Design of a Low-Power Medical Data Recorder Using the MAXQ2010

Many medical applications require portable, self-powered devices that do not require external power and data cables. The most obvious example is a portable data logger that patients carry with them to measure heart rate, body temperature, and other health indicators.

Of course, there are many complex applications that require a small battery device for safety redundancy and equipment monitoring even if powered by an external power supply, such as hospital wards, patient rooms, environmentally controlled laboratories or storage equipment. Environmental parameters (including temperature and humidity) need to be continuously monitored; in addition, portable devices are more convenient and flexible to install and use than devices that require external power and network cables. In some cases, such as medical equipment that patients need to carry with them, temperature detection equipment in cold storage, etc., it is impossible to connect external power cables and network cables.

What functions do portable medical devices require? First, they must have their own power supply. Usually, they can be powered by rechargeable or non-rechargeable batteries, although there are other ways (such as solar power), but this depends on the voltage and current requirements. Regardless of the power supply used, the efficiency of the power supply must be high enough, and battery-powered portable devices should be able to enter "sleep" mode to reduce power consumption as much as possible when they do not need to work at full load. The sleeping device can be "woke up" by an external trigger signal or periodically, and then increase the computing speed (of course, power consumption will also increase) to enter the normal working mode. The device should also have some working modes between full-load working and "sleep" modes to perform some simple tasks (such as accessing memory or refreshing LCD and LED display data), because the device usually only needs full-load computing power under certain conditions (such as filtering and decoding sensor data), so that a certain degree of balance can be made between power consumption and computing speed.

Even if a portable device supports wireless communication, it is not always guaranteed to be able to access the wireless network. Depending on the network conditions, the device may be working in an environment with a wireless network at one moment, and may be moved to an environment without a wireless network at the next moment, or the wireless network may be temporarily shut down due to a power outage. In these cases, if the device itself does not support wireless communication, the device needs to store the data collected at any time for future upload to the upper-level system for data processing. Some critical data (such as environmental safety failure data, configuration data or device drivers) must be stored securely and cannot be lost even if the battery fails or is removed.

Other features of portable devices depend on the specific application requirements. Data can be collected directly through analog sensors or read through a local area network access subsystem. Portable devices can only passively collect data or actively alert someone by sounding an alarm or sending a signal under certain conditions. Some simple data collection devices do not require user intervention before uploading data, while other devices (such as handheld blood glucose meters or wristband heart monitors) may require additional input and output devices rather than the host system to change configuration or view data.

Designing a Portable Data Logger Using the MAXQ2010

Although there are many microcontrollers to choose from in the industry, the features of Maxim's MAXQ series low-power mixed-signal RISC microcontroller MAXQ2010 are very suitable for designing battery-powered data acquisition devices. MAXQ2010 has extremely low power consumption, extremely high MIPS/mA ratio, and only requires a small battery current to support portable applications. The integrated 12-bit 8-channel ADC can collect many types of sensor data. In addition, it supports many types of local serial interfaces (such as I2C, SPI, synchronous/asynchronous UART) that can be used to access the host system and serial non-volatile storage devices, or communicate with other subsystems in this device.

MAXQ2010 can change power consumption by dynamically adjusting the clock frequency according to the computing power requirements of the current task, and when it has processed all data and events, the portable device can enter the lowest power sleep (stop) mode until it is awakened by the application again. The core voltage of MAXQ2010 is only 1.8V, which can greatly reduce power consumption. The 3V independently powered I/O can communicate with external high-voltage logic. If you want to use a single power supply such as a 3V button lithium battery instead of dual power supplies, you can use the built-in integrated regulator to power the core voltage. In stop mode, the regulator can be turned off to reduce power consumption.

