Role of MCUs in wearable electronics
With the current revolution in the wearable device’s industry, the need for smaller, more intuitive devices is rapidly increasing. Some of the current trends of devices that we are seeing in this new industry are the smart-watches, smart-glasses, sports and fitness activity trackers. Apart from consumer electronics, it is also creating an interesting demand in the medical industry.
It is obvious that electronics that goes inside these devices need to become smaller than they used to be. The most important electronic component would be the microcontroller. As these MCUs need to be small and also perform more functions, integration becomes another important factor. In this article, we will be looking at,
- The different requirements for a wearable electronic system
- How the market can be segmented based on the requirements
- Different components in a typical wearable device
- And finally we will take a look at how MCUs can help address these requirements
Requirements of Wearable Devices
Aesthetics:
The most important requirement in a wearable device would be the aesthetic factor. The end-product needs to be stylish, fashionable, and they need to blend with the existing fashion accessories such as ornaments, watches, glasses etc. The fact that the top semiconductor companies such as Intel partnering with the fashion industry to make these devices fashionable tells us why this requirement is very important.
Capacitive touch sensing is a key technology that enables improving the aesthetics. Important requirements for the capacitive UI here will be to be able to work on a variety of form factors including curved surfaces, to be tolerant liquids, and to be able to sense under thick overlays. Cypress’ CapSense and TrueTouch technologies make such requirements practically realizable.
Size:
The requirement for these devices is to be small in size so that they can easily fit on to a wearable. Nevertheless, the features that they exhibit shouldn’t be reduced or minimized. So the components used in these devices should be small in size and at the same time integrate more features in the same space. Technologies such as System-on-Chip (SoC) and chip scale packages (CSP) help to shrink the size.
Water tolerance:
Wearable devices are going to be everywhere where the human-body can go. Therefore it is important to design these devices to be tolerant to the environmental conditions such as water droplets, moisture, sweat etc.
Power consumption:
Wearable devices are battery powered devices, reducing the power consumption of these devices poses unique challenges due to the following factors.
- The wearable devices unlike other mobile devices are required to be always on and always connected because most of these are monitoring devices. For example, a smart watch needs to be always showing the time, be connected to a mobile phone through a wireless link such as Bluetooth, required to continuously count steps and report it back to a device and much more.
- Battery capacity is inherently limited due to the requirement to reduce the overall size
These devices need to operate at ultra-low power to conserve the battery life. This requirement drives special needs in MCU and firmware algorithm. 32-bit ARM architecture is a popular CPU technology for wearable devices as it provides best performance and energy efficiency. Also wireless technologies such as ANT+, Bluetooth Low Energy (BLE) are designed to consume low power.
Wireless communication:
Wireless connectivity has become the natural characteristic of modern electronic devices as it provides greater flexibility and freedom. It is even more important for wearable devices as they need to interact with one or more devices. Depending on the type and features offered, the device is required to support different wireless protocols such as Wi-Fi, ANT+, Bluetooth Low Energy (BLE), IEEE 802.15.4 based proprietary protocol etc. Some devices are required to support multiple protocols. For example, a wrist watch communicates with a heart rate monitoring chest strap using a proprietary wireless protocol and also it communicates to a running application in a mobile phone using BLE.
Application Processor/Embedded Controller:
The selection of the main processor is purely driven by the type and features of the device. For example, an ARM cortex-M controller can power a simple wrist band but a smart watch requires an application processor in order to run a complex operating system such as Android.
As explained earlier, 32-bit ARM processors are very popular in powering wearable devices as it provides best computing performance and energy efficiency. Modern controllers such as Cypress’ PSoC integrate sophisticated analog, programmable digital functionality along with ARM cortex-M core in a single chip truly utilizing the power of ARM architecture.
