Low power Bluetooth brings high-energy communication to wearable devices
Time:2022-10-28
Views:1653
Author: Richard Dham
Wearable devices are widely used, including medical care, sports and fitness, games, lifestyle, industry and military. They monitor various parts of the body, including eyes (smart glasses), neck (necklace or collar headset), hands (gloves), wrists (activity monitors and sleep sensors), feet (smart socks and shoes) and special areas, such as areas required for tracking devices or motion sensors. Wearables are typically equipped with sensors, processors, storage, connection links (for uploading data and downloading updates), displays, and batteries. Figure 1 shows a block diagram of a typical activity monitor.
Figure 1: Block diagram of wearable activity monitor.
Wearable devices introduce several design factors that must be considered and may differ from other types of embedded devices. Because these devices are worn, size and weight are critical. Average battery life is also important because wearables must operate with limited battery power. For consumer based applications, low cost is critical. The type of processor required and the amount of storage required depend on the use cases that the wearable device must support. For example, the motion sensor provides a continuous data stream that must be transmitted; In contrast, the Activity Monitor continuously collects data, processes it to determine the currently executing activity, and then records this metadata for later downloading.
Low power communication
The communication mode of wearable devices has a significant impact on key design factors. OEM has many communication protocols available for wearable devices. Classic Bluetooth, ZigBee, Wi Fi and other mature standards have strong market penetration, but their design intent is not to take low power consumption as the main design consideration. As a result, many OEMs have turned to proprietary agreements to achieve the necessary energy efficiency. However, proprietary protocols may limit the flexibility and market scope of wearables, as they limit interoperability to only devices that support the same proprietary protocol.
In order to meet the requirements of wearable devices and other low-power applications, the Bluetooth Special Interest Group has developed low-power Bluetooth (BLE). BLE focuses on achieving the lowest power for short distance communication. BLE operates in the 2.4 GHz ISM band used by classic Bluetooth, enabling devices to use existing Bluetooth radio technology to reduce costs.
BLE provides 1 Mbps bandwidth, which is more than enough for most wearable applications. Generally, wearable applications also need to provide status information without recording a large amount of data between transmissions.
To minimize power consumption, the BLE architecture is optimized at each layer:
• Physical layer – increase PHY modulation index to reduce transmit and receive currents
• Link layer – fast reconnection reduces overall transmission time
• Controller layer – Smarter controllers handle tasks such as establishing connections and ignoring duplicate packets. Uninstalling the host processor in this way keeps the processor in standby or sleep mode for longer
• Protocol layer – The connection setup time for data exchange is reduced to a few milliseconds. The protocol is also optimized to burst small data blocks regularly. This allows the host processor to maximize the time in standby or sleep mode when no information is transmitted.
• Broadcasting company mode – wearable devices can only be operated in broadcasting company mode, and no device connection program is required
• Broadcasting company mode – wearable devices can only be operated in broadcasting company mode, and no device connection program is required
• Robust architecture – BLE supports adaptive frequency hopping of 32-bit CRC to ensure more reliable transmission
BLE‘s ultra-low power consumption makes it an ideal choice for wearable devices. Its efficiency can reduce the size of the battery, thereby reducing the cost, size and weight of the equipment.
Although low-power Bluetooth is based on Bluetooth technology, it is not compatible with standard Bluetooth radios. However, dual mode radios can be used to support Bluetooth Classic and BLE. Dual mode devices (called Bluetooth smart ready hosts) do not need to use dongles, which is required when using proprietary protocols. The ease of use of BLE smart ready host in smart phones provides consumers with a simple and cost-effective way to connect to wearable devices.
Complex full package design
Communication is only part of the wearable architecture. These devices must have, among other components:
• Analog front end for processing raw sensor signals
• Digital signal processing function can filter noise and provide advanced post-processing
• Storage
• Processors for advanced system functions
• Charger
Figure 2 details the optical heart rate monitor implemented as a wrist strap. This type of device uses LEDs to illuminate tissues, and the reflected signals measured by photodiodes carry information about changes in blood volume. The transimpedance amplifier converts the photodiode current into a voltage, which is converted from ADC to a digital signal. Before heartbeat is detected, digital signals need to be filtered to eliminate DC offset and high-frequency noise. This information is transferred to the BLE controller for transmission. Alternatively, the heart rate can be calculated by the wearable device before transmission.
Figure 2: Block diagram of optical heart rate monitor with wrist strap.
Multiple discrete components complicate the system design. Each additional component also increases power consumption, system size, and cost. To minimize these factors, OEMs can utilize the System on Chip (SoC) architecture to integrate the controller with the necessary analog and digital components. For example, Cypress‘s PSoC BLE is designed to meet the stringent requirements of the wearable market. It integrates a 40 MHz Cortex M0 CPU and configurable analog and digital resources, and has a built-in BLE subsystem.
Figure 3 shows the implementation of a heart rate monitor using PSoC BLE. For the analog front-end, four unconfigured operational amplifiers, two low-power comparators, a high-speed SAR ADC and a dedicated capacitive detection module can realize an advanced touch based user interface. For digital processing, two serial communication modules 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 general digital modules for implementing digital logic in hardware, similar to the way logic is implemented in FPGA.
Figure 3: Block diagram of optical heart rate monitor with wrist strap using PSoC 4 BLE system on chip.
For this application, the only external components required outside the controller are some passive components, a transistor used to drive the LED, and the components required for RF matching. One advantage of integrating other components is better control of the system power supply. For example, developers can disable the emulated front end when it is not used.
The ready availability of Bluetooth Smart Ready in smart phones, tablets and other portable devices makes low-power Bluetooth an excellent choice for communication protocols in wearable applications. With the SoC based BLE controller, OEMs can minimize power consumption, device size and system costs, making their wearable designs more attractive and competitive.
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