The MAXQ2010 can read data from sensors in a variety of ways. If you collect analog sensor data, you can use the built-in 12-bit multi-channel ADC, which supports 8-channel single-ended input. The data collected by the MAXQ2010 from external sensors can be stored in RAM powered by a backup battery or in internal flash as needed. The on-chip 32kHz real-time clock (RTC) can also work in stop mode to provide time stamps for data as needed. If the user needs to enter data or display information to the user, the MAXQ2010 can achieve it. It has a set of general-purpose input/output pins (56 in the largest package) that can drive LEDs, read mechanical switch settings, or connect to the switch matrix through row and column scanning. The MAXQ2010 also has an LCD controller that can directly drive a 3V segment LCD, supporting up to 1/4 cycle multiplexing (COM1~COM4). Its largest package provides 40 dedicated drive pins, which can drive 160-segment LCD displays in 4x multiplexing mode.

Design Example of Data Recorder Based on MAXQ2010

Like many electronic devices used to collect or store data, the MAXQ2010-based data logger uses a USB interface to communicate with a host (such as a personal computer). However, since the MAXQ2010 itself does not have a USB interface, we use the FTDI chip FT232R to implement the USB-UART conversion.

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Using the FT232R brings many benefits to the data logger design. First, when the USB bus is active, the data logger can be powered by the FT232R's 3.3V regulator output. Only a pair of diodes are needed to achieve automatic switching from battery power. Because the regulator output voltage (minus the 0.2V diode forward voltage drop) is always higher than the battery voltage minus the diode voltage drop, the logger is powered by the USB Vbus instead of the battery when connected to the USB bus. The two diodes (Figure 1) are used to prevent charging the battery, and the output capacitor is used to reduce the effect of load transients on the battery. Second, the MAXQ2010 can communicate directly with the application running on the PC using one of the two serial ports (UART) without any additional drivers. The two serial ports are connected through a virtual COM port established on the USB interface. This design uses the MAXQ2010's 32kHz crystal-based FLL as its own clock source (and can also provide the time base for the RTC if necessary), which is much cheaper than other crystal or resonant circuits. The FLL circuit is equivalent to a frequency multiplier with a multiplication factor of 256, which changes the 32kHz crystal oscillation frequency to 8.388MHz as the clock of the MAXQ2010.

Figure 1 3 using FT232R

To calculate how much current a MAXQ2010-based data logger consumes, consider the following: First, an external signal (such as a key press or a sudden increase in sensor voltage) wakes up the microcontroller from stop mode; the system then reads the analog sensor voltage through a single-ended ADC channel and stores the acquired sensor voltage value in data RAM; at this time, in order to save power, the microcontroller returns to stop mode, and after about 60 seconds, the microcontroller wakes up again (return to step 1). Therefore, to calculate the average current consumption and estimate the battery life, the following parameters of the microcontroller need to be substituted into formula (1): tActive (the time required to complete all the above operations, including the time to enter stop mode), iActive (the typical current value during the above operations), tStop (the time to stay in stop mode), iStop (the typical current in stop mode), tExit (the time required to wake up from stop mode), iExit (the typical current when waking up).

(tActive × iActive) + (tStop × iStop) + (tExit + iExit)

tActive + tStop + tExit

Based on the above parameter values, the average current can be calculated to be about 202nA; that is, if the power source is a common CR2032 button lithium battery, the battery life can be estimated to be 1138 hours. The characteristics of batteries produced by different battery manufacturers will vary. The voltage drop of CR2032 batteries in the 90% discharge range does not exceed 0.3V, which means that before the battery voltage drops to 2.7V (2.5V after a diode voltage drop, which meets the minimum voltage when working with a single power supply), the microcontroller can work for 1024 hours.

增加电池容量或数量、用可充电电池，或当连接到USB时自动充电等许多措施均以用来延长电池寿命。一般平均电流仅略高于停止模式的待机电流，这是因为停止模式的时间远长于程序运行时间，停止模式的电流起主导作用。程序循环体代码可以被扩展，如测量多个传感器值或增加其它功能并不会显著改变电池寿命。当然，使用其它外设功能，如LCD显示，LED指示或串口等都会增加功耗，设计者在计算实际电池寿命时需要综合考虑这些功能可能增加的功耗。