Design and implementation of digital barometer based on 51 single chip microcomputer

1. Introduction

A barometer is a device that uses a pressure-sensitive element to directly convert the measured air pressure into a current or voltage signal that is easy to detect and transmit, and then processes it through subsequent circuits and displays it in real time. The core component is the air pressure sensor, which plays an important role in monitoring pressure, controlling pressure changes, and measuring physical parameters. The air pressure sensors used in barometers basically rely on the changes in air pressure at different altitudes to obtain air pressure values.

Meteorological research shows that in the vertical direction, air pressure decreases as altitude increases. For example, in the lower layers, the air pressure decreases by 10 hPa for every 100 m rise; at an altitude of 5 to 6 km, the air pressure decreases by 7 hPa for every 100 m increase in altitude; and when the altitude increases further, that is, after reaching an altitude of 9 to 10 km, the air pressure decreases by 5 hPa for every 100 m increase in altitude; similarly, if there is a downdraft in the air, the air pressure will increase; if there is an updraft in the air, the air pressure acting on the bottom of the air column will decrease. The weight of the air column acting on a unit area is generally called atmospheric pressure.

2 The structure of a barometer

The structure of the barometer studied in this paper is shown in Figure 1. The pressure sensor is used to convert the measured air pressure into a voltage signal; the V/F converter can convert the voltage signal output by the pressure sensor into a pulse signal with a certain frequency; so that the pulse signal can be received by the single-chip microcomputer, and the corresponding air pressure value can be calculated according to the linear relationship between voltage and frequency based on the number of pulses obtained per unit time, and finally displayed by the LED under the control of the single-chip microcomputer.

This barometer can accurately measure the corresponding air pressure value within the linear range of the air pressure sensor. It should be noted that its measured value is the absolute air pressure value. The technical indicators of the barometer studied in this article are as follows:

●Measurement range: 300hPa~1050hPa;

●Measurement accuracy: 0.1% FS (20℃);

●Display accuracy: 0.1%, achieved by 4 8-segment LED displays;

●Working temperature range: 0～85℃;

●Power supply voltage: 9V.

3 System implementation

In the process of system construction, factors such as stability, complexity, cost, and ease of debugging need to be considered. Each part in the block diagram shown in Figure 1 is a unit circuit that can complete its own function. There is no complex signal transmission between modules, and there is little interference, so the overall system is relatively stable.

3.1 Air pressure sensor

The pressure sensor occupies a core position in the barometer. When designing, the pressure sensor can be selected based on several performance indicators such as measurement accuracy, measurement range, temperature compensation, and measurement of absolute pressure value.

Since the barometer displays the absolute pressure value, it is necessary to select a pressure sensor that measures the absolute pressure value. At the same time, in order to simplify the circuit and improve stability and anti-interference ability, the pressure sensor is required to have temperature compensation.

For this purpose, the author selected Motorola's MAX4100A pressure sensor to measure the absolute pressure value. The temperature compensation range of this sensor is -40~+125℃; the pressure range is 20kPa~1050kPa; the output voltage signal (Vs=5.0V) range is 0.3~4.65V; the measurement accuracy is 0.1%VFSS, and it has good linearity at 20kPa~1050kPa. The specific output relationship is as follows:

Ｖｏｕｔ＝Ｖｓ(０．０１０５９ Ｐ－０．１５２８)±Ｅｒｒｏｒ

Where Vs is the operating voltage, P is the atmospheric pressure, and Vout is the output voltage.

3.2 V/F conversion

The function of V/F device is to convert the amplitude of input voltage into a pulse train whose frequency is proportional to the amplitude of input voltage. Although V/F itself cannot be regarded as a quantizer, it can also realize A/D conversion after adding timer and counter. Its outstanding feature is that it converts analog voltage into a pulse train with strong anti-interference ability, which can be transmitted over long distances and directly input into the computer, thereby realizing A/D conversion function by measuring the output frequency of V/F.

Considering the difficulty of peripheral circuit implementation and the corresponding performance indicators, the author selected the LM331 voltage/frequency conversion chip. The device uses a temperature-compensated bandgap reference circuit, so it has excellent temperature stability, with a maximum temperature drift of 50ppm/℃. At the same time, the pulse output of the device is compatible with any logic form; LM331 can be powered by single or dual power supplies, with a voltage range of 5~40V; full-scale range of 1Hz~100kHz; maximum nonlinear error of 0.01%. Figure 2 shows the peripheral circuit of LM331 in this system. In this circuit, the voltage-frequency conversion relationship based on LM331 is:

ｆｏ＝ＫＶｉ

Where, K = Rs/(2.09 Rt Ct RL)?, Rs = Rs1 + Rs2

In fact, Rs in the circuit is mainly used to adjust the conversion gain of the circuit. The typical values ​​of Rt, Ct, and RL are 6.8kΩ, 0.01pF, and 100kΩ, respectively, and the K value can be determined by the designer. In this design, K=2000, Rs=28.424kΩ, mainly considering that the single-chip microcomputer part uses the frequency measurement method to measure fo to ensure the measurement accuracy of the frequency signal. Since Rs, RL, Rt and capacitor Ct will directly affect the conversion result of fo. Therefore, there are certain requirements for the parameters of these components, and they should be appropriately selected according to the conversion accuracy during design. Although capacitor CL has no direct effect on the conversion result, a capacitor with small leakage current should be selected. Using resistor R1 and capacitor C1 to form a low-pass filter can reduce interference pulses in the input voltage and improve conversion accuracy.

