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MCU-Bluetooth design: Making smartwatch talk to IoT devices

Posted: 18 Sep 2014     Print Version  Bookmark and Share

Keywords:wearable  smartwatch  IoT  low power  Bluetooth 

Editor's note: Shrinking semiconductors have led to the development of wireless connectivity-enabled devices that can be worn, and whether or not wearables in the form of smartwatches will attract mainstream adoption, the technology is expected to evolve with new use cases over the next few years. ARM's Diya Soubra shines the light on the combination of low-power MCUs and Bluetooth LE technology in designing wearables that can communicate with IoT devices.

The fast evolving market for wearable products will see explosive growth and many new use cases over the next few years. Market research company IHS predicts this market will be worth $30 billion in revenues and 210 million units by 2018.

Devices worn on the wrist such as fitness activity trackers like Misfit or Misfit Shine, smartwatches like the Pebble mobile smartwatch or the recently announced Omate X smartwatch or products with the potential to form a new category like wristbands that authenticate a user's identity through their electrocardiogram (ECG) are likely to make up a majority of shipments.

But there are number of other imaginative use cases such as T-shirts with embedded displays that potentially could show a video that is running on the wearer's smartphone, along with a whole host of new applications that will fully grasp the possibilities offered by wearables as part of the Internet of Things, linking devices to cloud computing.

Bluetooth Low Energy

A key element in wearables development is low power wireless connectivity. Whether for a relatively simple and single-sensor-based wearable device such as an activity tracker, or a high-end product that integrates data from many environmental sensors such as a pair of snow goggles that has a built-in head-up display for GPS/mapping and distance/speed indication, Bluetooth LE—or Bluetooth Smart, as it is now branded by the Bluetooth SIG (Special Interest Group)—is a significant piece in the jigsaw of enabling technologies for wearables.

While Bluetooth LE as a wireless standard is not without competition, it is well placed to become the preferred connectivity. As a low-power technology, it will make a significant contribution to enabling wearable products to work for weeks, months or even years from a small coin-cell battery. In addition, Bluetooth LE has already being integrated into the latest smartphones and tablets.

All of today's wearable products are "appcessory" (application accessory) products that connect via Bluetooth to an application running on a smartphone or tablet and utilise the device's user interface or display. Typically, that app will connect to the internet, to enable the wearable to become categorised as an "Internet of Things" device.

Devices that employ Bluetooth LE features incorporate the Bluetooth Core Specification Version 4.0 (or higher). Bluetooth LE 4.0 is designed for sending small amounts of data in burst, thanks to a unique packet format with low latency (connection setup and data transfer can be achieved as quickly as 3ms), allowing for ultra-low peak-, average- and idle-mode power consumption.

For reference, a product that implements only the low-energy feature is known as a single-mode device. A product that implements both the low-energy feature and the original Bluetooth 'Classic' mode with Enhanced Data Rate (EDR) is known as a dual-mode device or 'Smart Ready'. According to the Bluetooth SIG, more than 90 per cent of Bluetooth-enabled smartphones, including iOS, Android and Windows based models, are expected to be 'Smart Ready' by 2018.

Low-power MCUs

Crucial to wearable product design is the use of small and efficient (performance/power) low-power microcontrollers. An essential ingredient is an 'always-on, always-aware' processor that handles motion sensors such as accelerometers or gyroscopes, or environmental sensors such as pressure or temperature sensing components. In multiple sensor designs, the processor performs the 'fusion' of data from the sensors to deliver better and more accurate information to a user.

As importantly, this approach reduces the amount of data transmitted up into to the cloud. The ARM Cortex-M3 processor has already proved to be a good choice for many such product designs.

The ARM Cortex-M series is an industry-leading family of 32bit processor cores that range in performance from the ultra-low-power Cortex-M0+, up to the top-of the-range Cortex-M4 processor, which incorporates highly efficient signal processing features for digital signal control, as well as accelerated SIMD (Single Instruction, Multiple Data) operation.


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