Wearable technology incorporates miniature devices that can be worn on the arms, legs, head, eyes, chest, or even clothing. Wearable electronics are based on the philosophy that a device can be smart enough to reduce the workload in one’s life, as opposed to just simplifying the work.

Wearables range from fashionable clothing items to smart devices, comfort devices, and medical devices. Looking at medical devices, one of the very first wearables was the pacemaker. Today, wearables abound in the medical field, as well as for consumer applications, including smartwatches, temperature adjustable T-shirts, virtual reality gear, fitness tracker bands, smart pendants, smart shoes, and many more.


Various Design Methodologies and Techniques

Of the different types of wearables, smartwatches have been grabbing a lot of attention. Figure 1 shows a typical smartwatch block diagram. A smartwatch requires not only a clock and connectivity, it also needs to be able to solve common human problems. A significant application is health monitoring. Smartwatches already monitor steps and track speed, numbers of kilometers run, and more, but monitoring heartbeat, blood pressure, and body temperature are features that make the device even more useful. Accelerometers have become common in devices, and programs to monitor the position of toddlers and the elderly, or monitoring sleep and wakeup time can add value to a product.


Figure 1: Wearable block diagram

One of the primary challenges in wearables design is choosing a battery. There have been several high-profile cases of batteries bursting due to lithium cells. A suitable battery is one that lasts long, is rechargeable, and has a low risk of overheating or catching on fire. A proper protection circuit also needs to be designed in order to make this possible.

Wearables designed for medical applications can detect life-threatening conditions like diabetes, blood pressure, and increased heart rate. Biomedical microelectromechanical systems – or Bio-MEMS – are bringing microfabrication and mechanical parts to the medical wearables sector. Bio-MEMS can convert chemical changes exposed to them into a kind of electrical signal. Bio-MEMS chip are extremely small in terms of form factor and are very reliable. These sensors are easy to interface with electronics and can be worn without any safety issues. Figure 2 shows a block diagram representation of a Bio-MEMS-based health monitoring wearable device.

Figure 2: Bio-MEMS-based wearable design block diagram

When something is being worn on the body, radiation safety becomes the major design factor. In such cases, the utmost attention is necessary to reduce EMI/EMC, radiation penetration, specific absorption rate, and thermal management. All such devices should undergo strict tests to comply with these requirements as it is a matter of life and health.

Aesthetics are another important aspect to consider designing a wearable. A design is expected to be sleek, lightweight, and easy to wear. It should not create any type of mark on body, and it should be comfortable. The device should not contain hazardous materials or those that irritate skin or cause allergic reactions. Consider the electric blanket. This is a kind of wearable but, if not designed properly, it can be life threatening. There are many cases of poorly designed electric blankets that caused fires.

Technology is growing at a rapid pace in the wearable industry, which makes innovation tougher. Original equipment manufacturers are adding so many features to wearables that they are no longer differentiating. What becomes important in a design is whether it adds value to a person’s life or reduces some of the work they have to do. Building fashionable wearables is good, but we need to also design wearables that serve us in our day-to-day lives. For example, adding a heartbeat-sensing system to a heart- or a blood pressure-monitoring system would certainly add value, adding to the probability of market success for the product. Fitness trackers are gaining attention due to this value, which they give us in terms of a complete health-monitoring system.


Fitness Trackers and Smartwatches

Fitness trackers and smartwatches have now become a trend not only in the fashion industry but also to the average person interested in improving their overall health. Various designs and form factors are available. Some are purely dedicated to fitness without a display that can be worn around the wrist as a band. With these devices, movement data, pulse rates, and number of steps is transferred to a mobile phone using Bluetooth technology.

Other type of bands also offer the added advantage of sending the calorie lost calculations or using GPS to track the locations covered by a runner or cyclist. These type of wrist bands are very much technology-specific. However, there is a viable market for people who wants to have wearables for fashion or to add more comfort to their life. In these cases, a display touch-based smart watch can track not only fitness but also enable users to access almost all mobile functions without taking their smart phone from their pocket or looking at it. Figure 3 shows the different categories of wearables available.

Figure 3: Smartwatches and fitness trackers Photo Credit: Mobilelap.com

Though the technology of wearables is attractive, the repetition of these features has become a problem for original equipment manufacturers. Wearables need to provide value at a low cost for the middle and lower classes to use them. Figure 4 shows some of the technologies which can add to the asset of smartwatches. Consider a built-in glucometer which can sample blood periodically. This would be an asset to diabetics. One may think of the injecting materials but Bio-MEMS technology can make this idea possible. One would only have to replace the syringes periodically. And all the data collected can be available in the cloud and accessed using any device simply by logging in.

Wearables can even be used to enhance safety. A panic alarm, for instance, would be a valuable feature in fashion devices.

Figure 4: Concept of smartwatch design


Smart Clothes

Smart clothes are one of the innovative wearable technologies that are just catching on. Consumers of such wearables include people living in places with extreme climate conditions, trekkers, mountaineers, and other sports enthusiasts. Apart from these, patients, elderly people, and children are a major market for smart clothing.

