With the recent attention given to wearable devices, one might think that wearables is a new market. However, the truth is people have always been experimenting with wearing small objects that serve both functional and aesthetic purposes. A bracelet, for example, can be transformed into a wristwatch; in addition to telling you the time of day, the device serves as an aesthetically pleasing piece of jewelry.
“Wearable” devices (as we know them) are miniature electronic devices worn on the body and often integrated into existing accessories. The wearables market segment is expanding fast thanks to Internet of Things (IoT) technology, growing 25 percent year over year (YOY) according to the International Data Corporation (IDC). At that rate, a business worth $22 million in 2013 is poised to hit $177 million by 2018.
Types of wearables include smart watches, fitness bands, glasses, smart clothing, smart shoes, and hearables. While some can tell you how fit you are, others can help you with navigation or tracking on the go. Wearables are not just about being portable, however; they are also about the cloud-based services that take the data from the devices and return insights and analytics to benefit the user.
Figure 3 illustrates a generic block diagram of a wearable device, along with its main functional blocks, such as the microprocessor, analog front-end (AFE), sensors (digital and analog), display, and power management, audio, and connectivity blocks. These need to be integrated into a device that also considers:
- Power. Wearables require battery power that is expected to last for a significant amount of time. As a result, power consumption is a critical design requirement and challenge.
- Connectivity. Wearables do not integrate full computing capability (like a smartphone), so they require interaction with one or more other devices via wireless connectivity. The common wireless protocols are Wi-Fi, Bluetooth Low Energy (BLE), and IEEE 802.15.4. Sometimes wearables can support multiple protocols too.
- Size. Wearable electronics rely on the miniaturization of their components, allowing powerful functions to be packed into a very small space. Sensors, processors, cameras, and speakers continue to grow smaller while becoming more capable. Packaging these components into the right form factor and level of manufacturability is a key challenge for wearable designers.
- Aesthetics. Wearables need to be stylish, fashionable, and blend well with ornaments, jewelry, watches, and glasses. This means they typically have small displays, limited space, and low-power requirements. Because they have to be compact, most wearables have a small touchscreen with gesture detection capabilities with single or no buttons. The various kinds of touchscreens include capacitive, resistive, surface acoustic wave, and optical imaging.
- Capacitive technology. This allows devices to implement multi-touch gestures, liquid tolerance, and multi-layer sensing, making wearables multi-functional. Capacitive technology is thinner and more accurate, as it reduces the visible distance between the user’s finger and what the user is touching on the screen, enabling taps, swipes, and gesture recognition. A capacitive touchscreen panel consists of an insulator that is glass-coated with a transparent conductor, such as indium tin oxide (ITO). Touching the screen’s surface results in a distortion of the screen’s electrostatic field, measurable as a change in capacitance.
- Tolerance. Depending on the type of wearable, a tolerance of water, heat, and vibration are often a necessity. For example, fitness bands are expected to tolerate sweat and moisture.
Processors in wearables need to offer a range of core options, from ultra-low power, to top of the range high-performance processing speed. The type of processor to be used in a wearable is driven by the device’s feature set. A few designs may also use an application processor. Recent wearable processors integrate a lot of functionality into a single chip, which is important in minimizing the size of the wearable.
All the design blocks continuously communicating with the processor would consume a lot of power, so an easy way to get around that problem is to use a switch that shuts out a section of the block and wakes it up only when needed. (Figure 4 shows an example in which the processor commands the switch to turn LEDs on and off.)
Switches allow multiplexing or de-multiplexing a great variety of signals. Wearable devices are experiencing sensor proliferation, and switches can also be used to route signals to or from them. An example is a wearable equipped to handle both USB and wireless charging. A switch is useful in this scenario, as the system doesn’t need the USB/wireless charging blocks to work simultaneously.
Moreover, all wearable devices need charging to maintain their portability. While some manufacturers opt for magnetically attached cables, others opt for contactless data connectors and wireless charging without a direct electrical connection. Contactless data connections use magnetic transceivers on both the device and cable to create a wireless connection. This approach can support high-speed I/O protocols, such as USB 2.0 and 3.0.
