Design of multi-point temperature measurement system based on 51 single chip microcomputer

1. Introduction to temperature sensor DS18B20   

DALLAS's single-wire digital temperature sensor DS18B20 is a new "one-wire device" with the characteristics of small size and wide applicable voltage. The unique and economical characteristics of the one-wire bus allow users to easily build a sensor network and introduce a new concept for the construction of the measurement system. DS18B20 supports the "one-wire bus" interface, and the measurement temperature range is -55℃～+125℃. In the range of -10℃～+85℃, the accuracy is ±0.5℃; 9-12-bit digital value reading mode can be realized through programming; the temperature value can be converted into 9-bit and 12-bit digital quantities within 93.75ms and 750ms respectively. Each DS18B20 has a unique 64-bit long serial number, which is stored in the internal ROM read-only memory of the DS18B20.   

The internal memory of the DS18B20 temperature sensor includes 1 high-speed temporary RAM and 1 non-volatile electrically erasable E2RAM, the latter of which stores high temperature and low temperature triggers TH, TL and structure registers. The temporary memory contains 8 consecutive bytes, the first 2 bytes are the measured temperature information, the first byte is the low 8 bits of the temperature, and the second byte is the high 8 bits of the temperature. Of the high 8 bits, the first 4 bits represent the positive (all "0") and negative (all "1") temperature; the third and fourth bytes are volatile copies of TH and TL; the fifth byte is a volatile copy of the structure register, and the contents of these three bytes are refreshed at each power-on reset; the sixth, seventh, and eighth bytes are used for internal calculations; the ninth byte is a redundant check byte. Therefore, reading the content in the temperature information byte can be converted into the corresponding temperature value accordingly. Table 1 lists the correspondence between temperature and temperature bytes.    Correspondence between temperature and temperature bytes

2. System hardware structure   

The system is divided into two parts: on-site temperature data acquisition and upper-level monitoring PC. Figure 1 is a structural diagram of the system. It should be pointed out that the lower computer can work independently from the upper PC. The purpose of adding an upper computer is to be able to more conveniently implement monitoring and management away from the site. The on-site temperature acquisition part uses an 8051 single-chip microcomputer as the central processor, and 10 DS18B20 sensors are connected to the P1.0 port to detect the temperature of 10 points. Non-volatile RAM is used as a buffer for system temperature acquisition and operating parameters. The upper PC communicates with the on-site single-chip microprocessor through the RS485 communication interface to comprehensively manage and control the system, and can complete data recording, printing reports and other tasks. The analysis of each module of the system is as follows:     2.1 Interface circuit between DS18B20 and single-chip microcomputer   

It is very simple to connect DS18B20 with 8051 microcontroller. You only need to connect the DS18B20 signal line to one I/O line of the microcontroller, and one I/O line can be connected to multiple DS18B20 to achieve single-point or multi-point temperature measurement. DS18B20 can be powered in two ways: external power supply and parasitic power supply. The former requires an external power supply, and the positive and negative poles of the power supply are connected to the VDD and GND of DS18B20 respectively. The latter uses a parasitic power supply, connecting the VDD and GND of DS18B20 together. When a high level appears on the bus, the pull-up resistor provides power; when the bus is low, the internal capacitor provides power. Since the use of an external power supply can enhance the anti-interference ability of DS18B20, this design adopts this method. In practical applications, the distance between the sensor and the microcontroller is often tens to hundreds of meters. The parasitic capacitance of the transmission line also has a certain impact on the operation of the DS18B20, so slight changes are often made to the interface to increase the chip's driving capability and reduce the impact of the transmission line capacitance effect to achieve the purpose of long-distance transmission.  

2.2 Keyboard and Display   

The keyboard can complete the following functions through programming: calibrate the temperature value, display the temperature value of each channel at a fixed time, display the temperature value of a certain channel separately, set the upper and lower limit alarm values ​​for each channel, etc. The LED can provide users with intuitive visual information. At the work site, users can determine the current working status of the system and the sampled temperature value information through the display data of the 6-digit LED.   

2.3 Alarm circuit   

When the measured temperature value exceeds the preset upper and lower limits, the alarm circuit responds and the buzzer sounds to inform the user of the temperature abnormality. The specific sensor temperature value that exceeds the limit can be determined by the software by querying the internal alarm flag of each DS18B20, and then adjusting the field temperature to achieve control of temperature fluctuations.   

3. Software design and process   

3.1 Lower computer software   

The lower-level temperature measurement software of the system is written in MCS51 assembly language, which mainly completes the read and write operations of DS18B20, realizes real-time data collection, and obtains the final temperature value and sends it to the microcontroller memory. However, it should be noted that due to the single bus mode of DS18B20, the reading and writing of data occupy the same line, so each operation must be performed strictly according to the timing. Figure 2 is a flow chart of the temperature measurement subsystem. The microcontroller first sends a reset pulse, which resets all DS18B20 chips on the signal line, and then sends a ROM operation command to activate the DS18B20 with matching serial number encoding. After being activated, the DS18B20 enters the state of receiving memory access commands, and the memory access commands complete temperature conversion, reading, etc. (the single bus storage command and control command do not work before the ROM command is sent).   

3.2 Host computer software   

The system host computer software is written in VC++6.0. The main functions include: real-time communication with the lower single-chip microcomputer; simulation and display of the temperature curve of each acquisition point; saving the temperature data of each temperature measurement point; statistics of the average temperature value of each acquisition point; printing of temperature statistics reports of each point,    etc.  

4 Conclusion  

This system has the following features:   

Simple structure, low cost and easy maintenance. 

By directly encoding the temperature data, the temperature data can be transmitted using only a single cable, which facilitates communication, has a long transmission distance and strong anti-interference ability. 

Flexible, convenient and easy to expand configuration. Multiple lower-level temperature acquisition subsystems can be expanded and networked with the upper-level PC via RS485 to form a multi-point temperature acquisition network. Temperature sensors can also be selectively added or removed in each subsystem. 

The system works stably and has high temperature measurement accuracy. Experiments show that when 24 DS18B20 temperature sensors are connected to a 200m long one-bit bus, the system can correctly collect temperature with a resolution of 0.5℃. 

It is suitable for on-site temperature measurement in harsh environments, such as environmental control, equipment or process control, temperature measurement consumer electronics, etc. It has very attractive application prospects in large-scale temperature multi-point monitoring systems.