Wearable devices cross a wide range of applications, including healthcare, sports fitness, gaming, lifestyle, industrial, and military. They monitor various parts of the body including the eyes (smart glasses), neck (necklace or collar headphones), hands (gloves), wrists (activity monitors and sleep sensors), feet (smart socks and shoes), and specialized areas, such as is required for tracking devices or motion sensors. Wearable devices are commonly equipped with sensors, a processor, storage, connectivity link (for uploading data and downloading updates), display, and battery. Figure 1 shows the block diagram for a typical activity monitor.
Wearables introduce several designs factors that must be considered and may differ from other types of embedded devices. Because these devices are worn, size and weight are crucial. Average battery life is important as well, given that wearables must operate on limited battery power. For consumer-based applications, low cost is essential. The type of processor required and amount of storage required depends upon the use cases the wearable device must support. For example, motion sensors provide a continuous data stream that must be transferred; in contrast, an activity monitor collects data continuously, processes it to identify what activity is currently being performed, and then logs this metadata for later downloading.
How wearable devices communicate has a major impact on key design factors. OEMs have a number of communication protocols available for use in wearables. Well-established standards like Bluetooth Classic, ZigBee, and Wi-Fi have strong market penetration, but were not designed with low power as their primary design consideration. As a result, many OEMs have turned to proprietary protocols to achieve the necessary energy efficiency. However, proprietary protocols can limit the flexibility and market reach of wearables since they have restricted interoperability to only devices supporting the same proprietary protocol.
To meet the requirements of wearable devices and other low power applications, the Bluetooth Special Interest Group has developed Bluetooth Low Energy (BLE). BLE focuses on achieving the lowest power for short-range communications. BLE operates in the 2.4 GHz ISM band that Bluetooth Classic uses, enabling devices to leverage existing Bluetooth radio technology to keep costs down.
BLE offers bandwidth of 1 Mbps, which is more than sufficient for most wearable applications. Typically, wearable applications also need to provide state information rather than having to log large amounts of data between transfers.
To minimize power consumption, the BLE architecture has been optimized at each layer:
- PHY layer – Increasing the PHY modulation index reduces transmit and receive current
- Link layer – Quick reconnections reduce overall transmit time
- Controller layer – A more intelligent controller handles tasks such as establishing the connection and ignoring duplication packets. Offloading the host processor in this way enables the processor to remain in standby or sleep mode longer
- Protocol layer – Connection setup time for exchanging data is reduced to a few ms. The protocol is also optimized to burst small blocks of data at regular intervals. This allows the host processor to maximize the time it can spend in standby or sleep mode when information is not being transmitted
- Broadcaster mode – Wearable devices can operate in broadcaster mode only, eliminating the need for devices to undergo a connection procedure
- Robust architecture – BLE supports Adaptive Frequency hopping with a 32-bit CRC to ensure more reliable transmissions
The ultra low power consumption of BLE makes it ideal for wearable devices. Its efficiency keeps battery size down, which reduces device cost, size, and weight.
While Bluetooth Low Energy is based on Bluetooth technology, it is not compatible with the standard Bluetooth radio. However, dual mode radios are available that support both Bluetooth Classic and BLE. Dual mode devices, known as Bluetooth Smart Ready hosts, eliminate the need for a dongle, as is required when using proprietary protocols. The readily availability of BLE Smart Ready hosts in smart phones gives consumers a simple and cost-effective way to connect to wearable devices.
A complex, full-package design
Communications is only one part of a wearable architecture. Among other components, these devices must also have:
- Analog front end to process raw sensor signals
- Digital signal processing capabilities to filter out noise and provide advanced post-processing
- Processor for high-level system functions
- Battery charger
Figure 2 details an optical heart rate monitor implemented as a wristband. This type of device uses an LED to illuminate tissue and the reflect signal, measured by a photodiode, carries information about changes in blood volume. A trans-impedance amplifier converts the photodiode current to a voltage, which is converted by an ADC into a digital signal. This digital signal needs filtering to remove DC offset and high frequency noise before heartbeats can be detected. This information is passed to the BLE controller for transmission. Optionally, the heart rate can be computed by the wearable device before transmission.
Multiple discrete components complicate system design. Each additional component also increases power consumption, system size, and cost. To minimize these factors, OEMs can utilize a system-on-chip (SoC) architecture that integrates a controller with the necessary analog and digital components. The PSoC BLE from Cypress, for example, has been designed to meet the strict requirements of the wearable market. It integrates a 40 MHz Cortex M0 CPU with configurable analog and digital resources and has a built-in BLE subsystem.
Figure 3 shows the implementation of a heart rate monitor using a PSoC BLE. For the analog front end, four unconfigured opamps, two low power comparators, one high-speed SAR ADC, and a dedicated capacitive sensing block enable advanced touch-based user interfaces. For digital processing, two serial communication blocks can be used to support I2C, UART, and SPI interfaces. The processor also has four 16-bit hardware timer counter pulse width modulators and four universal digital blocks for implementing digital logic in hardware similar to how logic is implementing in an FPGA.
For this application, the only external components required outside of the controller are a few passive components, a transistor for driving the LED, and those required for RF matching. One advantage of having the other components integrated is greater control over system power. For example, developers can turn disable the analog front when it is not in use.
The ready availability of Bluetooth Smart Ready in smart phones, tablets, and other portable devices makes Bluetooth Low Energy an excellent choice as the communication protocol in wearable applications. With SoC-based BLE controllers, OEMs can minimize power consumption, device size, and system cost, making their wearable designs even more attractive and competitive.