The short distance between transceivers provides a power-efficient connection. Wireless charging batteries, on the other hand, can be charged without a direct electrical connection. Wireless charging uses induction coils in the charging unit to charge the device. The electromagnetic field created by the charging coil allows energy to be transferred to the receiving coil, which acts like a transformer.
Either way, the device needs to monitor current consumption. First generation wearables consumed around 80 mA current, while today’s more advanced wearables consume up to 100 mA. Signal switches with built-in charger detection technology have greatly simplified detecting the kind of charger being used to manage the current limit (figure 5). Typically, charger-detecting signal switches are placed before the battery to detect the type of charger inserted, detect the impedance, and then control the current intake.
Depending on the target application, wearables have various types of sensors to collect different kinds of data. Figure 6 shows the various sensors in a typical smart band. The sensors collect data about the physical or chemical properties of the body and environment, providing insightful feedback. Examples of common sensors for smartwatches and fitness bands are: biometric sensors, which measure heart rate, heart rate variability (HRV), galvanic skin response (GSR) or blood pressure; environmental sensors, which measure temperature, pressure or UV light; motion sensors, such as a three-axis accelerometer, gyroscope, magnetometer or barometric altimeter; and human interface sensors, which measure proximity, gesture or position.
Smart clothing represents the next evolution of wearable technology and is an area where multiple sensors can be used. Unlike smart watches and fitness bands, smart clothing is still in its experimental stages. Overall, the market for smart clothing and body sensors is moving in the right direction, although the body sensors market is estimated to be larger in the long run due to a wider variety of device types and application markets. (In addition, healthcare is estimated to be one of the biggest drivers for body sensors, particularly connected wearable patches.) In smart clothing, some common sensors include heart straps worn on the chest, electrocardiogram (ECG) headbands that measure brain activity, posture monitors for detecting posture, and sensors that measure blood flow, pulse, blood pressure, blood oxygen levels, muscle movement, body fat, and body weight.
Sensors can be digital- or analog-based. Motion sensors tend to be digital and are controlled using well known communication standards, such as I2C or serial peripheral interface (SPI). Analog sensors are widely used in medical and healthcare devices. They need an AFE consisting of operational amplifiers (op amps), filters, and analog-to-digital convertors (ADCs) to convert the analog signal to a digital signal that can then be processed by the microcontroller.
The various functional blocks of a wearable, be it over SPI or I2C, may operate at different voltages: 0.8 V, 1.8 V or 3.3 V. In these scenarios, level shifters or voltage translators need to be used to communicate with the various blocks. For example, a microcontroller might support 3.3 V but needs to communicate with a peripheral that operates at 1.8 V. Figure 7 shows how a level shifter is used in this case.
User Interface Systems
A user interface (UI) system is a common way for us to interact with wearable devices. These include LEDs, buzzers, and vibrating motors (figures 4 and 6) that provide feedback to and alert the user. Pulse width modulation (PWM) drives these functionalities and often runs off of I2C buses. Input/output (IO) expanders often prove useful in such scenarios where the processor IOs cannot be dedicated for user interface activities.
Hearables is another wearable market that is growing very fast. To some people, hearables may seem like wireless earbuds, but these in-ear, computational earpieces represent a step in the direction of achieving advanced hearing aids. Essentially, a hearable is a microcomputer that fits in your ear canal and utilizes wireless technology. Many also provide additional features, such as monitoring heart rate, tracking brain waves through electroencephalogram (EEG), and providing real-time feedback on focus levels, stress, sleep patterns, and relaxation. The ear is an optimal location for gathering biometric data with minimal environmental noise, and hearables rely on haptics, making it an autonomous system.
The wearables market is poised to grow bigger as the market moves to more integration between watches, phones, glasses, headsets, and clothing. With various types of wearables being introduced to the market, the need for a differentiated feature set is also on the rise to match market expectation. With more feature sets comes the need for more design blocks and sensors—and hence a greater need to use switches for signal routing simplification and level shifters for voltage compatibilities.
Filed Under: M2M (machine to machine)