Design of Home Temperature Measuring Device Based on DS18B20 and AT89C2051

1. Design Overview

This design uses the USB port as the power supply port, uses the DS18B20 temperature sensor to collect temperature information, uses the AT89C2051 single-chip microcomputer for control, and uses a four-digit common anode digital tube display to achieve temperature measurement and display (the system block diagram is shown in Figure 1). This design can cultivate students' interest in learning single-chip microcomputers and improve their production and programming capabilities.

Figure 1 System Block Diagram

2. Circuit Principle

The PROteUS simulation software is used for schematic design and program simulation. The circuit is shown in Figure 2.

Figure 2 Circuit diagram

1. Power supply

The USB port is used for power supply. The USB adapter can be connected to the USB port of the circuit board or directly to the USB port of the computer. In this way, resources can be saved and a more ideal working voltage can be obtained. The appearance of the USB port and the definition of the power port are shown in Figure 3.

Figure 3 USB port appearance and power port definition

2. Temperature signal acquisition

The DS18B20 (appearance see Figure 4) intelligent digital temperature sensor is used as the temperature signal acquisition device.

Figure 4 DS18B20 appearance [page]

(1) Working principle of DS18B20

The read/write timing and temperature measurement principle of DS18B20 are the same as those of DS1820, except that the number of bits of the obtained temperature value is different, and the delay time of temperature conversion is reduced from 2s to 750ms. The temperature measurement principle of DS18B20 is shown in Figure 5. Among them, the oscillation frequency of the low temperature coefficient crystal oscillator is little affected by temperature, and is used to generate a fixed frequency pulse signal to counter 1. The oscillation frequency of the high temperature coefficient crystal oscillator changes significantly with temperature, and the generated signal is used as the pulse input of counter 2. Counter 1 and the temperature register are preset to a base value corresponding to -55℃. Counter 1 subtracts the pulse signal generated by the low temperature coefficient crystal oscillator. When the preset value of counter 1 is reduced to 0, the value of the temperature register will be increased by 1, counter 1 will be re-preset, and the pulse signal generated by the low temperature coefficient crystal oscillator will be counted again. This cycle continues until counter 2 counts to OH and stops accumulating the temperature register value. At this time, the value in the temperature register is the measured temperature. The slope accumulator in FIG5 is used to compensate and correct the nonlinearity in the temperature measurement process, and its output is used to correct the preset value of counter 1.

Figure 5 DS18B20 temperature measurement schematic

(2) Main features of DS18B20

1) The adaptable voltage range is 3.0V～5.5V, and it can be powered by the data line in parasitic power supply mode.

2) Only one line is needed between DS18B20 and microprocessor for two-way communication.

3) Support multi-point networking function, multiple DS18B20 can be connected in parallel on only three lines to achieve multi-point temperature measurement in networking.

4) No peripheral components are required, and all sensor elements and conversion circuits are integrated in a circuit that looks like a transistor.

5) The temperature measurement range is -55℃～+125℃, and the accuracy is ±0.5℃ at -10℃～+85℃.

6) The programmable resolution is 9 to 12 bits, and the corresponding resolvable temperatures are 0.5°C, 0.25°C, 0.125°C and 0.0625°C, respectively, which can achieve high-precision temperature measurement.

7) At 9-bit resolution, it takes a maximum of 93.75ms to convert the temperature to digital, and at 12-bit resolution, it takes a maximum of 750ms to convert the temperature value to digital.

8) Directly output digital temperature signal and transmit it serially to CPU via a one-wire bus. CRC check code can also be transmitted at the same time, which has strong anti-interference and error correction capabilities.

9) When the power polarity is reversed, the chip will not burn out due to heat, but it will not work normally.

DS18B20 follows the single bus protocol. Each temperature measurement must go through four processes: initialization, transmission of ROM commands, transmission of RAM commands, and data exchange.

3. AT89C2051 microcontroller

The AT89C2051 single-chip microcomputer is used as the main control component (see Figure 2).

4. Digital tube display

A four-digit common anode digital tube is used for dynamic display, and the temperature display is retained to one decimal place. When programming, P3.2~P3.5 is used as the bit selection end of the dynamic display of the digital tube, and Pl.0~Pl.7 is used as the segment selection bit of the dynamic display of the digital tube. When P3.2 outputs a high level, the "1" digital tube is selected, and when P3.3 outputs a high level, the "2" digital tube is selected, and so on. In the circuit, P3.2~P3.5 are connected to 4 NPN transistors as drivers. Pl.0~Pl.7 are connected to 8 resistors for current limiting.

3. Reference Program

This design uses the microcontroller C language for programming. Due to space limitations, the reference programs are not listed here one by one.

4. Production and debugging

The design is relatively simple to debug. As long as the installation and welding are correct and the program is written accurately and completely, it is generally easy to realize the function.

The debugged object is shown in Figure 6.

Figure 6 Actual picture