要約

This article walks through the capabilities of MAX32664 ultra-low power microcontroller to act as a biometric sensor hub to measure heart rate, blood oxygen, SpO₂, sleep, stress, etc. Maxim's bio measurement algorithms integrated with the sensor hub, accurate to US FDA standards, provide raw or processed data easily taking load off of the host microcontroller.

Introduction

Optical measurement solutions based on the photoplethysmography (PPG) technology are becoming more widely used in the increasingly popular wearable market. Maxim Integrated has introduced some comprehensive optical biosensing solutions designed to support the health-monitoring functions of these devices. This article introduces these sensor solutions, with a focus on the performance and advantages of the Maxim Integrated optical human physiological index measurement algorithm and MAX32664 low-power biometric sensor hub.

Maxim offers complete optical biosensing solutions

Figure 1 shows a complete optical biosensing solution for wearables with four main parts:

• Excellent performance sensors: Maxim Integrated introduced a series of optical measurement sensors suitable for human ears, wrists, fingertips, forehead, abdomen, and other measurement body locations. These sensors measure physiological characteristics such as heart rate (HR), heart rate variability (HRV), blood-pressure trending (BPT), and blood-oxygen saturation (SpO₂). This series of products features small sizes, low power, and high signal-to-noise ratio (SNR) to provide high-quality raw data for measurement systems.
• High-performance, high-accuracy human physiological index measurement algorithm: The Maxim Integrated algorithm team completed the measurement of basic physiological indexes (HR, HRV, SpO₂, and BPT), and determined that the algorithm output accuracy can meet the U.S. Food and Drug Administration (FDA) measurement standards. Also, the Maxim algorithm can create outputs that support advanced applications such as behavioral and motion detection, and sleep monitoring. Customers can select different algorithm modules for specific applications. All algorithms are integrated into the MAX32664 low-power biosensor hub to leave the host microcontroller unit (MCU) free to do other tasks in the system, as well as to remove all types of algorithm licensing burden.
• System-level design simulation and verification: The measurement of physiological characteristics needs to cover different populations, along with differences in skin color, body hair, blood volume, and other differentiating biological factors that can affect the measurement results. Optical measurement methods are structurally susceptible to interference from ambient light, and their tiny signals can also be impacted by interference from other high-frequency signals in the hardware design. These factors greatly increase the difficulty of product design. Maxim Integrated can provide complete design specifications and reference designs to support the structural and hardware design process. Customers can complete customized designs by simply modifying the reference design, greatly reducing the product design period.
• Professional clinical validation guidance: Maxim Integrated has accumulated rich clinical verification experience during product development and maintains a good cooperative relationship with authoritative certification bodies in different parts of the world. These valuable experiences can be used to guide customers in the process of clinical validation and certification by relevant authoritative bodies.

To recap, the Maxim Integrated complete optical biosensing development solution provides customers the flexibility to select product features, which greatly reduces the product design period and verification time. They can also seamlessly integrate the solutions into their own development platforms, reducing time to market.

Introducing the Sensor Hub

The MAX32664 is a part of the Maxim Integrated human physiological index measurement solution, with embedded firmware and algorithms for wearables. It seamlessly enables customer-desired sensor functionality, which includes communicating with the Maxim Integrated optical sensor solutions and outputting raw data or algorithm data. The host only needs to read the output of the MAX32664 through the I²C interface to obtain the required biometric measurement results. Figure 2 provides a system block diagram.

Introducing the Hardware of the Sensor Hub

The MAX32664 is one of the Maxim Integrated next-generation, ultra-low-power MCUs. The MAX32664 (Figure 3) features:

• Superior computing power: Ultra-low-power Arm^® Cortex^®-M4 MCU with floating-point unit (FPU) and the highest operating clock (96MHz).
• High integration: Internal 256KB flash, 96KB RAM, 16KB instruction cache, and 14 general-purpose I/O pins.
• Peripheral resources: One SPI/I²C interface for communication with the sensor and one I²C interface for communication with host; real-time clock (RTC) and UART support.
• Tiny size: WLP (1.6mm x 1.6mm x 0.65mm).
• Ultra-low power: 45µW/MHz @ working mode, 85nW/KB @ data retention mode.

