Building ARM-based Bluetooth-enabled wearables
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 low-energy (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 utilize the device’s user interface or display. Typically, that app will connect to the internet, to enable the wearable to become categorized as an “Internet of Things” (IoT) 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 percent 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 32-bit 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.
The Cortex-M series has been implemented in an extremely wide range of general-purpose microcontrollers from many of the world’s leading semiconductor companies. However, a lot of existing wearable products have used the highly flexible ARM Cortex-M3, as it consumes a small amount of power while delivering optimal performance and code density.
In fact, the high availability and sheer cost competitiveness of Cortex-M3 based MCU solutions from so many chipmakers has meant it has become ubiquitous and highly attractive to developers. An example of a Cortex-M3 based MCU is ST’s STM32 MCU family, which has already seen wide deployment in many wearable products such as the Fitbit Flex activity tracker and the Pebble smartwatch.
MCUs based on the Cortex-M0 and Cortex-M0+ are not only enabling ultra-low-power coupled with performance but also provide advantages for developers in terms of size and integration. One example is the Freescale Kinetis KL03 MCU, which is claimed to be the world’s smallest ARM-powered MCU.
Based on a Cortex-M0+ processor running at 48MHz, the device is available in an ultra-small 1.6 x 2.0mm2 wafer-level chip-scale package (CSP). According to Freescale, the KL03 consumes 35 percent less PCB area, yet delivers 60 percent more GPIO (general-purpose input/output) than the nearest competing MCU.
One difference between the Cortex-M processors is instruction set support. The Cortex-M0, Cortex-M0+ has a reduced set of instructions which reduces the complexity (and size) of the core. The richer instruction set of the Cortex-M3 and -M4 is better suited to more complex data processing. The Cortex-M4 also offers DSP instructions and an optional single-precision Floating Point Unit (FPU). Minimizing power consumption is absolutely critical in wearable products. The Cortex-M processors feature two, architecture-defined sleep modes which deliver static power figures of less than 0.7μW/MHz for the Cortex-M3 and Cortex-M4.
Figure 1 – The ARM Cortex-M Series of processor cores.
MCU and Bluetooth combination
While Cortex-M based MCUs can clearly be combined with a low-power single- (Bluetooth LE) or dual-mode Bluetooth module in a tiny, yet powerful, wearable product, an alternative approach that delivers greater integration and targets more complex designs is perhaps the use of a System-on-Chip (SoC) device that combines both a Cortex-M processor core and Bluetooth LE transceiver functionality.
One example is the nRF51822 from Nordic Semiconductor, which is a highly flexible multi-protocol SoC that is ideally suited for Bluetooth LE and 2.4GHz ultra-low-power wireless applications. The nRF51822 is built around a 32-bit Cortex-M0 core with 256kB Flash and 16kB RAM. The embedded 2.4GHz transceiver supports Bluetooth LE version 4.0, in addition to proprietary 2.4GHz operation.
Figure 2 – Block diagram of the Nordic Semiconductor nRF51822.
A second example is Dialog Semiconductor’s SmartBond DA14580, which is a single-mode Bluetooth LE version 4.0/4.1 SoC that integrates the Cortex-M0 processor. The DA14580 draws only 4.9mA during transmission and reception and less than 600nA at 3V in deep sleep mode.
The device can also run from voltages as low as 0.9V and is also ideal for energy harvesting in completely autonomous systems. Available in a wafer-level CSP, with dimensions of just 2.5×2.5×0.5mm, the DA14580 requires only five external components and can operate from a single coin cell, enabling use in the smallest of wearables.
And at the higher end are the Toshiba TZ1001MBG and TZ1011MBG application processors, which integrate the Cortex-M4 processor core, and a Bluetooth LE controller plus RF circuitry, along with Flash memory and sensors.
While the TZ1001MBG integrates an accelerometer, the TZ1011MBG is expected to integrate a magnetometer and gyroscope in addition to an accelerometer. Use of the high-performance Cortex-M4 with DSP and floating-point unit (FPU) allows the combination of data from multiple sensors that are both internal and external to the chip. The processors also integrate a high-resolution ADC to convert signals from external devices such as pulse wave and electrocardiogram sensors into digital data.
Figure 3 – Block diagram of the Toshiba TZ1000.
Development platform
To aid rapid prototyping, product development, and innovation opportunities that wearables bring, it is essential to have easy access to the latest technology: ARM mbed™ (mbed.org) is a key open-source platform for the development of Cortex-M series based products and applications and offers a host of development kits and boards along with free online tools and open source libraries.
The mbed platform enables developers to mix and match components such as MCUs, radios and sensors, and provides software stacks for wireless connectivity including Bluetooth LE among others such as WiFi and cellular. In addition, it also simplifies integration with IP (Internet Protocol) based services providing APIs (Application Programming Interfaces) for cloud services.
