With the rapid development of microelectronics technology and advancements in manufacturing processes in recent years, commonly used medical monitoring devices in the medical electronics field are showing a new trend towards portability, miniaturization, and home-use applications. Smaller, user-friendly monitoring instruments are playing an increasingly important role in community healthcare and home care. Among these developments, novel microcontrollers based on low-power architectures and new devices with significant low-power advantages have provided new impetus and laid a solid foundation for this trend.
System Design
To meet the needs of community and home-based medical monitoring, portable medical monitoring systems often need to collect and store relevant data on human physiological signals on-site, such as electrocardiogram (ECG), blood oxygen saturation, heart rate, and body temperature, in order to monitor important physiological parameters. These systems should be able to identify and process these signals (e.g., automatically alarm when key indicators are abnormal), provide a user-friendly human-computer interface, and transmit data to a PC for further processing. The structural block diagram of a portable medical monitoring system is shown in Figure 1.
Considering the application requirements of this system, the overall system design must especially meet the requirements of low power consumption, miniaturization, and reliability. The circuit design, circuit form, component selection, and circuit board fabrication within the system should all be closely centered around these three points.
In Figure 1, the sensor, blood oxygen module, and signal conditioning module constitute the forward path within the system. The sensor (such as an electrode for measuring electrocardiogram signals) converts weak physiological signals into electrical signals, and its output signal needs to be amplified and filtered by the signal conditioning module. The blood oxygen module can directly detect blood oxygen saturation, heart rate, etc., through a probe. Currently, there are integrated circuit boards available on the market for monitoring blood oxygen saturation, heart rate, etc., which often integrate the corresponding signal processing kernel. The output of such integrated circuit boards is a digital signal conforming to certain format requirements, which can be directly received by the central control unit.
To effectively amplify the useful differential signal output by the sensor, the signal amplification circuit in the signal conditioning module should have strong common-mode rejection and differential amplification capabilities. Simultaneously, the filter in the signal conditioning module should employ a high-order active filter composed of a precision operational amplifier with a non-inverting structure and an RC network. The signal conditioning module should also be low-power, capable of operating under a single power supply, and its signal amplification range should match the signal amplitude required for A/D conversion.
The central control unit can use a new type of 16-bit microcontroller, which has a high-precision multi-channel A/D function module, a large-capacity FLASH ROM and data RAM on-chip, and can realize the acquisition, reception and processing of physiological signals without the need for an external A/D chip.
Data storage modules can select different types of data storage chips according to different storage capacity requirements.
The display and keyboard interface module provides a keyboard interface for settings and the operating system, and uses a graphic dot-matrix LCD to display Chinese character menus, display physiological parameters, and play back waveforms, providing the system with an intelligent human-computer interaction interface.
The clock module provides the system with real-time time coordinates.
The data communication module provides a means of data exchange between this system and the PC. It can be a wired interface such as serial, USB, or TCP/IP network communication; or it can build a wireless communication network at a fixed frequency (such as 433MHz) through a wireless transceiver chip.
The power module supplies power to the analog and digital circuits in the system separately, providing different operating voltages and certain power zoning management functions. Its output quality is directly related to the accuracy and reliability of the system.
Portable medical monitoring systems require low current consumption for field usability, in order to reduce system power consumption and extend battery life. Therefore, low-power design is an important aspect of the overall system design. The core of low-power design is the design of a system with minimal power consumption, which not only reduces system power consumption but also enables the system to have lower electromagnetic radiation and higher reliability.
microcontroller
TI's MSP430F149 is a 16-bit microcontroller with ultra-low power consumption, reaching the microampere level.
The F149's software architecture is also designed for low power consumption, requiring only 6μS to wake the MCU from standby mode. Interrupts and subroutine calls have no hierarchical restrictions; this rich interrupt capability reduces the need for system polling and facilitates the design of interrupt-based data acquisition and storage programs.
Using the MSP430F149 as the central control unit of the portable medical monitoring system enhances the system's sophistication, reliability, and integration.