Rich High-Performance Algorithms

The Maxim Integrated algorithm team has been working diligently for more than five years on physiological index algorithms. It has developed a wealth of basic and advanced application algorithms. The algorithm team has a long-term plan in this area, not only to continuously optimize existing algorithms, but also to develop new application algorithms. The above algorithm modules can be integrated into the MAX32664 sensor hub, and the host reads the algorithm result through the I²C interface, which greatly reduces the development period. Also, each sensor hub variant has reference designs, which dramatically reduce design time and instantly collect data for evaluation.

The algorithm modules (Figure 4) have:

• Basic algorithms:
  
  
  • Analog front-end (AFE) management: Control optical sensing, calculate signal quality, achieve the lowest power consumption without sacrificing algorithm accuracy, and choose the optimal optical sensor driving mode.
  • Behavior detection and motion compensation: Classify basic activities, such as rest, running, riding, etc., and perform motion compensation on physiological indexes.
  • Heart rate detection: HR and HRV.
  • SpO₂ measurement: Suitable for wrists, foreheads, and ears, etc. This is the basic index to measure sleep quality.
• Advanced application algorithms:
  
  
  • Sleep quality and state.
  • BPT measurement: Only the fingertip solution is supported now, and the accuracy can reach the FDA measurement standard.
  • Respiration rate.

Body Location	Feature	Sensor Hub	Sensor IC	Demo
Finger Tip	HR, IBI, SpO2	MAX32665A	MAX30101/2	MAXREFDES220 (Same HW, Different FWs for A and D)
	HR, IBI, SpO2, Blood Pressure Trending	MAX32664D
Wrist	HR, IBI, HRV, Sleep Monitoring, Respiration, Stress	MAX32664B	MAX86140/1	MAXREFDES101 (Not Recommended for PPG)
	HR, IBI, HRV, Sleep Monitoring, Respiration, Stress, SpO2	MAX32664C	MAX86140/1	MAXREFDES103
			MAXM86146 Integrated Sensor Hub	MAXM86146EVSYS
Ear	HR, IBI, HRV, Sleep Monitoring, Respiration, Street, SpO2	MAX32664C	MAXM86161	MAXM86161EVSYS

Maxim Integrated introduced four different algorithm solutions based on different test sites and with different optical measurement sensors to meet customer needs (Figure 5):

• MAX32664 A Version: With the MAX30101/2 optical measurement module, this version measures HR, HRV, and SpO₂ from the fingertip.
• MAX32664 D Version: With the MAX30101/2 optical measurement module, this version measures HR, HRV, SpO₂, and BPT from the fingertip.
• MAX32664 B Version: Equipped with the MAX86140/1 optical measurement AFE, this version measures multiple physiological indexes based on the wrist or forehead, including HR, HRV, step counting, calorie calculation, and behavior classification.
• MAX32664 C Version: Equipped with the MAX86140/1 optical measurement AFE, this version measures multiple physiological indexes based on the wrist or forehead, including HR, HRV, SpO₂, step counting, calorie calculation, and behavior classification.

Designing Ultra-Low-Power Applications with the Sensor Hub

• The MAX32664 supports two ways to supply power (Table 1). With different ways to supply, the power consumption of MAX32664 in the active mode is quite different. If the power supply of the host is flexible and feasible, choose the dual-supply mode.
• Thanks to the highly efficient algorithm design from the Maxim Integrated algorithm team and the high computing power of the MAX32664, the MAX32664 can calculate the algorithm results in a very short time and automatically enter the ultra-low-power DEEP SLEEP mode. Table 2 shows the average calculation time of the sensor hub is less than 5% and the average power consumption is lower than 0.8mW, down to 0.43mW.
• After the MAX32664 enters the DEEP SLEEP mode, the host only has to pull down the sleep wake-up pin MFIO to wake up, which is easy to implement.

(V_DD: 1.8V, V_CORE: 1.1V, CPU Clock: 96MHZ, Sensor Hub Measurement Interval: 1s)

Online Upgrade of Algorithms

The MAX32664 has a factory-set bootloader that supports the I²C interface, allowing the online upgrade of the algorithms. In other words, the biometric sensor hub can be upgraded to new or more optimized algorithms after leaving the factory.