Designers of portable medical devices are facing some special challenges. The areas they are involved in are well known for their strict regulatory review, long-lasting design and life cycle, and unusual requirements for the robustness of the final product. In addition, some design goals that are common to all electronic devices have special significance once they involve medical devices. For example, low power consumption has always been the goal of designers of portable electronic devices. Lower power consumption means that the battery can become smaller and lighter, which can improve the portability of the product. For medical devices, the improvement of portability is of great significance to the quality of life of patients. Even the patient's life may directly depend on battery life. This article will explain how designers can use microcontrollers (MCUs) to reduce the power requirements of medical devices.
Voltage and battery life
In low-power applications, the static power consumption of the MCU is an important quality factor. Some MCUs with advanced processing technology may consume less than 50nA in sleep mode. In order to be suitable for various low-power designs, it is important that the MCU can work in a wide voltage range. For example, when using alkaline batteries, the operating voltage of 1.8V is usually specified, because the final voltage of each battery is 0.9V, and two batteries are usually used in the application. Choosing an MCU that can operate over a wide voltage range can extend the working life of portable devices. However, the operating voltage range of the MCU is not the only determinant. The operating voltage range of the entire system must be considered, including peripherals on the MCU. If a single peripheral in the system needs to consume most of the energy consumption, then only reducing the power consumption of the MCU has little effect on the total power consumption of the system.
Ways to reduce power consumption
1. Peripheral power switch
The basic principle of portable embedded system power management is to allow the MCU to control the power consumption of internal and external peripherals. When designing a portable medical device, first determine the necessary physical mode or state, and then decompose the design to close unnecessary circuits. Choosing the right MCU from many different suppliers can help you remove external components and reduce costs. As mentioned earlier, MCUs that can operate over a wide voltage range can enhance the functionality of system design.
Figure 1: In this medical data logger application, the I / O pins of the MCU can be used to power EEPROM and sensors.
Let's take an MCU-based data recording medical monitor as an example to illustrate how to minimize the power consumption of the entire system. This monitor contains sensors, EEPROM and batteries (see Figure 1). In practical applications, the sensor can measure temperature, oxygen saturation, blood pressure, blood glucose concentration, or many other quantities. The medical device will be used to monitor the patient's condition for a few hours or longer. In this example, the MCU takes sensor readings every 2 seconds, converts the sensor data, stores the data in an external EEPROM memory, and then waits for the next sensor reading. If you do not need to consider power consumption, you can always supply power to the EEPROM, sensor and its bias circuit. However, since it is a portable medical device, it is very important to use available power efficiently. So, what can be done to reduce the power consumption of such systems? The solution is to let the MCU shut down these peripherals through program control when they are not needed. As shown in Figure 1, designers can use the MCU's I / O pins and some code bytes to power the EEPROM and sensors when needed. Because the I / O pins of the selected MCU can provide up to 20mA, no additional components are required to switch the power supply.
2. MCU power management mode
A common way to save power in embedded applications is to periodically put the MCU into sleep mode when the system's resource requirements for the MCU are low. In our example, the system takes measurements every 2 seconds. If you actually need 11 ms to measure and store the results, the MCU can sleep 1989 ms between measurements. The longer the MCU is allowed to sleep, the lower the average power consumption consumed by the application. The system's MCU wakes up through an interrupt or through a watchdog timer timeout event. It is important to ensure that the application has the appropriate watchdog timeout duration. Generally, it works as follows: If the application requires the MCU to process data samples every fixed period of time, then the watchdog timer should wake up the MCU once in the required time period. When using this function, you need to select the MCU that supports the corresponding watchdog cycle.
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