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Figure 3

3.3 Microcontroller

This barometer implementation requires the use of the microcontroller's P1 port and part of the P3 port, as well as an interrupt source, a timer and a counter. Therefore, the author selected ATMEL's AT89C2051 microcontroller, which is compatible with the 89C51, has 2kB of reprogrammable flash memory, a 2.7V to 6V operating voltage range, 128Byte internal RAM, two I/O ports (P1, P3), two 16-bit counters/timers and six interrupt sources, and can directly drive LED outputs, and has a programmable serial communication port. In addition, the microcontroller is also small in size and low in price.

3.4 LED Display

A single LED is a display unit composed of 7 segments of light-emitting diodes. There are 10 pins, corresponding to 7 segments, a decimal point and two common terminals. In the display circuit, these light-emitting diodes have two ways of connection: common anode connection and common cathode connection. In this design, 4 LEDs are needed to form a display unit, and a dynamic display method is adopted. Since the connection of 4 single LEDs for display is relatively complicated, and the port driving capability of the single-chip microcomputer is difficult to guarantee, a special driver chip needs to be added. Therefore, the author uses a common anode LED with 4 LEDs connected together and the corresponding segments are connected inside. It has 12 pins, including 7 segments and 4 common terminals. In order to increase the brightness of the digital tube, a transistor drive circuit can be added to the bit selection line.

The display circuit controlled by AT89C2051 is shown in Figure 3. The display circuit needs to select appropriate resistors R and Ra to ensure the brightness of the LED. Too large or too small will not allow the LED to display normally. It is ideal to take R as 4.7kΩ and Ra as 510Ω during design. If the convenience of printed circuit board wiring is considered, chip resistors and resistor arrays can be used to save space. In addition, 74LS244 and 74LS06 can also be used to form a driving display circuit, but current limiting resistors must also be added. Because 74LS06 is an open-drain device, a pull-up resistor needs to be added at the output.

4 Software Implementation

Through the above design, the size of P can be calculated by fo to obtain the real-time air pressure value. After the hardware circuit design is completed, it can be simulated using the simulation environment of the AEDK5196PH simulator, and the processing program can be written in C51 language. The basic program flow is shown in Figure 4.

Program setting: T0 is a timer, and the basic timing time base is 50ms. T1 is a counter. Using internal interrupt 0 can ensure that the value of the counter is read after T0 is set to 500ms to calculate the pressure value. If T1 and T0 are both working in mode 1, and the font code is sent at port P1, and P3.0~P3.3 can be used as bit selection lines, then the corresponding function is as follows:

(1) Timer T0 interrupt function:

ｖｏｉｄ ｔｉｍｅｒ０(ｖｏｉｄ) ｉｎｔｅｒｒｕｐｔ １ ｕｓｉｎｇ １

{ ｉｎ ｔ ｘ, ｙ;

ｕｉｎｔ ｃｏｕｎｔ＿ ｐｌｕｓｅ;

ＥＴ０＝０; //Disable T／C0 interrupt

Tcount++; //interrupt times

ｉｆ?Ｔｃｏｕｎｔ ＝＝ １０){

TR1=0; //Stop the counter counting

Ｔｃｏｕｎｔ＝０;

ｘ＝ＴＨ１;

ｙ＝ＴＬ１;

ｃｏｕｎｔ_ｐｕｌｓｅ＝(ｘ＊２５６＋ｙ)＊２;

ph＝(uint)(10 ＊ ((float)(count pressure＋1520)／105．9? ?? //Calculate the air pressure

ＴＨ１＝０ｘ００; //Reset the initial value

ＴＬ１＝０ｘ００;

}

ＴＨ０ = -50000/256; //Reset to 50ms initial value

ＴＬ０ ＝ －５００００％２５６;

ｉｆ(ＴＬ０!＝ ０) ＴＨ０－－;

ＥＴ０＝１;

ＴＲ１＝１;

ｒｅｔｕｒｎ;

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This interrupt function is mainly used to complete the reading of pulses and the calculation of air pressure values. ph is a global variable that can be used to save air pressure values.

(2) In the display function, the air pressure value is first separated by bit and saved in an array, and then the segment code and the corresponding bit selection are sent to display the corresponding air pressure value. The specific procedure is as follows:

ｖｏｉｄ ｄｉｓｐｌａｙ(ｕｉｎｔ ｐｈ_ｉｎ)

{ ｕｃｈａｒ ｉ＝０;

ｕｃｈａｒ ｊ＝０;

uchar select_bit=0; //bit selection

ｄｏ {

ｃｕｒ_ｂｕｆ[ｉ]＝ｐｈ_ｉｎ％１０;

ｉ＋＋;

ｊ＝ｉ;

} while (ph_in = ph_in / 10);? //When the high bit is zero?End the loop

ｉ＝０;

ｓｅｌｅｃｔ_ｂｉｔ＝０ｘｆｅ;

ｄｏ

{ Ｐ１＝ｔａｂ[＊ｐ];

Ｐ３＝ｓｅｌｅｃｔ_ｂｉｔ;

ｄｌ_ｍｓ();?

ｓｅｌｅｃｔ_ｂｉｔ＝(ｓｅｌｅｃｔ ｂｉｔ＜＜１)＋１;

//Start displaying from the rightmost digit, circularly shift left

ｐ＋＋;

ｉ＋＋;

}ｗｈｉｌｅ(ｉ＜ｊ);

p = cur_buf; // pointer reset

ｒｅｔｕｒｎ;}

In this way, in the main program, you only need to initialize it when the program runs for the first time, and then call the display function in a loop to achieve the real-time display function.

5. Conclusion

The author has designed a barometer using pure hardware circuits. Practice shows that due to the influence of temperature and the limitation of hardware parameters, the stability of real-time display is poor and the accuracy is not high. However, the use of V/F conversion signal and programming method to achieve the measurement completely overcomes the above shortcomings. The results show that this method has the advantages of high accuracy, good stability, and easy function expansion, which can provide a new idea for the design of instruments and electronic products.