FPGA Design in Automatic Temperature Control System of Diffusion Furnace

1 Introduction
At present, most of the temperature control equipment at home and abroad is single-channel control, which can only control one heating device. In China, the research and development of multi-channel temperature monitoring systems for high-temperature equipment is still relatively lagging. Most equipment communicates with the host computer through RS232 interface or other wired interface, and the wireless monitoring part is rarely involved. The design method proposed here is bold and innovative on the basis of existing technology and is challenging. The design of the hardware circuit is implemented by FPGA programming, which is convenient for circuit modification. The FPGA method is used to realize automatic control of the entire system, reduce costs, improve accuracy, and use ZigBee short-distance wireless transmission protocol to achieve wireless remote control.

2 FPGA Hardware Design and Implementation
2.1 Overview
The FPGA design uses ACTEL's Libero IDE integrated development environment. The internal circuit of the FPGA consists of an A/D conversion module, a PWM module, a 10-channel PWM control signal selection module, a PS2 module, a 50 Hz clock signal generation module, an alarm circuit module (FPGA implementation), an LCD display module, and a Core8051 module. Figure 1 shows the system circuit block diagram. These hardware modules form the entire control system, among which the Core 8051 module is the core of the entire FPGA internal circuit, and all data is processed and displayed through the 8051.

2.2 Design and implementation of circuit modules
(1) A/D conversion module The A/D conversion module in ACTEL FPGA embeds analog-to-digital conversion in FPGA and realizes different A/D conversion accuracy through software configuration. In the design of the diffusion furnace temperature control system, the analog signal input is set to 20 channels, including 10 amplified temperature sensor voltage signals and 10 manual control input voltage signals. This module uses the time-sharing sampling method to perform A/D conversion on the 20 analog signals, and transmits the conversion results, channel numbers and valid signals to the I/O port of 8051, and then reads the required channel conversion digital signals in the software.
(2) Glitch filtering module When the output result after A/D conversion is tested with a logic analyzer, it is found that the conversion valid signal DATAVALID has glitches. In order to ensure the accuracy of the 8051 signal input, the valid signal must be processed to ensure that the conversion result of MD is correctly collected to avoid signal collection errors. Since the frequency of DATAVALID is 2 MHz, the high-frequency 10 MHz clock signal can filter out the glitches. The design idea of ​​this module is to make the clock signal a high-frequency signal of 10 MHz, and filter out the burrs through the D flip-flop.
(3) According to the communication protocol of PS2, the PS2 module converts the input serial data into parallel data and a one-bit conversion valid enable signal. These signals are transmitted to 8051, and the software program is run for processing to realize the setting data input of the entire system, that is, the setting of each channel parameter. Since the effective signal generated by the high-frequency clock has a very narrow pulse signal, this signal is widened to 12 times the working clock cycle of the PS2 module during the hardware circuit design, so that this effective signal can be collected during the execution of the 8051 program. The connection of the PS2 module in the system is shown in Figure 2.

(4) 10-channel PWM control signal selection module The reset control signal PWMRST in the PWM module is used to control whether the channel is turned on. This reset signal is sent to the corresponding PWM channel to realize the on-off control of the channel. The control signal PWMDATA is used to control the duty cycle data, and this control data is sent to the corresponding PWM module.
(5) PWM module width adjustable pulse module (abbreviated as PWM) is used to control the on-off of the thyristor. The input signal of this module includes a 50Hz clock signal and a pulse duty cycle control signal, which is transmitted to the PWM module to control the duty cycle of the output signal to adjust the on-off ratio of the voltage loaded on both ends of the electric furnace wire within a clock cycle, so as to achieve the purpose of adjusting the heating power of the electric furnace wire. This module is a PWM with reset. The reset signal can be used as a port closing signal to directly control whether the channel is heated.
(6) Alarm circuit module The alarm circuit module includes two parts: sound alarm and light-emitting diode indication. Its control signals are all given by the 8051 soft core. One control instruction is directly output to the LED, and the other is connected to the 50 Hz signal selection end. When the 8051 gives the alarm data, the LED is high level, the red light is on, and the 50 Hz signal is selected and output to the buzzer end to complete the alarm.
(7) 50 Hz clock signal generation module The PWM module requires a 50 Hz working clock. The module generates the required clock by frequency division. The counter frequency division method is used to use the 2 MHz input signal as the counting pulse signal. The output pulse is the highest bit of the counter to achieve frequency division. The circuit is simple and occupies relatively few resources.
(8) LCD display module system design uses a 640x480 dot matrix LCD display screen. The control board made of MAX-EPM3128ACT CPLD replaces the LCD controller. The control board with 16 address ports, 8 data ports and 4 control ports is connected to the peripheral processor. Because there is no need to read data from the LCD screen, the 4 control ports only use the command/data selection control signal CMD and the write signal WR. The operation of LCD in display is to read and write a RAM, and the dot matrix information to be displayed can be written into the corresponding RAM address. There are two identical RAM modules inside, which can operate different parts of RAM, so as to increase the refresh frequency of LCD. The display memory data can be read and written alternately, and can be displayed while writing. The data/command selection signal of LCD controller is CMD signal. Figure 3 shows the connection principle diagram of LCD interface and FPGA.

(9) ZigBee wireless transmission circuit uses MCl3192 RF transceiver as ZigBee wireless transmission module. The software protocol embedded in the module supports collision-free serial communication function, which enables transparent transmission of multi-point TTL/2RS232/RS485 data streams without affecting each other. Data is sent from the serial port output of 8051 to the wireless transmission module. The feedback signal of success or failure of transmission is received at the serial data input of 8051. Both the transmission and reception of 8051 are TTL level signals. The other end of the ZigBee module is directly connected to the RS232 serial port of the PC to receive the data sent from 8051. Figure 4 shows the connection block diagram between modules.

(10) Instantiate Core8051 module The system software of this control system is implemented by 8051 programming. The 8051 soft core provided by ACTEL is the core of this control system. The system design is to directly call the soft core and then instantiate the 8051 soft core. The temperature control algorithm in the software is written using the incremental PID algorithm to achieve high-precision temperature control. Figure 5 shows the interface block diagram between the 8051 module inside the FPGA and the internal memory. Figure 6 shows the connection block diagram between Core8051 and the program memory. Figure 7 shows the connection block diagram between Core8051 and the A/D converter.

3 Conclusion
The research on the automatic temperature monitoring system of the diffusion furnace can not only be widely used in drying equipment, diffusion furnaces, and heat treatment equipment in the semiconductor manufacturing industry, but can also be transplanted and applied to temperature control systems in other industries such as textiles and chemicals. The design of the temperature control system of the diffusion furnace is implemented using Fusion FPGA, which is easy to change the circuit and has strong portability. Compared with the original system, it puts multiple A/D conversion, Core8051 core, PWM and other modules into one chip, which greatly reduces the cost; it adopts PID algorithm control to improve the accuracy; it completes the multi-channel temperature control and wireless transmission functions. The system has been applied to the temperature control system of the diffusion furnace in the integrated circuit process laboratory, and the control accuracy has been improved by 30% compared with the previous one. The digitalization process of the entire system has been completed, and good practical effects have been achieved.