Licensed under the Apache 2.0 free software licence, the mbed Software Development Kit (SDK) is an open-source C/C++ software platform. As well as being powerful enough to build complex projects, the mbed SDK also provides a hardware abstraction layer built on the low-level ARM CMSIS (Cortex Microcontroller Software Interface Standard) APIs.
These enable consistent and simple software interfaces to the processor for interface peripherals, real-time operating systems (RTOSes) and middleware, simplifying software re-use and reducing the learning curve for new developers.
Based on the industry-standard ARM professional C/C++ compiler, the web-based mbed compiler is a powerful online IDE (Integrated Development Environment) that is free for use with hardware that implements the mbed Hardware Development Kit (HDK). The mbed compiler also supports full export to different toolchains. In addition, the mbed component database has reusable libraries for a range of hardware that includes sensors, plus middleware and IoT services.
The mbed Hardware Development Kit (HDK) provides full MCU subsystem design files and firmware for building development boards and custom products. Development boards based on the HDK are usually the quickest way to get started with the mbed platform, which include Cortex-M based boards from leading semiconductor companies such as Freescale, NXP, ST and Nordic Semiconductor.
In particular, one key development kit for Bluetooth LE is the nRF51822-mKIT from Nordic, which is based on the above mentioned nRF51822 SoC and is the first mbed development platform specifically aimed at Bluetooth LE applications. The kit is fully compatible with the mbed Bluetooth LE APIs, simplifying use of the protocol stack, and a range of mbed software libraries. The combination of the nRF51822-mKIT, plus Bluetooth LE APIs via the ARM mbed ecosystem is an excellent example of a platform that enables a quick and easy start to wearable product development.
User interface and App development
In terms of fast software user-interface development for wearables, the Koru OS from Korulab offers a user interface that looks much like the Android OS, but is highly optimized in terms of code density and is targeted at wearables that run on MCU-powered hardware.
The memory footprint for core system code is as small as 118KB, yet it can also handle 60fps (frames per second) performance on a Cortex-M4 processor. There is kernel support for OSes such as Android, FreeRTOS, NetBSD and Linux and graphics engine support including OpenGL ES, Open VG and Framebuffer. The Koru UI framework makes it easy for software coding with tools that can automatically generate all the necessary graphical assets. The OS is also highly scalable for different size screens for various wearable form factors such as a watch, amulet or armband.
And for developers looking to develop a wearable product within a few days or perhaps even hours, all that is required is the development of a mobile app based on MetaWear, which started life as a Kickstarter project.
This is a production-ready platform based on an SoC that combines a Cortex-M0 –based MCU with a Bluetooth LE transceiver, together with sensors such as an accelerometer and temperature sensor.
The MetaWear firmware comes pre-loaded on the platform and exposes Bluetooth services and characteristics for all peripherals and sensors. MetaWear Android or iOS APIs are available for download and even sample iOS and Android apps are available to get developers started. In addition, 3D CAD designs for enclosures for the MetaWear hardware are also available for generation on a 3D printer.
Conclusion
The axis of this story of development building blocks for the quick creation of wearable and accessory products revolves around the reality that there is an exceptionally wide choice of ARM Cortex-M based hardware available today, in conjunction with the extensive ARM and mbed software ecosystem. This breadth of choice, market familiarity of the technology and software support has enabled the Cortex-M family to be the processor architecture on which the wearable market revolution is based.
Google’s AndroidWear operating system, announced at the GoogleIO conference in June 2014, is being brought into the market on Cortex-A processors in devices like the Moto360 watch.
The ubiquity of high-performance and low-power ARM based solutions for mobile and wearable markets means that developers will not have to remain with one supplier of ARM-based MCUs and/or Bluetooth LE controller/RF chips or modules, or indeed sensors. There is a wide range of Cortex-M based MCUs from many leading chipmakers at ever more attractive price points, ranging from the ultra-low-power Cortex-M0+ through to the highly versatile and popular Cortex-M3, right up to the high-end Cortex-M4.
While the processors that ARM provides deliver very compelling performance, power and cost points, ARM’s focus has turned to assembling an ecosystem of additional system building blocks, centred on enabling customers to deploy (internet-connected) systems as efficiently and quickly as possible.
On the software side, there is a range of operating systems, compilers, middleware, code libraries and other tools that support Cortex-M processors via the ARM mbed ecosystem. This rich collection of resources makes it possible and perhaps surprisingly easy to design a wearable device or appcessory featuring low-energy Bluetooth connectivity and ARM Cortex processing capabilities within weeks or even days.
About the author:
Diya Soubra is CPU Product Marketing Manager for the Cortex-M3 processor and IoT subsystems at ARM – www.arm.com
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