Figure 5 shows a block diagram of what a clothing wearable device might actually look like. In a clothing-based wearable, one would want to have all-in-all safety. The heating radiation produced by electronics, especially radiofrequency specific devices, would detract from the value of the product. Therefore, choosing Bluetooth would be a good decision in such devices as these are going to be ultimately close to the consumer. Bluetooth could be a concern in terms of battery consumption, depending upon processing, and data transmission requirements. Intelligent power consumption design is needed. EZ-BLE technology from Cypress sends data in bursts and remains in sleep mode when not in use. Technologies like EZ-BLE help keep overall power consumption down.

The features of a wearable device are important as well. Consider people living in extreme climate conditions. A heating element as well as a cooling element would be quite welcome. This should be controllable with an android or iOS app and should also have a temperature sensor that can automatically make adjustments to maintain body temperature. For mountaineers and trekkers, a wearable bladder that can pump water at intervals and can be consumed by the user through a pipe would add an extra advantage. This water pump can be made adjustable and user controllable with embedded electronics operated through a mobile app. Many deaths occur in areas of extreme climate changes due to skin diseases or excessive water loss. This can be detected using humidity sensor, and feedback from this humidity sensor could control the water bladder pump to alert the user to consume more water under such extreme conditions.

Oxygen level detection in the body would also be useful. For babies, a humidity sensor embedded in diapers would help alert parents to when they need to be changed. For heart disease patients, a heart rate sensor embedded on their clothing could give them an early warning of pending heart trouble.

An accelerometer and GPS would certainly add value to wearables. GPS could help loved ones track the user’s position. An accelerometer would help them know whether the user is sitting, sleeping, walking, or has fallen down. All this data can be sent through Bluetooth as well as Wi-Fi. Wi-Fi is typically not an ideal choice for wearables as it produces heat as well as radiation which is not recommended for close proximity to the human body. Some time ago, use of GSM technology was on the rise for sending data through the cellular networks and SMS would be used to send information. However, because of radiation issues, these technologies have dropped down. This leaves Bluetooth as the most viable option for wireless communications, although range always remains an issue.

Heating and cooling blankets have also been a very successful wearable in the extreme climate conditions. We just need to adapt this same technology and embed it into our T-shirts, pants, and socks. Though these wearables have many advantages, one cannot neglect to address the risks they bring with them. Strict product certification should be adopted to prevent incidents.

Figure 5: Smart clothing design

Choosing SoCs and Microcontrollers in Wearables

The selection of microcontrollers and systems-on-chip is a critical task as one needs to balance cost, performance, and power consumption. Microcontroller selection involves many factors, including:

GPIO (General Purpose Inputs and Outputs) Availability

A good number of GPIOs are needed because the microcontroller will be handling a great many sensors, LCDs, Bluetooth radio, etc. Multiplexing of data lines is an option to reduce GPIOs. Another option is to use I2C-based devices. While these options require fewer GPIOs, designing firmware to implement them becomes more challenging. See Using GPIO Pins Effectively for more on how to get the most out of an MCU’s I/O.

Power Consumption

Power consumption is a very important criterion because no one wants to have to continuously charge his or her smartwatch or fitness tracker. Users expect to charge at most once every day or so. See Low Power Modes and Power Reduction Techniques for ways to improve battery operating life using an MCU’s built-in low power modes.

Internal Data Conversion Features Available

Devices with integrated ADC and DAC peripherals reduce circuit space as well as cost. A microcontroller with integrated ADCs and DACs can shrink a wearable’s form factor, allowing for a sleeker design and lower weight. An MCU suitable for wearables will also offer analog peripherals, including op amps and instrumentation amplifiers, to further reduce design size and complexity.

Other important considerations are speed, SNR, cost, performance, and linearity. It makes sense to list out these factors and compare them when selecting an MCU for a wearable application. Apart from microcontroller devices, a good regulator design with universally pluggable power charging connectors and pre/post compatible software are also good to make sure are available before your device design is finalized.  If your design will utilize a touch interface, you’ll want to familiarize yourself with Capacitive Touch Sensing Design.

Die Size and Chip Package

Die size is a major concern as wearables are expected to take the minimum space possible. Today, 20 nm to 40 nm technology silicon chips can be integrated into BGA packages. Developers need to combine efficient board design with low component count. For this reason, programmable system-on-chip devices are a good choice since they integrate all the necessary functionality into the MCU except for biasing passive components.

We have explored the philosophy behind designing wearables, available technologies that can be used to design wearables, critical design decisions, architectures, and peripherals. By selecting the right processor to meet the requirements of the application, you can reduce device size, development time, and bill of materials cost without compromising functionality.

Nishant Mittal is systems engineer at Cypress Semiconductor, Bangalore. He has done his masters in technology with specialty of Electronics Systems from IIT Bombay, Mumbai, India.

Ronak Desai is a staff engineer at Cypress Semiconductor with nine years of industry experience. He has a BE in Electronics and Communication from Mumbai University, India. He is part of the Development Kits Group and is based out of Bangalore, India. You can reach Ronak at [email protected]