer wishing to wirelessly connect their wearable application equipped with sensors to a smartphone to decide between BLE and ANT. To ensure maximum compatibility, both ULP wireless protocols can be used. Consequently, there are no downsides. The nRF51422 or the corresponding N5 modules will operate with all of Nordic’s stack variants.
The S110 stack is for BLE peripheral operation, the S120 designates the BLE master function, the S130 permits all four BLE modes: Peripheral, master, observer and broadcaster. S210 designates the pure ANT stack. In order to connect both technologies, only the S310 stack needs to be flashed in the transceiver. It includes both the ANT and BLE peripheral modes.
In addition, the Gazell open-source protocol can be used which, like BLE, is intended only for smaller star topologies. This shows: The nRF51 range is characterised by an excellent hardware structure and is a true superchip thanks to the mature and tried-and-tested protocol stacks.
NFC – Near Field Communication
Near Field Communication (NFC) is suitable for data transmission over very short distances. This enables phone numbers, photos, navigation data and even digital permissions, for example, to be transmitted from one wearable garment to another, either by both wearables being in close proximity or briefly touching each other.
NFC also functions similarly when used for simple electronic payment transactions, for example when a smartwatch is passed over the payment terminal on public transport, at petrol stations or in supermarkets. The short distance between the devices during the payment process ensures a high level of security for the transmission of data.
However, due to the extremely short range of just a few centimetres and a very low data transmission rate, NFC is not considered a competitor to Bluetooth or ANT. But it offers interesting options for wearable applications in relation to energy consumption.
The active NFC chips do not just communicate with the passive NFC tags over a short distance, they also transmit energy. Some NFC tags, such as those produced by STMicroelectronics and Panasonic, can even use this energy to activate their microcontroller and execute a few smaller applications without requiring their own energy source.
Near Field Communications chips are already installed in many smartphones. This trend is already used in Bluetooth speakers and headphones as well as wi-fi access points. NFC’s automated registration and coupling procedures do away with the need to enter a password, PIN code or manual network selection.
Wearable applications are an ideal means of replacing keys, membership cards and various ID cards and tickets. It therefore makes sense for the data memory to be accessible both by NFC and the application controller. For only when both worlds have access to the same data can such ‘smart’ applications be realised. With the M24SR and M24LR, STMicroelectronics is also supplying two product ranges.
They differ primarily in range, data throughput and energy harvesting capability. Thanks to the EEPROM technology used, they are both available at a very attractive price: Each device is part of the internet of things for around 30 cents.
With the NFC connection to the smartphone, not only is it possible to use the internet connection, but also the touchscreen, microphone, GPS, loudspeakers and temperature, compass and 3-axis sensors. Consequently, additional components are not required and the integration of NFC makes a significant contribution to cost savings.
Developments in the field of wireless technology indicate that more and more semiconductor manufacturers are becoming aware of the potential offered by wearables. The trend is towards ever smaller and more economical wireless modules in order to ensure maximum mobility for the end user.
Writer is Lan Hong, product sales manager for wireless products at Rutronik Elektronische Bauelemente GmbH
Tags: Bluetooth, iot, NFC, Rutronik, wireless