Design of digital thermometer based on AT89C51&DS18820

1 Introduction

With the rapid development of modern information technology and the gradual realization of traditional industrial transformation, temperature detection and display systems that can work independently are applied in many fields. Traditional temperature detection uses thermistors as temperature sensitive elements. Thermistors have low cost, but require subsequent signal processing circuits, and their reliability is relatively poor, the temperature measurement accuracy is low, and the detection system also has certain errors. Compared with traditional thermometers, the digital thermometer designed here has the characteristics of convenient reading, wide temperature measurement range, accurate temperature measurement, digital display, and wide application range. AT89C51 single-chip microcomputer is selected as the main control device, and DSl8820 is used as the temperature sensor to transmit data through the 4-bit common anode LED digital tube serial port to realize temperature display. The measured temperature value is directly read through DSl8820 for data conversion. The physical and chemical properties of the device are stable, the linearity is good, and the maximum linear deviation is less than 0.1℃ at 0℃~100℃. The device can directly transmit digital signals to the single-chip microcomputer, which is convenient for single-chip microcomputer processing and control. In addition, the thermometer can also directly use the temperature measuring device to measure the temperature, thereby simplifying the data transmission and processing process.

2 System Hardware Design

According to the system functional requirements, the system principle structure block diagram shown in Figure 1 is constructed.

2.1 Selection of MCU

AT89C51 is the core device of the temperature test system design. This device is the basic product of the MCS-51 series of single-chip microcomputers produced by INTEL. It adopts reliable CMOS process manufacturing technology, has high-performance 8-bit single-chip microcomputers, and belongs to the standard MCS-51 CMOS product. It not only combines the high-speed and high-density technology of HMOS and the low-power consumption characteristics of CHMOS, but also inherits and expands the architecture and instruction system of the MCS-48 single-chip microcomputer.

2.2 Connection between temperature sensor and microcontroller

The temperature sensor's single bus (1-Wire) is connected to the microcontroller's P2.0, which is the microcontroller's high-order address line A8. The P2 port is an 8-bit bidirectional I/O with an internal pull-up resistor, and its output buffer stage can drive (absorb or output current) 4 TTL logic gate circuits. Writing "1" to this port can pull its port to a high level through the internal pull-up resistor, and it can be used as an input port at this time. This is because there is an internal pull-up resistor, and a current will be output when a pin is pulled low by an external signal. When accessing the external program memory or the external data memory with a 16-bit address. If the MOVX DPTR instruction is executed, it means that the P2 port sends out the high 8-bit address data. When accessing the external data memory with an 8-bit address, the MOVX RI instruction can be executed, and the content of the P2 port is the content of the R2 register in the special function register (SFR) area, which does not change during the entire access period. During Flash programming and program verification, the P2 port also receives high-order addresses and other control signals. Figure 2 shows the internal structure of the DS18820. Figure 3 shows the interface circuit between DS18820 and MCU. [page]

2.3 Reset signal and external reset circuit

The P1.6 port of the microcontroller is the input terminal of the dog feeding signal in the MAX813 watchdog circuit, that is, the microcontroller sets the dog feeding signal once every time it executes a program, and clears the watchdog device. If the program is abnormal, the microcontroller pin RST will appear high for more than two machine cycles to reset it. The reset signal is valid at a high level, and its effective time should last for 24 oscillation pulse cycles, that is, more than two machine cycles. If a crystal oscillator with a frequency of 12 MHz is used, the reset signal should last for more than 2μs to complete the reset operation.

2.4 Single chip microcomputer and alarm circuit

The alarm circuit in the system is composed of a light-emitting diode and a current-limiting resistor, and is connected to the P1.2 port of the microcontroller. The function and connection method of the P1 port are the same as those of the P2 port, except that during Flash programming and program verification, P1 receives the lower 8-bit address data.

2.5 Power supply circuit

Since the system requires a stable 5 V power supply, a power supply that can meet the voltage, current and stability requirements must be used in the design. The power supply uses a three-terminal integrated voltage regulator LM7805. It has only three pins: input, output and common. It has overcurrent protection, overheat protection and adjustment tube safety protection circuits. Since it requires few external components, it is easy to use and reliable, so it can be used as a voltage-stabilized power supply. Figure 4 is a power supply circuit connection diagram.

2.6 Display Circuit

The mature technology 74HCl64 is used to realize serial-to-parallel conversion. LED display is divided into static display and dynamic display. Static display is used here, and the system realizes static display through the serial port of the single-chip microcomputer. The serial port is in mode zero state, that is, it works in shift register mode, and the baud rate is 1/12 of the oscillation frequency. When the device executes any command with SBUF as the destination register, data begins to be sent from the RXD terminal. When the write signal is valid, the send control terminal SEND is valid after one machine cycle, that is, RXD is allowed to send data, and shift pulses are allowed to be output from the TXD terminal. Figure 5 is the connection diagram of the display circuit. [page]