This paper introduces a temperature measurement and heating control system based on an ATmega8 microcontroller. The working principle of the system is analyzed, and the hardware circuitry and software flow are discussed in detail. Experiments demonstrate that the system is reliable and highly stable. 1 Introduction Temperature is one of the main controlled parameters in industrial production, and various temperature control systems are widely used in metallurgy, chemical industry, machinery, food processing, and other fields. The temperature measurement and heating control system introduced in this paper uses the ATmega8 AVR series microcontroller as its core component. Through reasonable planning of the system software and hardware design, the advantages of the microcontroller's integration of numerous system-level functional units are utilized, effectively reducing hardware costs without reducing functionality. The system is easy to operate. Experiments demonstrate that this temperature control system has high reliability and stability. 2 System Structure and Control Algorithm 2.1 Overall System Structure The overall structure of the temperature measurement and heating control system is shown in Figure 1. The system mainly includes on-site temperature acquisition, real-time temperature display, heating control parameter setting, heating circuit control output, serial communication with the host computer, and the system core ATmega8 microcontroller. The temperature acquisition circuit transmits the on-site temperature to the microcontroller in the form of analog voltage. The microcontroller converts analog voltage into digital quantities usable by the control system through its integrated A/D converter. Combining the ambient temperature with the user-set target temperature, the microcontroller calculates the real-time control quantity according to a pre-programmed incremental PID control algorithm. This control quantity enables the opto-isolated drive circuit, determining the operating state of the heating circuit and gradually stabilizing the furnace temperature to the user-set target value. Various status parameters during system operation are displayed in real-time on a digital tube and communicate with the host computer via an RS232 serial port in full-duplex mode. The user directly completes all temperature measurement and heating control operations on the host computer. 2.2 System Control Algorithm The system adopts a pulse width modulation (PWM) control method based on the incremental PID algorithm, where the duty cycle of the PWM square wave is obtained by the incremental PID algorithm. The output of the incremental PID algorithm is given by the formula: where en, en-1, and en-2 are the deviation values ​​of the nth, n-1th, and n-2th iterations, respectively; Kp, Ti, and Td are the proportional, integral, and derivative coefficients, respectively; and T is the sampling period. The microcontroller inputs the difference between the ambient temperature and the user-set target temperature into the incremental PID algorithm formula at fixed intervals T. The output of the formula determines the duty cycle of the PWM square wave, and the subsequent heating circuit determines the heating power based on this PWM square wave duty cycle. A larger deviation between the ambient temperature and the target temperature results in a larger duty cycle and a higher heating power, rapidly reducing the deviation between the measured and set temperatures. Conversely, a smaller deviation results in a smaller duty cycle and a lower heating power, until the target and measured values ​​are equal, achieving automatic control. 3 Hardware Design 3.1 ATmega8 Microcontroller The ATmega8 microcontroller is a high-end Flash microcontroller based on the AVR RISC architecture, launched by Atmel. Its core connects 32 working registers and the instruction set together. All working registers are directly connected to the ALU (Arithmetic Logic Unit), enabling the execution of one instruction per clock cycle while simultaneously accessing (reading) two independent registers. This structure improves code efficiency, allowing most instructions to execute in just one clock cycle. Therefore, the ATmega8 boasts performance approaching 1 MHz/s/MHz, operating at speeds 10 times faster than ordinary CISC microcontrollers. The ATmega8 microcontroller integrates a hardware multiplier with an execution speed of two clock cycles, 8 KB of Flash program memory, 512 bytes of E2PROM, two 8-bit timers with compare mode, one 16-bit timer with compare and capture modes, three PWM outputs with a maximum precision of 16 bits, an 8-channel 10-bit A/D converter, a PI/TWI synchronous serial port, and a USART asynchronous serial port. The numerous system-level functional units integrated into the ATmega8 greatly facilitate the development of the control system. During the design process, hardware circuitry was simplified as much as possible through software programming, effectively shortening the development cycle. 3.2 On-site Temperature Acquisition The on-site temperature is obtained by a temperature sensor. In this system, a Pt100 platinum resistor is selected as the temperature sensor, utilizing the characteristic that the resistance of platinum metal changes with temperature to measure the temperature. The platinum resistance thermometer is amplified and linearized by the transmitter, outputting a standard DC signal of 4 mA to 20 mA, corresponding to an ambient temperature of 0℃ to 400℃. This signal is then converted into a DC voltage signal of 0.6 V to 3 V through a 150 Ω high-precision metal film resistor. This analog voltage signal meets the input requirements of the ATmega8's built-in A/D converter and can be connected to the ATmega8's PC0 for A/D conversion. The ATmega8 integrates an 8-channel 10-bit high-speed A/D converter. This system uses only a single-channel PCO as the analog voltage input for A/D conversion. The reference voltage for A/D conversion uses the system's built-in Vcc. Based on the above, the analog voltage of 0.6V to 3V input to the 10-bit A/D converter represents 0℃ to 400℃. Therefore, the ambient temperature T can be expressed as: Where A/D_Data is the 10-bit digital value obtained after A/D conversion, and AD_Max is the digital value corresponding to the 10-bit A/D converter reference voltage, which is 0x03ff here. The absolute error of the temperature measurement is 400/210℃, and the relative error is less than 0.1%, which meets the system accuracy requirements. At this point, the temperature value is converted from a non-electrical analog quantity to a digital quantity, which can be directly used for calculations in the microcontroller's internal heating control algorithm. In actual programming, to reduce the interference of transient errors during the sampling process, an arithmetic mean filtering method is used; that is, the final temperature value T used in the control calculation is obtained by calculating the arithmetic mean of 10 sampled temperature values. 