Portable medical equipment MCU selection essential book

November 05, 2020

< p> introduction

The rapid development of electronic technology and the gradual improvement of personal health awareness have made human demand for portable medical electronic instruments increasingly growing. Biometrics and medical instruments are becoming one of the pillar industries of the global economy in the 21st century. On the contrary, with the advancement and popularization of various portable biomedical instruments, the improvement of measurement accuracy, power consumption, computing power, cost and integration has put forward higher requirements for electronic devices, especially microcontrollers (MCUs).

Driven by the gradual opening up of the home medical market, the needs of remote local residents and military field training, the need for clinical diagnosis within the hospital, and the need for long-term monitoring, some traditional professional-grade medical devices have gradually become portable. The rapid development of portable medical electronic instruments has made mobile medical, telemedicine, personal daily health monitoring and treatment possible. The trend of portability puts low power consumption, high integration and ease of use in the design of medical electronic instruments. In addition, the intelligent design of the instrument and the complexity of the algorithm make the core processor must have higher computing power.

As the core controller and processing center of instrumentation, the reasonable selection of MCU is the first step in instrument design. Semiconductor chip companies represented by TI, Freescale, ST, Silicon Labs, Microchip and Renesas have launched their own low-power, high-integration mixed-signal processors for the portable medical electronics market. When all kinds of MCU manufacturers claim that their products have the lowest power consumption, rich peripherals and superior performance, designers need to combine the existing needs of their own research and products with the upgrade plan for the next few years to make a reasonable analysis, so as to choose the appropriate MCU. Perform instrument design.

1 Introduction to Portable Medical Electronic Instruments

1.1 Household and clinical portable medical instruments

The competition for home medical electronic products represented by electronic thermometers, blood glucose meters, digital sphygmomanometers, and low-frequency physiotherapy devices is increasingly fierce, and the market demand can be greatly increased. Such instrument design is very cost sensitive. The MCU with integrated peripherals on the chip can effectively reduce the system cost, and also help to reduce the size and improve the stability. The performance of the MCU is not high, and the traditional 8-bit or 16-bit core is basically Meet the design needs.

In addition, the clinical portable medical equipment represented by the dynamic electrocardiograph and the dynamic blood pressure monitor has the characteristics of high measurement accuracy, long continuous operation time, relatively complicated operation and diverse communication functions. For such instrument design, mixed MCUs with high analog performance, high computational efficiency, and integrated peripherals will play a huge role.

1.2 Wearable medical electronic equipment

In recent years, various types of wearable medical electronic devices have emerged in an endless stream, and wearable features have put forward higher requirements for instrument portability and low power consumption design. On the one hand, reduce the battery size, choose a more energy-efficient MCU: reduce the effective data volume by increasing the computing power and algorithm complexity, thereby reducing the load pressure of data storage and communication, and ultimately reducing power consumption; on the other hand, pay attention to the on-chip The rich and superior MCU family can reduce the size of the equipment and improve the stability of the whole machine and reduce the system cost.

2 Typical MCU selection analysis

Mainly around the two characteristics of low power consumption and high integration of portable medical electronic instruments, the MCU selection is analyzed.

2.1 Low Power MCU Family

With the improvement of semiconductor technology and integrated circuit design level, major semiconductor manufacturers have launched their own low-power series MCU products, as shown in Table 1. Low-power MCUs often have some common features: a streamlined and efficient CPU core that maintains a balance of performance, power, and cost; CMOS circuit technology, low-voltage power supply systems; and flexible, low-power management modes. The simple and fast sleep wake-up mechanism allows the MCU to quickly switch to different depths of sleep during idle periods and wake up in time; independent peripheral clock control switches, multiple internal and external clock source selections; rich energy-saving analog and digital Set up to select specific integrated peripherals and corresponding amounts of on-chip memory for your specific application.

