A guide to meeting key functional requirements for successful wearable designs


By Ariel Lasry, member of the MIPI Alliance Board of Directors and chief engineer, Toshiba Electronics Europe, with contributions from Peter Lefkin, managing director of the MIPI Alliance

Wearables are opening up imaginative opportunities for product innovation. The market is new, different and exciting, but it will be extremely competitive and pose new challenges for product designers who must integrate very specific functionalities and performance capabilities into miniaturised form factors. The purpose of this article is to summarise design considerations that developers can use to conceptualise and produce successful wearable products.

The compelling growth market for wearables

The wearables market is substantial and diverse. About 274 million wearable devices will be sold this year, an 18% increase compared to 2015 when 232 million units were sold.

Devices already introduced to the market illustrate the diversity and potential of wearable products. There are dozens of smart watch designs from Apple and other vendors; sport watches like the Fitbit Blaze; virtual reality head-mounted displays such as Oculus Rift; in-sight display applications, including Garmin’s Varia Vision for bicyclists; and smart clothing, such as the Cityzen Sciences “D-Shirt” that monitors health, wellness and fitness metrics.

Four basic functions of wearable designs

Despite the diversity of wearables, certain fundamental functions must be built into all designs. Every wearable, for example, will have 1) one or more sensors to detect physical or environmental conditions; 2) a microcontroller and sensor fusion software to make intelligent use of data from multiple sensors; 3) capability to communicate the data to the outside world via wireless or other network technologies; and 4) capability to interact with the user via keypad, touch, LED notification lights, proximity or other sensor-based applications, a display, or voice-enabled controls or speech input.

Two categories of wearable devices

While all wearables have the four key functions, individual products can be segmented according to two general categories: “small sensing” or “larger feature-rich” devices. Developers can use these classifications to identify the main design and performance characteristics they’ll need for their products.

Small sensing devices include fitness bands, biomedical patches and other products for which sensing is the key feature. These devices have modest CPU performance requirements and can operate on a coin-sized battery even though the analog sensing performance requirements can be very demanding. The devices employ a microcontroller that typically runs a real-time-operating system (RTOS) or no operating system. The software is relatively simple and the user interface is limited. Minimal data is communicated to a hub device, requiring very little bandwidth.

Larger wearables that have feature-rich UIs offer imaging as a key feature. These products include smart watches, sport watches, and head-mounted virtual reality devices. These products have higher-end microcontrollers or optimised application processors and operating systems; sophisticated software to support sensors, cameras, audio, graphics and high-resolution displays; and rechargeable batteries. Some applications, such as high definition video streaming, can require wireless broadband connectivity. 

Optimising performance to overcome common constraints

When developing product specifications for “simple” wearables, designers will need to balance the key features they want in the device against performance and system requirements associated with very compact form factors. Typically, there will be some tradeoffs.

Battery life is a good example of this. While a developer might want to minimise the frequency with which a customer will have to recharge or change the battery, the need to support sensor accuracy or sensor software might require a tradeoff in battery life.  Developers will encounter these tradeoffs when building biomedical sensing wearables that must have the capability to accurately measure blood pressure, heart rate, galvanic skin response (GSR), or arterial oxygen saturation (SpO2), for example. Measurement precision can actually reach down to the mV range for the “sophisticated” analog front-end of the device when the AFE embeds sensors that have direct contact to the human body, such as skin patches.

To optimise battery life, developers can employ low-power messaging techniques and advanced power management methods that put the device in a low-power or sleep state when not receiving or transmitting data. Bluetooth Smart is typically used as a wireless control interface in wearables. It is also used to transmit small amounts of data for wearable applications.

Electromagnetic interference (EMI) is an important concern when multiple components are embedded in small form factors because the interference can impact a product’s performance. MIPI® Alliance interface specifications control emissions to help designers minimise interference and optimise the performance of their implementations. In small sensing devices like skin patches that embed sensors into the AFE to measure galvanic skin response (GSR) or other biomedical parameters, EMI can also impact the accuracy of the measurements or the reliability of the communications signal. These risks, too, can be minimised by interconnecting all components with low-EMI interfaces.

The shortage of board space in wearables generally requires high levels of integration to make the best possible use of available space. Fortunately, highly integrated microcontroller devices as well as Bluetooth Smart, sensors and storage technologies can provide advantages for product designs that use numerous “discrete” sensors such as accelerometers, magnetometers, and gyro components.

Integration of discrete sensors can benefit significantly from advanced techniques for interconnecting sensors to the microcontroller. Designers need to minimise the number of I/Os to save board space but they often want the flexibility to add more sensors. The MIPI I3C specification, forthcoming from the MIPI Alliance, will make it easier for designers to connect and manage multiple sensors in a device while ensuring low-power operation and minimal EMI.

Optimising performance in feature-rich wearables

Many wearables have demanding performance requirements. The market is rapidly adopting smart- and sport-watches that have the capability to display 2D as well as 3D graphics, and bandwidth requirements are increasing accordingly.  A head-mounted device, for example, might have additional cameras to capture high-speed video input or larger displays with high-resolution visual output. For these types of applications, the MIPI CSI-2 interface can be used to transmit video from the camera to the processor. MIPI DSI can be used to transmit the visual content from the processor to the display in these devices as well as smart watches and glass applications. If audio features are needed for speech input, voice triggers as well as routine audio applications like music, MIPI SoundWire can optimise the performance of amplifiers and microphones. MIPI SoundWire enables smart audio devices and voice trigger/audio sensing features optimised for power and package size.

Storage is essential. In “simple” devices that have low storage capacity requirements, Serial NOR flash is adequate.  When higher capacity is necessary, Serial NAND is cost-effective. As devices become more complex, with higher bandwidth requirements, eMMC is typically used.  Next, UFS memory can help developers when eMMC performance or capacity is not sufficient (as when faster booting is required). UFS devices are built with standard specifications, from the JEDEC organisation, which use MIPI UniPro and M-PHY®interfaces.  

More innovation and miniaturisation to come

The mobile design community is continually finding ways to advance the performance of small devices and we can expect continued advances in wearables too. It’s easy to envision 4K video, already available on smartphones, making its way into wearables. Likewise, some of today’s larger wearables, such as head-mounted displays, will become smaller while improving their performance characteristics.

Developers can increase their chances of success in current and future wearable markets by crafting products to meet key functional needs and integrating the components to optimise performance within the specific contexts of small sensing and larger feature-rich devices. 



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