Application of MSP430F449 in miniaturized low-power data acquisition module

Introduction

With the development of low-power microcontroller technology, various applications have more stringent requirements for microcontroller systems. The MSP430 series microcontrollers have low operating voltage, ultra-low power consumption, powerful processing capabilities, stable system operation, integrated a large number of peripheral devices, rich on-chip peripherals and convenient and efficient development environment, etc., and are widely used in battery-powered miniaturized low-power devices and instruments. In particular, most of the MSP430 series microcontrollers have built-in analog-to-digital converter modules, and the conversion accuracy ranges from 10 bits, 12 bits and 14 bits, so the MSP430 series is very convenient to use in low-power data acquisition and other needs.

MSP4.30F449 is the most advanced model in the MSP430 series. Main features: can work at a low voltage of 1.8 to 3.6V; in low power mode, the CPU can be awakened by interrupts, and the response time is less than 6μs; 12-bit A/D converter with internal reference source, sampling, holding, and automatic scanning features; FLASH memory up to 60kB; convenient and efficient development environment, etc.

2 Design

The low-power data acquisition module mainly completes the data acquisition and data storage functions in the miniaturized system. On-site use requires that its current consumption is as small as possible to reduce the power consumption of the system and extend the battery power supply time. This paper designs a miniaturized data acquisition module based on the low-power microcontroller MSP430F449 . The entire system consists of MSP430F449, sensors, signal conditioning, serial communication and other circuits, and is powered by ±3V batteries, as shown in Figure 1.

2.1 Signal conditioning circuit

Since the input charge signal of the sensor is very small and there is a 50Hz AC coupled to the input terminal through stray capacitance, a relatively large feedback capacitor is selected during the design to enhance the anti-interference ability of the sensor. At the same time, according to the amplitude of the measured signal, an amplifier circuit is designed, and the AD620 chip is selected. It has the characteristics of low noise, low input bias current and low power consumption, and is ideal for accurate data acquisition systems (such as sensor interfaces). The extremely wide power supply operating range: ±2.3~±18V meets the battery power supply requirements. Only one external resistor is required to obtain any gain range within 1~1000, that is, G=49.4kΩ/RG+1. The amplified signal is then anti-aliased filtered.

Since the above-mentioned circuits are all working under zero bias conditions, the output signal amplitude is positive and negative, and the signal before entering the ADC must be single-ended, so it is necessary to shift the double-ended signal to make it a single-ended signal. The signal shift circuit is shown in Figure 2. Because the battery continuously consumes energy during use, the application of the low-dropout regulator TPS76318 can stabilize the voltage at point M in Figure 2 at 1.8V (the voltage provided by the battery is not less than 2.7V), thereby improving the accuracy of the system.

The conditioned signal is directly connected to the A/D input of the MSP430F449 chip to achieve A/D conversion.

2.2 Data collection and conversion

MSP430F449 has a built-in analog-to-digital converter module with a conversion accuracy of 12 bits, which converts analog signals into 12-bit data and stores them in the conversion storage register. Main features:

(1) Fast sampling speed, up to 200KSPS

(2) 12-bit conversion accuracy, 1-bit nonlinear differential error, 1-bit nonlinear integral error;

(3) There are multiple clock sources available for the ADC12 module, and the module itself has a built-in clock generator;

(4) Equipped with 8 external channels and 4 internal channels;

(5) Built-in reference power supply, and the reference voltage has 6 programmable combinations;

(6) There are 4 modes for analog-to-digital conversion, which can be used flexibly to save software and time. [page]

The external analog signal enters the A0 channel for analog-to-digital conversion after amplification, filtering and shifting, as shown in Figure 3. The conversion mode is single-channel multiple conversion mode.

The system performs multiple conversions on the selected channel A0 until it is stopped by software. Each time the conversion is completed, the conversion result is stored in the corresponding ADC12MEM0, and the corresponding interrupt flag ADC12IFG.0 is set to mark the end of the conversion. The ADC core completes the conversion of the analog signal into 12-bit data and stores it in the conversion storage register. The core uses two reference levels, VR+ and VR-, as the upper and lower limits of the conversion range and the full-scale value and "0" value of the reading. The reference level is defined by the conversion storage controller. The design is defined as VR+=2.5V, VR-=0V. The conversion formula is:

NADC=4095×(VIN-VR-)／(VR+VR-)

The sampling process is shown in Figure 4.

The MSP430F449 microcontroller has an on-chip FLASH memory that can store programs or important data, information, and other data that will not be lost after power failure. By programming the FLASH, the A/D converted data can be written into the FLASH memory.

2.3 Serial communication

The microcontroller mainly completes real-time data acquisition. The collected data is initially processed and transmitted to the host through the asynchronous serial communication interface. The design uses the MAX232 chip to realize the mutual conversion between the 3-5V level and the serial port level, as shown in Figure 3. Asynchronous serial communication is to automatically realize the data shifting in and out by hardware after a series of register settings to complete the communication function. The baud rate is set to 9600b/s.

The PC receives data from the serial port through a serial communication control in the VB program, and the VB program displays the data on the control interface. After processing, the data waveform is obtained, as shown in Figure 5, and is used for research and analysis.

3 System Debugging

As an important part of the data acquisition system, the miniaturization and low power consumption design of the low power data acquisition module is a systematic overall project. After the successful circuit design and debugging, the test shows that the data acquisition module with MSP430F449 as the core can well capture the output signal of the sensor and meet the design requirements. The system is easy to use, stable in operation, and the measured data is real-time, which has been affirmed by relevant technicians.