3.3 Digital Tube Display Circuit During the heating process, parameters such as the actual temperature of the controlled object and the target furnace temperature set by the user are displayed in real time through the digital tube display circuit. The principle of the digital tube display circuit is shown in Figure 2. This display circuit adopts a dynamic display technology of "microcontroller → serial-in parallel-out chip → digital tube". The microcontroller and the 74HCl64 serial-in parallel-out circuit are connected using a synchronous serial port SPI method. The microcontroller operates in host mode, with the clock output terminal SCK connected to the CLK pin of the 74HCl64, and the data output terminal MOSI connected to the data input pin AB of the 74HCl64. The microcontroller transmits the 8-bit segment code to be displayed to the 74HCl64 via SPI, and the 74HCl64 outputs 8-bit parallel logic levels to drive the display. By using synchronous serial SPI and the 74HCl64 serial-in parallel-out chip to drive the 8-bit segment code of the digital tube, six microcontroller I/O ports are saved compared to the traditional parallel driving method. Furthermore, by utilizing the ATmega8's built-in hardware SPI unit, no software simulation of SPI communication is required. Due to the use of dynamic display technology, during programming, it is crucial to ensure that the segment to be displayed is first sent to the 74HCl64 before each update of the displayed value, and then a specific point on the digital tube is enabled via PC1-PC4; otherwise, misalignment will occur. 3.4 Heating Drive Circuit The ATmega8's I/O port output load capacity is a maximum of 40mA, which cannot directly drive high-power equipment such as electric furnaces and motors used in industrial environments. An intermediate drive circuit is necessary to achieve microcontroller control of the power equipment's operating status. In practical applications, relays or AC contactors are typically used for indirect driving. Because relays or AC contactors have mechanical contact characteristics, they significantly reduce the overall stability and reliability of the control system. To avoid the drawbacks of mechanical contact switches, this system uses a fully opto-isolated intermediate drive circuit based on a thyristor. A thyristor is a high-power switching semiconductor device capable of operating under high voltage and high current conditions. It offers advantages such as no mechanical contact, small size, and easy installation, and is widely used in power electronic equipment. A schematic diagram of the heating drive circuit is shown in Figure 3. The ATmega8 calculates the real-time control quantity based on the ambient temperature, the user-set target temperature, and relevant control parameters. This control quantity is written to the OClA register of the microcontroller's timer 1 to determine the duty cycle of the output PWM wave. During the high level of the PWM wave, the bidirectional optocoupler is powered on through the current-limiting protection resistor R4. The gate of the bidirectional thyristor TRIACl is triggered and turned on by the signal via R1, R2, and the bidirectional optocoupler, energizing the heating circuit. During the low level of the PWM wave, the bidirectional optocoupler is turned off, the gate of the bidirectional thyristor TRIACl is turned off without a trigger signal, and the heating circuit is de-energized and stops working. R3 and C2 in the circuit form a resistor-capacitor absorption unit, which can reduce the overvoltage impact on the thyristor caused by the self-induced electromotive force generated by the inductive element in the heating circuit when the thyristor is turned off. R1 and C1 form a low-pass filter unit, which can reduce the impact of false triggering of the bidirectional optocoupler on subsequent circuits. At the same time, the use of the bidirectional optocoupler completely isolates the strong and weak circuits, avoiding interference from high-power devices to the microcontroller. 4 Software Design The system program consists of a main program, a temperature acquisition subroutine, a heating control subroutine, a keyboard scanning subroutine, a serial communication subroutine, and an interrupt subroutine. The main program mainly completes the initialization and self-test of each component of the heating control system, as well as the coordination of various functional modules in actual measurement. The keyboard scanning and control algorithm subroutines utilize the rich interrupt resources of the ATmega8 to complete the above work in the external interrupt and timer overflow interrupt subroutines. Serial communication with the host computer uses the ATmega8's built-in UART hardware transmission interrupt to meet the asynchronous and real-time requirements of bidirectional data transmission. The flowcharts of the microcontroller's temperature acquisition subroutine and heating control subroutine are shown in Figure 4. The host computer monitoring program was developed using the Visual C++ 6.0 environment. The use of the Microsoft-provided MsComm control effectively avoids the cumbersome programming drawbacks of directly calling Win32 APIs, achieving the full-duplex asynchronous communication required by the system with less code. Users can complete operations such as setting temperature control parameters, saving temperature data, and offline analysis through the host computer program. 5. Conclusion The temperature measurement and heating control system designed by the author fully utilizes the features of the ATmega8 microcontroller and, combined with existing technology, greatly reduces the design complexity of the hardware circuit. The system has been designed and manufactured, and has achieved good results in experiments simulating a deep-sea high-temperature hydrothermal environment, exhibiting advantages such as accurate temperature control, a user-friendly interface, high stability, and strong anti-interference capabilities.