Today’s wearables market is crowded, competitive and successful product design is complex, with manufacturers having to make trade-offs between a product’s diverse elements in order to meet customer needs. Greg Fyke, Director of Marketing, IoT Wireless products, Silicon Labs explains how.
A typical embedded system typically begins with defining the functions and capabilities that serve as the project’s top level drivers. Similarly, a successful wearable design demands focus on the ‘user experience’ that it needs to produce. These requirements include how the wearable looks, feels and interacts with the end user, as well as the impressions, feelings and emotions it evokes.
Today, there are numerous wearable products that monitor health and biometrics, track distance travelled, logs routes, estimates calories burned, and notifies us of incoming calls and messages while seamlessly integrating and communicating with our smartphones. The wearables market can be divided into three broad product categories, each posing design trade-offs as shown in Figure 1 (below).
A key component choice for a wearable product is the microcontroller (MCU). Selecting MCUs that excel in low power operation is critical for most wearable applications. Among the 32-bit architectures available today, the ARM Cortex-M series has emerged as a leading low power processing platform. The Cortex-M0+ is a two-stage pipelined architecture that trades-off some performance efficiency to realise lower active mode current consumption.
The Cortex-M3 and M4 processors offer a three-stage pipeline and a good balance of power and performance. The M4 processor’s single precision floating point unit and DSP extensions can dramatically shorten execution time and energy consumption for software algorithms such as Kalman filtering, which is used to extract information from noisy sensor data. Smart watches demand more advanced processors such as the Cortex-M7 and proprietary cores that trade-off some power efficiency for greater processing capabilities and high bandwidth memory interfaces.
Choosing the right battery technology is also an important design consideration. Disposable batteries have the advantage of not requiring any specialised charging circuitry. The downside is that they complicate the mechanical design and limit the overall ease of use of the product. Rechargeable batteries enable slimmer designs but add cost and design complexity. Regardless of the choice, wearables demand a small form factor. This limits the size of the battery and the amount of energy to work with.
In the quest for a longer battery life, designers of wearables cannot sacrifice a good user experience for energy efficiency. Fortunately, MCUs are now available that balance optimal performance with low power optimisations for long battery life. In addition to a low sleep current, one of the more critical features is a fast wake-up time. A quick transition from sleep to active states results in a more responsive solution and less wasted energy.
Advanced MCUs provide peripherals that can perform even when the MCU is asleep.
CMOS-based sensors used in wearables provide the foundation for a rich user experience, enabling new applications and use cases. There are three main sensor categories for wearables - motion sensors, environmental sensors and biometric sensors. Each sensor type offers unique insights into the activity, surroundings and health of the end user.
Mobile apps are a critical part of any wearable solution, and Bluetooth Smart has quickly become the dominant wireless solution for connecting wearables to iOS- and Android-based mobile devices. Smartphones provide a flexible user interface, and apps open up personalisation capabilities that could not otherwise be realised with a wearable alone. However, adding and using wireless connectivity comes at a design cost. A winning wearable design requires a careful balance. Wireless transactions are often the highest power consumer in a wearable system. Determining how much and how often information is sent or how often the wearable synchronises with a smartphone can have a dramatic impact on the battery life of the end product.
We are now on the threshold of a new era of wearable innovations. With easier access to more accurate sensor driven end user data, a wearable device may be able to reliably and uniquely identify you. This shift from health and activity tracking to secure and reliable user identification will open up a wealth of new opportunities in healthcare, security, mobile payments and social networking. The first wearable products to realise compelling new user experiences in these areas at the right cost points, will be the next winners in the marketplace.