Major low-power MCU families and their manufacturers

Table 1 main low-power MCU family and its manufacturers

Tables 2 and 3 select mainstream low-power MCU products on the market today, comparing their power consumption parameters with system specifications. It can be seen that the 32-bit MCU of ARM's high-efficiency and low-cost Cortex-M series core has the same power consumption level as the traditional 8-bit, 16-bit low-power MCU, and even specializes in low power consumption. The energy-oriented EFM32 series of MCUs in the field have exceeded the MSP430 family of products at different main frequencies, and their power consumption in the 32-bit MCU field is also quite obvious. Secondly, Freescale launched the world's first KineTIs L (KL) series of microcontrollers based on the ARM Cortex-M0+ core in March 2012. The 32-bit entry-level KL series MCUs combine excellent energy efficiency and ease of use. A combination of rich peripherals, the overall power consumption level is comparable to various 8-bit, 16-bit low-power MCUs, while the 32-bit core's ultra-high computational efficiency, high-performance peripherals (such as the industry's unique 16-bit SAR ADC, The price advantage of the highest rate of about 0.5 Msps) and (0.5~2) USD per tablet will have a strong impact on the 8-bit and 16-bit MCU market. In addition, ST's STM32L series of low-power MCUs lags behind the above two 32-bit MCUs in power consumption, but since it launched the world's first Cortex-M core 32-bit MCU as early as June 2007, Subsequently, in May 2010, the industry's first ultra-low-power ARM Cortex-M3 microcontroller was released. The leading market promotion has led to the STM32's market share. STM32-based system solutions, hardware and software finished modules, and chip distribution channels have been Form a relatively complete ecosystem. Compared with other 32-bit low-power MCUs, the three have a small gap in computing performance, power consumption and peripheral integration. The biggest advantage of using STM32L is the low development threshold, rich reference resources and direct experience sharing.

Absolute power consumption comparison of typical low-power microprocessors

Table 2 Comparison of CPU absolute power consumption of a typical low-power microprocessor

Typical Low Power Microprocessor System Indicator Comparison

Table 3 Comparison of typical low-power microprocessor system indicators

TI's latest MSP430FR series MCU, code-named "Wolverine", integrates ferroelectric memory into the chip, replacing the original Flash as a program memory, improving memory read and write speed while significantly reducing power consumption. From the comparison of the data in Table 1, the "Wolverine" series has injected new vitality into the MSP430 ultra-low-power MCU product line, leading the major low-power MCU series in the energy consumption level. However, under the pressure of the low-cost, high-performance 32-bit ultra-low-power MCU cluster, the power consumption parameters have gradually changed, and the weak power consumption advantage has become possible for most portable instrument designs. Neglected minor contradictions. For portable medical electronic instruments with increasingly high tasks and algorithms, 32-bit MCUs with higher computational efficiency and frequency can complete work faster in the same time, thus gaining more time for sleep and reducing power consumption from the system. And make tasks and events get a faster response.

For some instrument designs that require extremely high power requirements, simple processing tasks, and cost-sensitive instrumentation, traditional 8-bit low-power MCUs are the best choice.

2.2 Integrated peripherals and their performance analysis

The development of semiconductor technology has promoted the system integration to become higher and higher, which has greatly improved the size, power consumption, cost and stability of hardware systems, and the design of hardware systems has become more and more simple. For portable medical electronic instruments, the main object is to face the human body. Reducing the volume of the instrument is a direct way to improve portability. The type, quantity and performance of on-chip peripherals have become the key reference factors for determining the final MCU selection. As shown in Figure 1, the peripherals that are typically integrated with low-power MCUs and their main functions are listed.

MCU integrated peripherals and their main functions

Figure 1 MCU integrated peripherals and their main functions

The main characteristics of physiological parameters are: many types, and there are many correlations between them; the signal spectrum is concentrated in the range of several tens of kHz; the signal is weak and the range of variation is large; the sudden and irregular nature of the characteristic variation is strong. Therefore, the main purpose of signal measurement for portable medical electronic instruments, the integration of MCU analog peripherals and their performance, deserves high attention. As shown in Table 4, firstly, the maximum number of ADCs in the EFM32 full range of ADCs is 8. For the traditional 12-lead Holter recorder design, there will be no room for expansion. Secondly, the KL series' unique 16-bit successive approximation ADC can save the first-level signal amplification in the measurement system, thus reducing the analog circuit scale, reducing the system cost and improving the stability and integration. Under the same conditions, the accuracy can be Increased by a factor of 16 and the dynamic range is expanded by 25 dB.

Because some instruments usually need to measure multi-parameter signals synchronously, the increase of data volume and recording time will cause frequent operation of the control module and communication interface. Table 4 shows that some models of STM32L contain SDIO interface, which is designed with SD card recording function. When the instrument is used, it can improve the reliability and speed of SD card reading and writing. In addition, in addition to considering whether their number, rate of rate and other indicators meet the design requirements, you must pay attention to the unit power consumption of each part. Table 5 illustrates two dual-power and 32-bit low-power MCUs with similar integration and functionality from Energy Micro and ST. It can be seen that the impact of peripheral power consumption on the overall power consumption of the system cannot be ignored.

                                

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