Wearable devices for consumer represent substantial market opportunities for design engineers who understand the implications of IoT, social media and ecosystem development.
BY JOY WRIGLEY, LATTICE SEMICONDUCTOR
The markets for consumer-oriented wearable devices are growing dramatically due in large part to new features and capabilities made possible by the confluence of ultra-low-power semiconductor components, short-range, low-power communications protocols and a new generation of sensor technology. These new devices often rely on wireless connectivity technologies such as Bluetooth or Wi-Fi for access to the Internet of Things (IoT), which allow them to communicate with user’s smartphones or other personal electronics as well as cloud-based services.
The smartwatch is perhaps the most hyped category of wearables, and with good reason given the marketing prowess of such industry heavyweights as Apple, Motorola, Pebble and Samsung – the key players in this evolving, but relatively nascent, category. Essentially, wearable smartphones and smartwatches offer users a plethora of features and functionality, including cellular phone, email, instant messaging, media player, advanced touch displays, activity monitoring, running apps and more. To support all these capabilities, smartwatches are inherently complicated, have intensive processing requirements and incorporate numerous sensors – much like smartphones. However the smaller size of smartwatches dictates the need for a much smaller battery, which makes maximizing battery life a key requirement.
Sports watches make up a separate, though related category. Professional athletes were the early adopters of these products, which are based on ultra-low power RF transceivers, inexpensive MCUs and short distance communication protocols such as Bluetooth Smart. Very light weight and ultra-low energy consumption are vitally important features because marathoners, tri-athletes and professional cyclists want to be monitored for many hours without changing batteries. As a result, sports watches from Garmin, Polar and Suunto are powered by coin-cell batteries. Although first-generation sports watches sent physiological data to a PC for analysis, using the IoT was a natural extension.
Fitness-oriented activity monitors are a fast growing segment of wearables. Lacking many of the “bells and whistles” of smartwatches and sports watches, these simple, dedicated devices monitor such activities as the number of steps taken during a day and send it via smartphone to a data center to provide users with updates and kudos when goals are reached.
Design Drivers for Today’s Competitive Products
Despite the many product categories and applications they span, there are several common denominators that wearables must address before they can hope to gain market traction. Unsurprisingly, ensuring optimal ease-of-use and durability under everyday living conditions is the chief goal for any wearable device.
Battery life is a major concern and while a precise definition of what’s considered an acceptable battery life remains elusive, the old adage that “more is better” is appropriate. Effective power management that maximizes a battery’s charge by minimizing the power required to operate the device is key to providing a good user experience. A key way of achieving this is by ensuring that power-hungry CPUs remain asleep when not needed and do not respond to false wake-up triggers
Another requirement is the transparent connectivity between wearables and any host platform (e.g., smartphone, tablet, PC) and IoT-capable cloud-based applications. Almost without exception, this involves a low-power wireless interface, such as one of the low-power variants of the Bluetooth or ZigBee standards. Some devices that require higher data rates or longer ranges use the low-power variant of Wi-Fi while some products with extreme cost or power constraints employ proprietary wireless protocols.
Wearables must also be interoperable with a large cohort of mobile platforms and other IoT-enabled devices. In addition to smartphones, tablets and other mobile devices, wearables may be required to work in parallel with other wearables that also share the same host platform. Today, smartwatches and fitness monitors sharing access to a smartphone are the most common examples of this type of M2M collaboration, but it’s likely that wearables will be expected to work seamlessly within body-area networks comprising many more devices in the near future.
It is also important that wearables be supported by a robust application ecosystem. This includes local apps, cloud-based apps and services, online presence and social media. The infamous maxim for embedded systems which states, “the processor architecture with the richest ecosystem wins,” is echoed in the wearable systems market. In order to be truly useful, a wearable must be able to port its data, either into a local application or out into an open ecosystem that facilitates information exchange and interactions with social networks. For example, activity trackers are now expected to be able to share the statistics they produce with other users, either via a private group or through public social media such as Facebook. In fact, social connectivity is often a primary consideration for consumers when they go to purchase an activity monitor or other wearable.
Different types of wearables are quite naturally built with different components and technologies. For example, because smartwatches depend on software apps to implement functionality, they require a high performance 32-bit microprocessor. On the other hand, the sports watches used by athletes are most frequently powered by 8-bit MCUs.
In certain IoT applications, 8-bit devices can deliver better performance than 32-bit processors. IoT applications using thin clients, for example, are good fits for 8-bit MCUs despite their limited flash memory and onboard RAM. Wherever there is direct port-to-port I/O, 8-bit devices almost always have lower latency than 32-bit devices, and 8-bit MCUs typically consume less power.
Sensors are always involved in activity monitors – Fitbit, for example, integrates a pedometer – and this usually means some sort of digital signal processing is required. Accuracy, duty cycle and sample rate will vary depending on the precision required by the application.
Low Power is a Common Denominator
One constraint shared by all these applications is power requirements, which must always be low – although the definition of “low” also depends on the device and its applications. For our purposes , it can be assumed that batteries are the primary energy source.
Sports watches typically perform their technology magic powered by a coin cell battery such as the CR2032, which has an energy capacity of about 225 mAh. That’s not nearly enough for smartwatches that run an OS and often have a color display. A capacity of over 2,000 mAh is required to meet the power requirements of the current generation of smartphones.
