Course Design of Intelligent Temperature Meter Based on Single Chip Microcomputer

This design uses AT89C51 as the core part of the entire control system, and collects temperature through DS18B20 through three buttons: mode switch key, plus key and minus key. The mode switch key can switch between normal display, upper limit temperature setting state, and lower limit temperature setting state. When the measured temperature is higher than the set upper limit value, the buzzer alarms and the LED red light flashes to alarm. When the measured temperature is higher than the set lower limit value, the buzzer alarms and the LED yellow light flashes to alarm. The system can easily realize temperature collection and alarm, and can set the upper and lower limit alarm temperatures as needed. It is very convenient to use, with the advantages of high accuracy, wide range, high sensitivity, small size, and low power consumption. It is suitable for temperature measurement in our daily life and industrial and agricultural production. It can also be used as a temperature processing module to sneak into other systems as an auxiliary expansion of other main systems.

3.1 Selection and Introduction of MCU

The CPU is the core of the entire control part. Considering the economy and meeting the needs, this design uses AT89C51 as the core of the entire control system. It contains a memory that can be used to program the system. Flash memory is powerful. When programming it, it can not only be operated online, but also can be operated using general methods. The compiled program is mainly stored in the chip, and its function is relatively powerful. Therefore, AT89C51 is selected as the control center of this design. The device pin diagram is shown in Figure 3-1:

Figure 3-1 AT89C51 pin diagram

AT89C51 Introduction: The erasable read-only memory of the microcontroller can be repeatedly erased 1000 times. The device is manufactured using ATMEL's high-density non-volatile memory manufacturing technology and is compatible with the industry-standard MCS-51 instruction set and output pins. Due to the combination of a multi-functional 8-bit CPU and flash memory in a single chip, ATMEL's AT89C51 is an efficient microcontroller, and the AT89C051 is a streamlined version of it. The AT89C51 microcontroller provides a highly flexible and inexpensive solution for many embedded control systems. The appearance and pin arrangement are shown in the figure.

3.2 Minimum system module

In this course design, according to the needs of the subject, the AT89C51 chip of the 51 series microcontroller of ATMEL Company is used as the control chip. In actual application, if only the microcontroller chip is used, it is impossible to achieve the designed purpose. There must also be matching circuits to form a complete control system to meet the design requirements. In this design, the AT89C51 chip and the matching circuits are mainly used to form the required modules for research. Some specific parameters in the module are mainly to use the P3 port as the 1602 command data control, clock, read and write control, and enable control interface, the P2 port as the key scan interface, and the P2.3 as the DS18B20 bus interface. P2.5 and P2.7 are used as alarm control interfaces. The circuit connection diagram 3-2 is as follows:

Figure 3-2 Data processing and control module

3.3 Temperature sensor design

3.3.1 Introduction to Temperature Sensor

DS18B20 can set 9~12 bit resolution, which has high accuracy. In order to meet the actual work needs, a relatively small sealing mode and a relatively wide voltage can also be used. The set resolution and alarm temperature are stored in EPROM and will not be lost after power failure.

The pinout of the temperature sensor DS18B20 is shown in Figure 3-3.

Figure 3-3 DS18B20TO-92 package temperature sensor

Pin function description:

VDD: Optional power pin, power supply voltage range 3~5.5V.

DQ: Data input/output pin. Drain is open circuit, high level in normal state.

GND: Ground power pin.

The components of DS18B20 mainly include: photolithography 64-bit ROM, temperature sensor, non-volatile temperature alarm triggers TH and TL, and configuration registers, which are arranged in a certain structure inside.　

The serial number arrangement of the photolithography 64-bit ROM has a certain pattern. Generally, the first one is the 8-digit product label of different types, followed by the 48-digit serial number of the DS18B20, and the last one is the check code of all the previous serial numbers. Through summary, we can know that the role of the 64 photolithography ROM is to ensure that each DS18B20 is different, so as to meet the requirements of connecting multiple DS18B20 on the bus.

The high-speed memory mainly included in the DS18B20 temperature sensor is very easy to lose. The memory RAM is mainly composed of eight bytes, and the contents of these eight bytes are very different. The first byte is the lower eight bits, the second byte is the upper eight bits, the third and fourth bytes are non-volatile copies of the highest and lowest bits, and the fifth byte is a non-volatile copy of the structure register. The third, fourth, and fifth bytes refresh the content at each power-on reset. The sixth, seventh, and eighth bytes are used for internal calculations. The ninth byte is a redundant test. The main function of E2RAM is to store high and low temperature triggers TH and TL.　

Through the communication protocol of DS18B20, we know that the temperature conversion when the host controls it mainly consists of three steps. Each time you read and write DS18B20, you need to reset it. After completing the setting, you need to send a command. Only after sending the corresponding command can you perform related operations. After receiving the reset request, the CPU needs to pull down and then release it. After a certain period of time, the signal can be reset.

3.3.2 Connection between temperature sensor and single-chip microcomputer The interface model used by the temperature sensor when connected to the single-chip microcomputer is P2.0, which belongs to the high-order address line in the single-chip microcomputer. The P2 port is an 8-bit bidirectional I/O with an internal pull-up resistor. In addition, it can drive 4 logic gate circuits when outputting buffers. The operation of the P2 port is not affected by external interference, and it can work normally even when the program is being written and checked. Figure 3-4 shows the composition of DS18820.

Figure 3-4 Interface connection between DS18B20 and microcontroller

3.4 LCD display module

The characters stored in this module are composed of different characters, including English letters, common mathematical symbols and Arabic numerals. Different characters have different codes. For example, the capital A in English letters represents 41h. If the display is used to send, the module will display 41h, but the letter A will be displayed on the LCD.　

The controller of the 1602 LCD module mainly has 11 instructions. A large number of control instructions can better control the program and meet its working needs.　

The LCD1602 liquid crystal module used is a standard 16-pin socket, and the interface circuit is shown in Figure 3.6

Figure 3.6 shows the connection diagram of the circuit

3.5 Overall block diagram of system hardware

The overall block diagram of the system hardware is shown in Figure 3.7

Figure 3.7 Overall block diagram of system hardware

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