Bluetooth low energy technology in wearable electronics

Wearable devices are widely recognized for their high penetration in many mature markets and the widespread use of lower cost MEMS sensors. Wearable devices are highly portable, can be worn or attached to the body, and measure/acquire information through one or more sensors. Figure 1 shows a general information flow diagram for a wearable device.

　　 Figure 1: Wearables - Information Process

We broadly classify wearable devices based on service segments or wearables. Table 1 shows the types and typical use cases of wearable devices.

Table 1: Classification of wearable devices (* Body: including arms, torso and legs)

Most wearable devices come with one or more sensors, processors, memory, connectors (radio controllers), displays, and batteries. Figure 2 shows an example of an activity monitor.

　　 Figure 2: Block Diagram - Activity Monitor (Wearable Device)

Because of the need to wear such devices on the body, in addition to the basic functions, there are other factors that determine whether the device can be accepted by consumers, including:

●Supported communication modes

● Average battery life

● low cost

● Product size and weight

These factors are detailed in the following sections.

There are many different communication protocols on the market, some are standard protocols, such as Bluetooth Classic, ZigBee, WiFi, and some are proprietary protocols developed by chip vendors. Standard protocols such as Bluetooth Classic, ZigBee, and WiFi do not have low power consumption as a primary design feature, so most OEMs choose to use proprietary protocols. The use of proprietary protocols greatly limits the flexibility of these wearable products to interoperate with devices that use the same proprietary protocol.

To eliminate this limitation, the Bluetooth Technology Alliance (SIG) introduced Bluetooth Low Energy (BLE) technology as the lowest power short-range wireless communication standard. Like Classic Bluetooth, BLE also operates in the 2.4 GHz ISM band with 1 Mbps bandwidth. The most notable features of BLE are as follows:

● Low data rate - Ideal for applications that only need to exchange status information.

• The protocol is capable of bursty transmission of short messages over a fixed time interval, so the host is in a low power mode when no information is sent.

• The protocol reduces the time required to establish a connection to data exchange to a few milliseconds.

Every layer in the architecture is carefully optimized to reduce power consumption

o The physical layer's modulation index is increased compared to classic Bluetooth, helping to reduce the transmit and receive currents.

o Optimized link layer for fast reconnection to reduce power consumption.

o The controller can perform a variety of mission-critical tasks, such as establishing a connection and ignoring duplicate packets, thus allowing the host to be in a low-power mode for a longer period of time.

● Adopts a robust and reliable architecture similar to Classic Bluetooth, supports adaptive frequency hopping, and has 32-bit CRC check function.

● Only broadcast mode is supported; there is no need to perform connection operations on the device.

BLE is not compatible with standard Bluetooth radios because they are different technologies. However, dual-mode Bluetooth devices support both BLE and Classic Bluetooth. With the Bluetooth Smart Ready host (dual mode device), the BLE runtime eliminates the need for a transceiver, which is in stark contrast to proprietary protocols.

The BLE protocol described above can be perfectly applied to wearable devices for the following reasons:

● The protocol has been carefully optimized for ultra-low power consumption.

● Low power consumption helps to reduce battery size, thereby reducing product cost, size and weight.

● Since the BLE smart-ready host is used in the smartphone, it is easy to implement the protocol.

● The wearable device exchanges a small amount of burst information for a long time interval.

The communication protocol is only part of the wearable device. In addition to the communication interface, the wearable device also contains a variety of other modules, such as sensors, analog front ends for processing sensor signals, digital signal processing modules for filtering out environmental noise, and A memory for recording information, a processor for performing various system related functions, a battery charger, and the like.

　　 Figure 3: Optical Heart Rate Monitor - Wristband

Figure 3 shows a typical implementation of an optical heart rate monitor wristband. The optical heart rate monitor uses the PPG principle to detect changes in blood volume using optical techniques. The technology uses LED lights to illuminate body tissue while using photodiodes to measure reflected signals that carry changes in blood volume. Transimpedance amplifiers (TIAs) can be used to convert photocurrent into voltage. The voltage signal is then converted to a digital signal by the ADC. The digital signal is then processed in firmware to eliminate DC offset and high frequency noise to detect heartbeat. In addition, the filtering process can also be implemented in the analog domain using active filters. The heartbeat information is sent to the BLE controller and then sent to the BLE-enabled device via the Bluetooth link. Some optical heart rate monitors use a separate controller to perform heart rate processing, while the controller communicates with the host processor via an I2C/SPI/IART communication protocol.

In such systems, the use of multiple discrete components not only complicates the system (electrical compatibility between the different components, but also requires testing), and increases power consumption (because there is a lack of control over the AFE when not in use) , bill of materials cost and PCB size.

To solve these problems, many vendors have introduced devices based on a system-on-a-chip (SoC) architecture. These devices not only have controllers, but also analog and digital subsystems that enable most of the basic analog front end and digital functions. Cypress's PSoC 4 BLE based on the Programmable System-on-Chip (PSoC) architecture is one such controller. The device is truly a SoC for the wearable market because it includes a 48MHz Cortex M0 CPU, configurable analog and digital resources, and a built-in BLE subsystem. Figure 4 shows the architecture of the PSoC 4 BLE device.

　　 Figure 4: PSoC 4 BLE Architecture

The analog front end portion of the device contains four unconfigured op amps, two low-power comparators, a high-speed SAR ADC, and a user interface-specific capacitive sensing module. The digital section contains two serial communication modules (SCBs) that can be used to implement the I2C/UART/SPI protocol; four 16-bit hardware timer counter PWMs (TCPWMs); and four general-purpose digital blocks (UDBs), which are like FPGAs can be used to implement digital logic in hardware.

Figure 5 shows the method of implementing the above wristband product using the PSoC 4 BLE device.

　　 Figure 5: Optical Heart Rate Monitor - Wristband - PSoC 4 BLE

In this implementation, the PSoC 4 BLE device can utilize its internal resources to implement all functions. The components required outside the controller consist of only a few passive components and a transistor that drives the LEDs and is part of the RF matching network. This integrated solution controls the power consumption of the AFE and disables the AFE when it is not in use, reducing bill of materials costs and PCB size. In addition to the above advantages, using the SoC architecture will also help speed up the time-to-market for the following reasons:

● Provides off-the-shelf firmware IP for system development

● Each module is on the same chip, and it takes a lot of time to interoperate.

Flexible and configurable environment allows changes to be implemented in the final phase

In some designs, the Cortex-M0 core does not meet processing performance requirements. In this case, the M3 core can be used to handle system-related functions. At the same time, BLE-based SoCs (such as PSoC 4 BLE) can be used to control Bluetooth communication and AFE and digital logic. .

in conclusion:

Bluetooth smart-ready devices such as smartphones and tablets are gaining popularity, and BLE has significant advantages. These factors make BLE a popular wearable product communication protocol. After recognizing the BLE niche concept, various chip vendors have also developed BLE controllers, and some have also produced BLE-enabled SoCs. BLE-enabled SoCs help reduce system power consumption, bill of materials costs, and product size. Make the wearable market more attractive and full of good prospects.

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