Simulation Design of Digital Voltmeter Based on 51 Single Chip Microcomputer

Abstract: The design uses AT89C51 single-chip microcomputer, A/D converter ADC0808 and common anode digital tube as the main hardware, and analyzes the digital voltmeter Proteus software simulation circuit design and programming method. The single-chip microcomputer is applied to measurement technology, ADC0808 is used to convert analog signals into digital signals, AT89C51 is used to process data, and the digital tube is used to complete the display in a scanning manner. The designed digital voltmeter can measure the voltage value of 0~5 V. AT89C51 is an 8-bit single-chip microcomputer. When the input voltage of ADC0808 is 5 V, the output digital value is +4.99 V. The circuit of this design is simple, low-cost and stable in performance.
Keywords: digital voltmeter; 51 single-chip microcomputer; ADC0808; digital tube LED

0 Introduction
With the development of electronic science and technology, electronic measurement has become a means that must be mastered by the majority of electronic workers, and the requirements for measurement accuracy and function are getting higher and higher, and voltage measurement is particularly prominent because voltage measurement is the most common. Digital voltmeter is a voltmeter designed using digital measurement technology. Compared with analog voltmeters, digital voltmeters have the characteristics of intuitive and accurate readings, wide display range, high resolution, large input impedance, high integration, low power consumption, strong anti-interference ability, and strong scalability. Therefore, they are widely used in voltage measurement and voltage calibration. This paper uses ADC0808 to convert the input analog signal, and the control core AT89C51 microcontroller calculates and processes the conversion results. Finally, the output device is driven to display the digital voltage signal. The interface circuit design is realized through Proteus simulation software, and real-time simulation is performed.
Proteus software is a circuit analysis and physical simulation software. It runs on the Windows operating system and can simulate and analyze (SPICE) various analog devices and integrated circuits. It is a simulation software that integrates microcontrollers and SPICE analysis. It is powerful, has rich system resources, low hardware investment, and intuitive images. It has been favored by users in recent years.

1 System Overview
1.1 Design Task
Design a digital voltmeter using the MCU AT89C51 and ADC0808, convert the voltage value of the analog signal between 0 and 5 V into a digital signal, display it with a two-digit digital tube, and observe the voltage value of the ADC0808 analog input signal through a virtual voltmeter, and the LED digital tube displays the corresponding numerical value in real time.
1.2 Overall Scheme
The circuit block diagram of the digital voltmeter is shown in Figure 1.

The circuits needed in this design include power supply circuit, analog/digital conversion circuit, single-chip control circuit, display circuit, etc. The chips needed in the design include AT89C51 single-chip microcomputer, ADC-0808, 74LS74, LED digital tube, etc.

2 Proteus software simulation circuit design of digital voltmeter
The voltage input signal to be measured is within the maximum working voltage range of the ADC0808 chip, and is converted by the analog/digital conversion circuit to achieve A/D conversion, and the program data is processed by the single-chip control circuit, and then the digital tube displays the input voltage through the seven-segment decoding/driving display circuit.
The hardware circuit schematic is shown in Figure 2.

2.1 Interface design of AT89C51 single-chip microcomputer and digital tube display circuit
A digital voltmeter is designed using the single-chip microcomputer AT89C51 and ADC0808 to convert the DC voltage value of the analog signal between 0 and 5 V into a digital signal 0 to FF, which is displayed with two digital tubes. Proteus software starts the simulation. The current input voltage is 2.5 V, which is converted into a digital value of 7FH. The voltage of the input analog/digital converter ADC0808 can be changed by adjusting the potentiometer RV1 with the mouse pointer, and the voltage value of the analog input signal of ADC0808 can be observed through the virtual voltmeter. The LED digital tube displays the corresponding numerical value in real time.
The crystal oscillator frequency of the AT89C51 single-chip microcomputer is set to 12 MHz in the Proteus software. The EA of this circuit is connected to a high level, and there is no external ROM expansion.
2.2 Interface design of A/D conversion circuit
The A/D converter uses the integrated circuit ADC0808. ADC0808 has 8 analog input signals IN0~IN7 (pins 1~5, 26~28), and the address lines C, B, and A (pins 23~25) determine which analog input signal is A/D converted. This circuit connects the address lines C, B, and A to ground, that is, selects channel 0 to input the analog voltage signal. Pin 22 ALE is the address latch enable control signal. When the input is high, the address signal is latched. Pin 6 START is the start control signal. When the input is high, the A/D conversion starts. This circuit connects the ALE pin and the START pin together, and they are controlled by the P2.0 pin and the WR pin of the microcontroller through an NOR gate. Pin 7 EOC is the A/D conversion end signal. When the A/D conversion ends, pin 7 outputs a positive pulse. This signal can be used as a detection signal to indicate whether the A/D conversion is over or as a signal to request an interrupt to the CPU. This circuit is connected to the P3.2 pin of the microcontroller through a NOR gate. Pin 9 OE is the A/D conversion data output enable control signal. When the OE pin is high, the digital quantity of the A/D conversion is allowed to be read. The OE pin is controlled by the P2.0 pin and the RD pin of the single-chip computer through the NOR gate. Pin 10 CLOCK is the real-time clock input terminal of ADC0808. The clock signal is obtained by using the six-frequency crystal oscillator frequency of the single-chip computer 30 pin ALE. The 8 digital output terminals are connected to the P0 port of the single-chip computer.

3 Software programming of digital voltmeter
When the system is powered on, the startup address of ADC0808 is initialized, the digital tube display is turned off, and the A/D conversion is started. After waiting for the startup to end, the digital quantity result of the analog input signal of channel 0 of ADC0808 is converted and output on the three-digit digital tube through the dynamic display of the digital tube.
According to the design requirements and the hardware circuit, a resistor voltage divider is used when inputting analog signals. The final sampled input voltage is only one tenth of the actual input voltage. Therefore, a data adjustment program should be written in the program. It should also be noted that the hardware display circuit uses dynamic scanning display. In the dynamic scanning display mode, there are certain requirements for the frequency of dynamic scanning. If the frequency is too low, the digital tube LED will flicker. Usually, the digital tube lighting time interval is generally about 5ms. This requires that when writing the program, it should be lit and maintained for a certain time. Summarizing the above analysis, the program flow chart is shown in Figure 3 and Figure 4.

The program design of this circuit mainly includes A/D conversion part, LED display, initialization and timer interrupt part. Part of the program code is shown below.



5 Conclusion
The digital voltmeter in this article can measure the voltage value of 0~5V. AT89C51 is an 8-bit single-chip microcomputer. When the input voltage of ADC0808 is 5V, the output digital value is +4.99V. If you want to get higher accuracy, you need to use I2-bit, I3-bit and other higher than 8-bit A/D converters. The display part of the digital voltmeter can add a BCD code adjustment program to display its data through a three-digit digital tube. The display deviation of this design can be solved by correcting the reference voltage of 0808, or by using software programming to correct its measured value. This system was debugged by Proteus simulation software during the design process, and has the characteristics of simple circuit, low cost, high accuracy, fast speed and stable performance.