Design of AT89C2051 Bidirectional Temperature Monitoring System

1 Introduction

Some applications require temperature monitoring and require the temperature to be kept within a certain range. For example, the temperature of crop seedlings is 20℃~30℃, the temperature of poultry incubation is 37℃~40℃, and the temperature of bacterial strains is 23℃~28℃. However, the ordinary one-way over-temperature alarm is difficult to meet the requirements. Therefore, a two-way over-temperature control system design using temperature sensors and single-chip microcomputer application systems is proposed. The system design can display the temperature exceeding the lower limit or the upper limit, and the over-temperature point can be preset.

2 System Design Principles

Figure 1 shows the system design block diagram. First, input the upper and lower limits of the temperature change allowed through the keyboard. Under normal circumstances, the temperature should be kept between the upper and lower limits. When the temperature is higher than the upper limit, the temperature is lowered and an alarm is sounded; when the temperature is lower than the lower limit, the temperature is raised and an alarm is sounded.

The internal timer of the single-chip controller samples the temperature value once at a certain interval and generates a timer interrupt. The timer interrupt service program mainly compares the sampled temperature value with the set upper and lower temperature limits to determine whether the upper and lower temperature limits are reached, thereby deciding whether to increase or decrease the temperature and alarm, realizing two-way automatic temperature control, and can also realize fault alarm and change the upper and lower temperature limits through the keyboard.

Thermistors use semiconductor temperature sensing elements, which have negative resistance temperature characteristics. When the temperature rises, the resistance value decreases. To this end, a constant current is applied to the thermistor, the voltage across the resistor is measured, and then the temperature is obtained through the temperature measurement formula: T=T0-KUT, where T is the measured temperature, T0 is a temperature parameter related to the thermistor characteristics, K is a coefficient related to the thermistor characteristics, and UT is the voltage across the thermistor. K and T0 are constants. The voltage across the thermistor can be measured to calculate the ambient temperature of the thermistor. In this way, the change of resistance with temperature is converted into the change of voltage with temperature, and the voltage is then converted into a digital quantity by an A/D converter, and the temperature value is calculated by software.

3 Hardware Circuit

This system design is composed of AT89C2051 single chip microcomputer, AD574A, programmable keyboard and interface device 8279, temperature sensor, LED display, watchdog circuit, etc. [page]

3.1 AT89C2051 Microcontroller

AT89C2051 is a cost-effective 8-bit single-chip microcomputer produced by ATMEL. The device adopts the 80C31 core instruction system and is fully compatible with MCS-51. KB of Flash can be used as user program memory and 128 bytes of random access data memory (RAM); it contains 2 program encryption bits to prevent illegal reading of the program, and has good security; its low-voltage, high-performance CMOS 8-bit machine is compatible with MCS-51 instructions and output pins, forming a minimum measurement system; it has 20 pins, 15 bidirectional input/output (I/O) ports, of which P1 is a complete 8-bit bidirectional I/O port, 2 external interrupt ports, 2 16-bit programmable timer counters, 2 full-duplex serial communication ports, and 1 analog comparison amplifier; the clock frequency of AT89C2051 can be zero, that is, it has a sleep power saving function that can be set by software. The system wake-up methods include RAM, timer/counter, serial port and external interrupt port. After the system wakes up, it will enter the continuous working state; in power saving mode, the on-chip RAM is frozen, the clock stops oscillating, and all functions stop until the system is reset by hardware to continue running. Based on the many performances of AT89C2051, this system design uses AT89C2051 as the controller.

3.2 A/D Converter AD574A

This system design uses the 12-bit successive approximation A/D converter AD574A produced by ADI. AD574A contains high-precision reference voltage and clock circuit, has a small number of external components, low power consumption, and has functions such as automatic zero calibration and automatic polarity conversion, so that it can complete A/D conversion without any external circuit and clock signal, and its conversion time is 35μs.

3.3 Power-off protection circuit and watchdog circuit

In order to make the system work reliably and save the set data, the system needs to have a reset function. The power-off protection circuit and the system reset circuit use the MAX813L monitoring circuit to monitor the power supply voltage and the working status of the CPU in cooperation with the software. Once a crash occurs, the program "runs away" in an infinite loop, or the power supply voltage drops to the system setting value, the system needs to be reset. The connection between MAX813L and AT89C205l is shown in Figure 2. MR is grounded through a reset button. The main functions of the monitoring circuit are: system power-on reset, when the power supply voltage exceeds the reset threshold voltage of 4.65 V, the reset terminal can output a reset signal of 200 ms, and the system is reset; monitor the +5 V power supply: when the +5 V power supply is normal, RESET outputs a low level, and the microcontroller works normally; when the +5 V power supply voltage drops below +4.65 V, RESET outputs a high level, and the microcontroller is reset; the watchdog timer is cleared, and WDO maintains a high level: when the program "runs away" or crashes, the CPU cannot give a "feed the dog" signal within 1.6s, and WDO jumps to a low level. Since the MR end has an internal 250 mA pull-up current, D is turned on, MR obtains a valid low level, RESET outputs a reset pulse, the microcontroller is reset, the watchdog timer is cleared, and WDO returns to a high level. Manual reset: If you need to manually reset the system, just press the manual reset button to effectively reset the system. [page]