"From smart watches to portable health and fitness trackers, wearable devices are increasingly changing the square of our daily lives. The desktop computer revolution in the 1980s brought unprecedented personal productivity outbreak. In the 1990s The appearance of the laptop, is consistent with the development of the Internet, and liberates us from the restraint of the power cord and the network cable. Then the bombing growth of the cellular phone and smartphone has brought us unprecedented mobility and wireless connection capabilities. Today's "Wrist Revolution", together with the IOT, the "Mobile" is pushing "mobile" to a new height: can wear calculation.
The revolution is rewriting the design history of portable electronic products. In this article, we will focus on design ideas hidden behind the User-Experience-Driven design method. By applying these design ideas, a variety of successful wearable products are already available. We will also discuss the characteristics and functions that affect the energy consumption budget and calculation needs of the product, including how to choose a microcontroller (MCU) suitable for product design requirements.
Status of the revolution on the wrist
Smart watches, active trackers, wearing GPS devices, heart rate monitors, and smart glasses are the main application forms of wearable products, according to Futuresource Consulting, the global sales of wearable equipment is approximately $ 8 billion in 2013. Through innovative combination of mature and perfect functionality, simple and easy-to-use connectivity, compact dimensions, ultra-low power treatment and wireless connectivity, wearable devices are becoming a new personal electronic device category, helping us become Health, more information and better equipment.
A few years ago, although some leading mobile phone manufacturers have begun experimental launch of wrist products based on their existing mobile phone products, they are quite cumbersome. Until 2012, when a new company (such as Pebble SmartWatch wrapped around the smartphone manufacturer, the revolution was really beginning to develop rapidly. These products allow end users to make more use of smartphones they already. Garmin, Samsung, Sony, Fitbit, Magellan (see Figure 1) and other consumer electronic equipment manufacturers have also launched their own smart watches, active trackers, and other wearable products, making a strong competitor of the revolution on the wrist.
Figure 1: Under a single CR2032 button battery power supply, Magellan Echo intelligent sport watches use Silicon Labs EFM32 Gecko MCU to achieve battery life to 11 months
This adequate competitive market environment also encourages miniaturized sensitive startups to launch a characteristic innovation product, such as Misfit SHINE fitness tracker (shown in Figure 2) is constantly grabbing the market share of competitors.
Figure 2: Misfit Shine is a design elegant fitness tracker that achieves excellent energy efficiency and long battery life by using EFM32 Gecko MCU.
A successful wearable device must provide suitable price, performance, functionality, and battery life, as well as unique appearance, touch and role to stand out from competitors. MCUs, sensors, wireless electronic devices, and attractive user interfaces must be loaded into a small size package, which can be comfortable to wear in a person's wrist or elsewhere. Since this shape size greatly limits the space occupied by the battery, the wearable system must be very energy-saving in order to allow the battery to replace and charge the time interval to maintain the longest run cycle.
User experience driver win design
To integrate these different elements into a product-recognized product, a complex design requires parameters such as balanced power consumption, performance, functionality, and shape size. By using a so-called "user experience driver" design method, some manufacturers have successfully spent the difficulties, and enter this unfamiliar area. Although these design methods have subverted the traditional thinking and practical methods of many embedded developers.
The design process of the embedded system usually begins with functionality and performance definition, which will be used as the top-level driving force of the project. Instead, designing a wearable product usually begins the final "user experience" definition that needs to be obtained. The definition of these product needs is reflected in the interaction between appearance, touch, and end users, as well as it brings, feelings, and emotions. The next step of the design process is to convert the user experience into a "use case" - a series of top-level features for defining the product hardware and software units.
Apple is one of the early practitioners of this strategy. They use this strategy to define a new market and have been in a leading position. If you have the importance of carefully designing the user experience, you will be suspicious, carefully consider the unique control roller, gem-like shell design and easy-to-use iTunes software help companies transform and ultimately dominate the entire digital music play Market.
Define user experience
User experience requirements for defining wearable products can be classified as the following two categories:
● Functionality - distinguishes out unique appearance, touch, characteristics, and functions of other wearable products.
● Easy-to-use - a series of requirements for easy setting, intuitive operation, and reduced maintenance. Long battery life has a very important role in ease of use, because if a wearable device has to recharge every few days, it will be a frustrating thing, and it is easy to cause users to abandon the product.
At the same time, these elements that define the user experience can easily convert the use case, thereby constitute the basis of product design. Defining the user experience may involve constructing a wearable product entity that makes it very attractive texture, in line with ergonomic shape and conveys a specific sensory design element. Other products may need to create special visual models for control and display, making complex operations simple and intuitive.
Use example
Once the product's user experience is clearly defined, it must be converted into a use case, the functional requirements of the use case will drive the design of the wearable product. Detailed use can provide important information that make it easier to perform accurate comparative studies for all aspects of wearable product design.
The use case should include the task of wearable devices expect the tasks to be executed, the resources and the environment being executed. These details typically include the data types to be collected, how to interact with users or other devices, expected operating environment (temperature, water resistance, impact resistance, etc.), mode of operation (data collection, and analysis, user interaction, communication, etc.), And the frequency of data synchronization with other devices.