Some advanced devices have a separate co-processor to offload the processing of sensor data from the main processor. This is required as the device may have loads of sensor data that needs to be analyzed together in real time requiring constant CPU attention. This function is called sensor hub or sensor fusion. The following figure illustrates the role of a sensor hub in a wearable system.
Operating System:
Depending on the type and features offered, a wearable device may or may not need a specific operating system. For example, a simple wrist watch that monitors temperature, measures movement using a 3-axis accelerometer, and displays time on a monochromatic segment LCD display can run with a lightweight RTOS whereas a smart watch that is designed to be an extension of your mobile phone needs to run an advanced operating system such as Android.
At the same time, sensor hubs require special firmware with context aware algorithms.
Market Segmentation
Now that we have understood the requirements of a typical wearable device, it is important to segment the market accordingly. Right market segmentation enables designers to create the right product and users to choose the best device. The table below segments the market based on the device features. The complexity of the segmentation increases from top to bottom of the table.
Components in a Wearable Electronic Device
Now we will look at the components of a typical wearable system. The figure below illustrates the block diagram of a wearable system combining all the features we discussed in the previous section.
Depending on the type of the main processor used, more peripheral functions can be integrated into a single processor chip. For example, most of Cypress’ PSoC devices readily integrate capacitive sensing function in them which removes the need for a separate touch controller. Similarly, Cypress’ PSoC4 (the flagship cortex M0 device) integrates segment LCD drive in it.
The important subsystem of a wearable device is the data acquisition or sensor subsystem. Depending on the type of the device, this could be a simple system having few MEMS sensors or a complex one having a dedicated sensor hub to interface with the sensors. MEMS sensors play key role in fitness and wellness devices helping to monitor the motion of a human body in all dimensions. These are also called motion sensors. Invariantly all these sensors provide motion information in digital over I2C or SPI communication interface. Examples of these sensors are: 3-axis accelerometer, Gyroscope, Magnetometer, and Barometric altimeter.
The other set of sensors are analog sensors which are widely used in medical and healthcare devices. Biometric sensors such as heart rate monitor, EEG are example of these sensors. The analog sensors require a special component called analog front end (AFE). AFE contains Op-Amps, Filters, and ADCs to condition and convert the analog signal to digital signal that can be readily processed by CPU.
Another important subsystem is the User Interface (UI) system. How a human interacts with a wearable device is a very important consideration. The interactions should be as intuitive as possible to minimize the complexity. The popular UI technology is the capacitive touch sensing Depending on the application, capacitive UI is implemented in many forms such as touch screen, buttons, and sliders etc. Also the UI elements such as LEDs, buzzers, and vibrating motors help to implement alert and feedback from the device to the user. For example, a smart watch that is connected to a mobile phone needs to alert the user when a message arrives. Pulse Width Modulation (PWM) is essential in driving these elements. PWM is used to implement various effects on the LED such as dimming and is used to provide various vibration effects for haptic feedback. These techniques require accurate timing and frequent CPU attention if implemented in firmware. So it is important to choose a processor/controller that supports hardware PWM.
MCU’s can solve these challenges!
Except for advanced infotainment devices which requires an application processor, MCUs can address most of the types of wearable devices. Also the latest MCUs integrate most of the functions in a single chip. This is important in reducing the overall size of a wearable device and BOM cost. For example Cypress’ PSoC can integrate the following functions:
- Power efficient 32-bit ARM architecture (ARM cortex M0 and M3 are supported)
- Flash up to 256 KB and RAM up to 64 KB
- Capacitive touch sensing
- Segment LCD drive
- Hardware PWM
- SPI/I2C/UART communication
- Analog Front End (Comparators, 12-bit SAR ADC)
- Full-Speed USB 2.0
Application Example of a Smart Watch
To conclude the below block diagram shows a typical Smart watch that can be implemented using PSoC and some external components. Gives an understanding of system level implementation and how PSoC can help develop a solution quickly. The blue box at the center represents the functions that can be integrated into a single PSoC.
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