In MCU-powered wearables, integrated peripherals play critical roles in making the most out of the limited amounts of space and energy available. While the RF transceiver is perhaps the most useful, careful consideration must be given to I/O and power management options. For example, small, low-power state machines can perform routine data collection and monitoring tasks while the relatively energy-hungry CPU remains inactive except when needed. In many applications, this can allow the CPU to remain in an ultra-low-power sleep mode for 98% or more of the time.
Many MCUs already include their own pre-integrated “smart I/O” peripherals but some applications may benefit from the addition of custom functions. Often, these functions are integrated with an ASIC or FPGA/PLD which may also contain connectivity, power management and higher-level functions (Figure 1). We will explore this further in the following section which addresses internal and external connectivity.
Compact low-power designs for wearable devices can often save energy by offloading routine functions to state machines or small blocks of logic implemented in ASICs or programmable devices, allowing the device’s MCU to spend most of its time in an energy-saving sleep mode.
Internal and External Connectivity
Further power and space savings can be achieved by careful integration of connectivity and power management elements. Connectivity may be divided into two categories–internal connections between subsystems and external connections to host devices or the IoT. In many instances, the new Bluetooth Smart standard is likely to provide connectivity between the sensor and the aggregating device, such as a smartphone, PC, set-top box or dedicated personal health system. An important exception is the sports watch category, which adopted the proprietary ANT+ protocol before Bluetooth Smart became a standard.
ANT+, which was developed and is maintained by Garmin’s Dynastream division, has created its own ecosystem, which can communicate with PCs and equipment typically found in gyms. Garmin Fitness and Polar devices utilize ANT+ in high-end training devices. A few sports watches connect directly to the smartphone, although the ANT protocol is part of the software suite of many smartphones. It only has to be activated.
Returning to Bluetooth Smart, the most important single feature of recently adopted v4.1 is “dual mode” topology. This permits a device such as a smartphone to act as a Bluetooth Smart Ready hub and a Bluetooth Smart peripheral at the same time.
The most obvious use scenario is the ability to pass data from a sensor or smartwatch to a mobile phone and then on to a PC if appropriate. Another attribute, which gives developers even greater freedom, is the ability to set up a scatternet.
In its pre-v4.1 mode of operation, Bluetooth enabled communication by creating piconets. But its three-bit address space limits the maximum size of a piconet to eight devices – one hub and seven peripherals, which could negatively affect usability as the IoT expands. Now that the device can assume either identity, it is possible for a hub to communicate with many more than eight devices.
Another important change for developers gives them more flexibility in maintaining communication sessions. With v4.0, the interval between connection “advertisements” from a Bluetooth Smart device to a Bluetooth Smart Ready device was fixed. Unfortunately, this meant that when an activity device such as a fitness monitor was physically separated from the hub, the connection could be quickly abandoned and had to be restored manually. Beginning with v4.1, the developer now sets the interval between connection advertisements.
Bluetooth connectivity may be incorporated into a design using a stand-alone Bluetooth radio, which requires internal connectivity such as an LVDS, I2C or a parallel port to exchange data and commands with its host MCU.
In designs based on one of the many single-chip radio/controller system-on-chip (SoC) products available today, some level of internal connectivity is often required to interface the controller with additional sensors or peripherals it is not equipped to support. This can include mobile I/O expansion and bridging or even higher-level interface functions, such as sensor function pre-processing and co-processing, or a display controller for a small LCD (Figure 2). These functions may be implemented in one of the new generation of low-power programmable logic devices that have recently become available, such as Lattice Semiconductor’s iCE40 Ultra FPGAs.
Even the most highly-integrated SoCs may require additional internal connectivity to add peripherals or external interfaces.
Energy management and production is probably where the wearable industry will see the most innovation over the next several years. Given the ultra-low power consumption that is a top priority for wearables, energy harvesting is a definite possibility, most likely by converting body heat or kinetic energy generated by movement during exercise into electrical energy. It should be noted that when energy harvesting is mentioned, it is almost always with the understanding that a rechargeable battery is part of the energy system.
The use of solar energy is also being investigated. Most of the power management of the MCU and its peripherals is accomplished within the MCU itself through the use of various levels of “sleep” or reduced activity states. At the system level, most wearables today have a very simple LED or LCD display that does not require more sophisticated power management than what an MCU can handle. As screens become more complex, however, power requirements will take on even greater importance.
As the total wearable market evolves, it will probably follow the usual path toward consolidation. Because of their ubiquity, smartwatches are likely to become the preferred aggregator of the physiological information collected by sensors.
Sports watch manufacturers are already differentiating themselves by expanding beyond activity tracking to include monitoring of traditionally “medical” vital signs such as blood pressure (BP), oxygen saturation (SpO2), heart rate (HR) and heart rate variability (HRV). The transition from stand-alone applications to broader interoperability cannot be far behind. This does not, however, mean that proprietary protocols will give way entirely to standards because smartphones can accommodate any number of protocols and proprietary systems can support robust ecosystems within a circumscribed application space.
Despite likely advances in battery technology, maximizing battery life will certainly remain a priority for wearables, particularly as new features and functionality are added. Ensuring that power-hungry CPUs run only when needed will remain one of the best approaches to accomplishing this.