Each new generation of wearable devices offers a wide range of features that require a suitable power source. The Apple Watch Series 3, for example, includes an accelerometer, gyroscope, heart rate sensor, microphone, speaker, barometric altimeter, ambient light sensor, Wi-Fi, Bluetooth, NFC, GPS, and optional LTE—all features that require efficient management of energy consumption. Low power is a critical factor in the design of wearable devices.
Wearable devices have a minimal form factor, which is why the batteries cannot perform in accordance to their physical size. The continuing trend toward smaller and more subtle packages, in turn, requires an essential estimate for power management systems. The batteries that are adapted are lithium ion, but they are struggling to withstand the increasingly vibrant market. They provide better energy capacity per volume ratios than any other chemical technology used in the battery. Most currently available smartwatches have a battery life measured in days. The advent of wearables requires batteries with particular technology both in form and in chemistry. When it comes to packaging and form factor, flexible and “printed” batteries are very promising for wearable devices.
To meet the demands of end users, designers must combine energy consumption with the smaller and smaller size of the battery. Ultra-low power devices, energy harvesting techniques, a dynamic approach to maximising efficient energy use, and careful PCB design are the main techniques that are required in the design phase.
Because of this, collecting energy from the environment could be a way to prolong battery life.
Thanks to Moore’s Law, electronics have seen a reduction in the size of their packaging. In this way, a whole series of system-on-chips has been realised.
Wearable systems are preparing to invade the market with billions of connected devices. The concept of intelligent and automatic control and monitoring in real time will become the most important paradigm of various applications. Many wearable devices will continuously monitor their physiological parameters in a non-invasive manner, programming their check-up automatically using AI algorithms.
Microcontrollers play a fundamental role in the development of remote control applications, data storage, and processing. This new nature of embedded systems and their applicability highlights more stringent requirements regarding ultra-low or even infinitesimal power consumption. Microcontrollers are an essential component of embedded systems because they allow most of the functionality to be implemented. The 32-bit ARM architecture is a more popular CPU technology for wearable devices, as it provides better performance and energy efficiency. The Cortex-M series is oriented towards mixed-signal and low power MCU devices. As such, the series is ideal for wearable applications and the Internet of Things (IoT). System-on-chips based on ARM Cortex-M cores are generating much curiosity for all MCU users, able to offer systems with high code execution and a wide range of peripherals and memories.
Modern controllers integrate sophisticated analogue and programmable digital features into a single chip, together with an ARM core, using all the power of their architecture. Some advanced devices have a separate co-processor to lighten the workload (see figure 2).
Microcontrollers (MCUs) with low operating currents, plus low-power suspension modes, are critical to the success of these projects. The reduced power consumption of the product allows wearable monitors to use smaller batteries.
Maxim Integrated has its microcontrollers based on ARM Cortex-M4F MAX32630 and MAX32631 to provide fast processing and extend battery life for high-performance wearable devices. MCU power management optimises runtime and provides the lowest power consumption in active mode (127 μW / MHz), DMA (32 μW / MHz), and sleep mode (3.5 μW).
Microchip, on the other hand, offers microcontrollers with Low-Power (XLP) eXtreme technology that has sleep currents up to 9 nA and operating currents up to 30 μA / MHz (see figure 3).
Another example of a wearable MCU solution belongs to STM32’s STM32 family of STMicroelectronics, which has been deployed in many wearable products, such as Fitbit Flex and the Pebble SmartWatch.
Intel is a healthy ARM competitor with its Intel Quark that targets the IoT and wearable market of Cortex-M. The Intel Quark 32-bit D2000 microcontroller with its development kit includes an ultra-low-power core running at 32 MHz, with 32 KB of integrated flash and 8 KB of SRAM.
Power management, sensors, and microcontrollers can be combined according to application usage. Since many of the technologies involved require very different production processes, integration is a natural step for miniaturisation and can offer some definite advantages to the system: power reduction in PCBs, reduced wiring for faster execution speed, reduced exposure of components for the environment, improvement of materials, and process steps to optimise system costs. Wearable electronics present some unique thermal design challenges for both the package and the system as a whole. Thermal design is a challenge for products such as smart watches and glasses that have high-thermal demands on the processor and package.
In addition to decisions on functional features related to battery, ultra-low power components, and connectivity management, there is another set of equally talented and interconnected considerations that define a high-performance design: security and privacy. Security is the prerequisite for privacy. In most cases, wearable devices communicate clinical data and therefore require support for encryption and data protection.
In the end, we need designers to continuously reduce the size of components, and reduce their power requirements, to keep pace with the increasingly smaller needs for wearable devices.