Data storage
In portable medical monitoring systems, where data storage capacity requirements are not high (within tens of kilobytes), data can be stored in the FLASH memory within the MCU through certain software design techniques. The F149 has 60KB of FLASH ROM, consisting of two information memory segments and 118 main memory segments. Besides the memory segment storing the control program, the physiological parameter data collected by the system can also be sequentially stored segment by segment in other available memory segments. This saves on dedicated data storage chips, allowing a single 16-bit microcontroller—the F149 chip—to replace the conventional combination of "microcontroller + data storage chip + A/D chip" in portable monitoring systems, thus reducing costs and effectively improving system integration.
For data storage capacities ranging from several hundred kilobytes to 1 megabyte, the AT24LC series data storage chips with an I2C interface can be used. In particular, the storage capacity can be significantly increased by cascading several AT24LC series chips.
For monitoring systems that require real-time storage of multi-channel physiological parameter data, ATMEL's AT45 series SPI serial interface low-voltage FLASH memory can be used.
For further data storage capacity requirements (above 1GB), you can consider the parallel high-capacity FLASH storage chip commonly used in USB flash drives. Its disadvantage is that it occupies too much MCU I/O resources and has slightly higher power consumption.
Data collection
The F149's A/D conversion module ADC12 has four conversion modes: single-channel single-transformation, single-channel repetitive transformation, sequential channel single-transformation, and sequential channel repetitive transformation. In monitoring systems, it is often necessary to acquire multi-channel, continuously changing data. Therefore, the sequential channel single-transformation mode offers simpler timing control and higher flexibility.
When using the sequence channel single-conversion mode, the SHP bit of the control register ADC12CTL1 should be set to 1, and the SHS bit (sampling input signal source selection control bit) should be set to 1, meaning the sampling timer is triggered by Timer_A.OUT1 in F149. Timer A can operate in increment mode, and its timing interval is the sampling time interval of the A/D conversion. The advantages of using the timer output signal to trigger sampling are: precise control of the sampling time interval; and because hardware timing is used, the MCU can enter a sleep state within the timing interval and be woken up by an interrupt, which fully complies with the interrupt-based low-power programming principle.
The following issues should be noted during the data collection process:
• To avoid potential frame drops, it's crucial to ensure proper timing coordination between data acquisition and data storage. The solution is to allocate two buffers within the F149's data RAM, each the same size as a page in the FLASH memory. By switching between these two buffers and utilizing the FLASH memory's batch page write function, sampled data can be stored promptly.
• Ensure a stable clock source during data acquisition. It is important to ensure the availability of the corresponding clock signal when the F149 enters low-power sleep mode; otherwise, it may cause serious errors or even misinterpretations in the sampling time interval, which are often difficult to detect.
Since many physiological signals in the human body are low-frequency signals, and many physiological parameters change slowly, high-frequency real-time data acquisition is often unnecessary. For slowly changing physiological parameters such as body temperature, data can be collected and recorded once per minute. Furthermore, employing a scalable storage method in the software can significantly save data storage space. This involves continuously acquiring physiological signals at a constant sampling rate and storing them in the F149's data buffer, but using relatively long storage intervals for identified continuous normal physiological signals, while continuously recording identified abnormal physiological signals in real time (i.e., storing only the amount of data collected). This effectively saves data storage space and reduces average power consumption.
Power module
Since portable medical monitoring systems often contain both analog and digital circuits, the power module must supply power to both analog and digital circuit sections simultaneously.
Portable monitoring systems are typically powered by battery packs consisting of alkaline or rechargeable batteries.
Figure 2 is a structural diagram of the power module of the portable monitoring system.
The system employs a power supply structure where the central control unit, analog circuits, and digital circuits are each powered separately. Within the system, the MCU controls the switching on and off of the DC-DC boost converter circuit to manage the power supply to the analog circuits; the MCU also controls the enable pin of the 3V low-dropout regulator circuit to control the power supply to the digital circuitry.
Conclusion
The powerful MSP430F149 chip provides a low-power, high-performance, and low-cost microcontroller option for portable medical monitoring systems. This paper, combined with the overall design concept of the monitoring system and the principles of low power consumption and reliability, discusses in detail the design ideas and specific implementation methods of the data acquisition and storage section capable of recording human physiological parameters for extended periods, as well as the power supply module capable of providing high-quality power output. These design solutions have achieved excellent results in practical applications.