With these guidelines, the design team can begin to determine the sensing, calculation, and communication components that meet the application needs. At the same time, the material list (BOM) cost and power budget can also be carried out simultaneously with the initial design demand, providing the team to provide the necessary parameters to centralize the general design.
Energy management
Because the service life of the battery plays such an important role in the wearable design, we need to carefully analyze the part of the use of energy management during the use of energy management.
In order to accurately simulate design how to affect the battery life of wearable devices, the use case should include a detailed description of the effects of energy consumption, for example:
● The data type and frequency must be collected from the external environment.
● Whether the user interacts with the device through the app, touch screen, buttons or above. If yes, the information type and frequency of communication are used.
● How to communicate with other wearable devices, smartphones, local networks or internets. The change in power supply depends on how the wireless interface (such as Bluetooth, Wi-Fi or Zigbee) is implemented.
● The device is synchronized with his homolog or host system or how the frequency is swapped. (Frequently synchronized with host systems such as smartphones will significantly reduce battery life.)
Once the information is collected, the use case should provide a detailed description of the various operating modes of the system and the runtime in each mode. This will become the basis for system energy score and for any design compromise to maximize battery life.
MCU selection and optimization
The use case and the energy consumption-related portion should be as much as possible including the information of the wearable device to perform sensing, control, and computing tasks, including which tasks are executed by the MCU, which tasks are executed by peripherals. This will help to choose the MCU product that best suits the wearable application requirements and development strategies, and take advantage of MCU energy-friendly features.
By identifying software features and logical algorithms that must be executed, and you can build an appropriate initial estimate or computing requirement for wearable applications. Suppose there is a fitness monitor, its MCU uses an IR proximity sensor to monitor the pulse through a multi-axis acceleration meter, using other sensors to detect temperature, humidity, blood oxygen level, and even ultraviolet (UV) intensity (see image 3). The MCU can then filter out noise and interference doped in the original sensor data prior to determining the real step and the frequency, or bind to heart rate data to distinguish between a specific activity type or other biometrics input.
Figure 3: Health and fitness trackers and other wearable devices contain all kinds of sensors to measure physical activity and other biometrics, such as UV irradiation intensity
Among several excellent 32-bit processor architectures for modern MCUs, the ARM Cortex Series 32-bit RISC CPU has become a leading processor core in embedded design, which is mainly due to its efficient architecture, easy to expand instructions. Set, a large number of development tools and code bases. In the past few years, ARM has created several series of Cortex CPUs, each of which is optimized for special needs. The ARM Cortex-M series processor kernel is developed for embedded MCUs, and performance requirements in these applications must be adapted to energy consumption and low-cost solutions. The Cortex-M Series provides kernel options to meet various wearable device properties, including price, battery life, processing requirements, and display types (see Table 1).
Table 1: Design ARM Cortex-M Series designed to meet diversity design needs
In the Cortex-M series, the M3 and M0 + kernels are designed for cost-sensitive applications and meet these applications, and external event rapid system responses, and low dynamics and static power consumption. More complex and powerful M4 kernels quickly complete the common calculation-intensive algorithms in biological monitoring applications. Its enhanced instruction set includes a powerful digital signal processing (DSP) function library. The single-precision floating point processing unit (FPU) of the M4 core can significantly shorten the runtime, reducing the time of the MCU being active, thereby minimizing overall energy consumption.
Deep sleep extended battery life
In order to reduce the impact of the MCU in the wearable platform energy calculation, it is important to minimize "the frequency and time" of any tasks that need to be woken up in low-power hibernation mode. So the use case should include the expected frequency of the various tasks on the MCU, and whether their execution is an event or planned drive.
One of the main ways to optimize low-power embedded design is to find the lowest sleep mode that is still capable of sufficiently responding to an external event. Most of the MCUs using the Cortex-M treatment kernel support multiple sleep modes.
Silicon Labs EFM32 Gecko Series products use standard 32-bit ARM Cortex-M kernel, combined with energy optimized peripherals and clock architectures. The EFM32 architecture is designed for power-sensitive applications. This architecture utilizes various power consumption patterns to provide optimal energy efficiency for wearable devices (see Table 2).
Sleep / Standby (ie EM1 mode of EFM32 MCU) - can quickly return to activity mode (usually by interrupt mode) with a slightly high power consumption. In this mode, the EFM32 power consumption is 45μA / MHz; and the other 32-bit MCUs are typically 200 μA.
Deep Sleep (EM2 mode of EFM32) - Key units that retain MCUs are active while disabling high frequency system clocks and other non-necessary loads. In this mode, the EFM32 power consumption is low to 900NA; and the other 32-bit MCUs are typically 10-50μA.
Stop (STOP) (EM3 mode of EFM32) - Enhanced version of depth sleep mode, can further save power, while maintaining limited autonomous peripheral activities and fast wake-up capabilities. In this mode, the EFM32 power consumption was 0.59μA; while the other 32-bit MCUs were typically 10-30 μA.
Shutdown (OFF) (EM4 or shutdown mode of EFM32) - This "Near-Death" state saves the smallest wake-up function that can be triggered by an external stimulus. This energy saving efficiency is significantly increased wake-up time. In this mode, the EFM32 power consumption is 20na (420NA at RTC); while the other 32-bit MCUs are typically 1.5μA. "
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