Design of Temperature Controller Based on ARM and CPLD

1 Introduction

With the rapid development of computer technology, people require more accurate measurement and control of analog physical quantities such as temperature in daily life and production. This not only meets the needs of real-time monitoring of industrial sites, remote observation, telemetry and remote control of host PCs, but also requires connection to the Internet to achieve remote monitoring and access to digital and intelligent sensor functions.

Here we propose a temperature control system with ARM microcontroller as the core and CPLD technology. The system converts the information collected by the temperature sensor into A/D and transmits it to the microprocessor for processing. The processed data is then remotely transmitted via the network interface 121. Or it communicates with the host PC through the RS232 serial interface to realize a distributed temperature monitoring system.

2 System Hardware Design

The system design is mainly developed for field instruments in the field of industrial control. Its hardware design block diagram is shown in Figure 1. The block diagram includes modules such as ARM microprocessor, power supply, monitoring reset, memory expansion (RAM, Flash and EEPROM), human-machine exchange interface (LED), network communication, temperature detection circuit, A/D conversion, D/A conversion output, RS232 communication and CPLD control circuit (decoding and configuration of peripheral devices to realize the hardware and software of the system).

The temperature sensor measures the external temperature information. Under the control of CPLD, the digital signal after A/D conversion is sent to the ARM microprocessor for processing. At the same time, the processing information is displayed through the LCD and remotely monitored by the network interface. Of course, the keyboard can also be used for real-time manual intervention on site. The data processed by the ARM microprocessor is transmitted to the host computer through the RS232 serial port for display and storage. Of course, if an error occurs during the working process, an audible and visual alarm will be generated. At the same time, the keyboard can also intervene in the setting on site and handle faults.

Here, the ARM microprocessor is the 32-bit AT91M40800 from ATMEL. In addition to the ARM7TDMI core, the AT91M40800 also integrates many peripherals, and a large number of internal registers can quickly complete interrupt processing. Since the AT91M40800 microprocessor is connected to the off-chip memory through the programmable EBI, it has a faster access speed; at the same time, it also has 8 priority vector interrupt controllers connected to the external data controller, thereby improving the interrupt response speed. Therefore, the AT91M40800 microprocessor is very suitable for the field of industrial real-time control and is the best choice for processors in embedded industrial controllers.

2.1 Network communication interface circuit design

AT91M40800 itself does not have an ETHERNET interface, and needs to use an external Ethernet controller to realize network functions. Considering the cost performance, the system uses the RTL8019AS Ethernet controller produced by ReaItek, which expands an ETHERNET interface. The RTL8019AS connection circuit is shown in Figure 2.

The JP pin of RTL8019AS is connected to VCC to make it work in jumper (configuration mode) mode, and read and write operations are performed in I/O mode. NETCS is the chip select signal of AT91M40800 to RTL8019AS, and the address is 0x03000300~0x0300031F. LED0 and LED1 are each connected to a light-emitting diode to indicate the communication status. [page]

The 10BASE-T wiring standard is used to achieve Ethernet communication through twisted pair cables. Since the RTL8019AS has a built-in 10BASE-T transceiver, the network interface circuit is relatively simple. Only an external isolation low-pass filter (LPF) 20F-01 is required to connect to the external network. TPIN± is the receiving line, and TPOUT± is the transmitting line. After isolation, they are connected to the RX± and TX± ends of the RJ-45 interface respectively.

RTL8019AS works in interrupt mode. When a data packet is received, NETINT (interrupt signal) outputs a low level and notifies the processor to read the data. The CPU starts the remote DMA, NETCS (chip select signal) and NETRD (read signal) are valid, and then reads data from the internal RAM of RTL8019AS. NETRST (reset signal) is generated by the control circuit CPLD, and the high level is valid to ensure the reliable reset of RTL8019AS.

2.2 CPLD control circuit

CPLD mainly decodes the RTL8019AS Ethernet controller, LCD display and MAX197 control circuit. Altera's CPLDEPM7032A is selected. The device has 600 available gates, 32 macro cells, 38 user I/O pins, uses 3.3 V CMOS technology, and has 5 V tolerant input. The MAX+PLUSII development platform is used to complete the required design in the graphical editor. The internal design of EPM7032A is shown in Figure 3.

The network interface module is also the focus of this design. Its main program is responsible for completing the initialization of system parameters and real-time calling tasks. The main program uses polling to continuously detect the status word and the return value of the function to determine whether an event has occurred. If it has occurred, it will process the event and return to the main program after processing to continue executing the following program. The main program flow of the network interface module is shown in Figure 5.

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The program first initializes the system and allocates a memory buffer pool for processing the TCP/IP protocol; then the network parameters are initialized; then the serial port, timer, ARP buffer and RTL8019AS are initialized in turn; finally, a while infinite loop detects the status word, and each subroutine performs related processing.

4 System Debugging

System debugging mainly includes hardware debugging, software debugging and system simulation comprehensive debugging. First, the experimental board is powered on and connected to the computer parallel port through its corresponding ARM and CPLD JTAG interface, and then the corresponding compiled and simulated programs are downloaded and burned into ARM and CPLD respectively, and each module is debugged in turn, which basically meets the functions of temperature signal acquisition and processing. The parameter results after debugging meet the requirements and the operation status is good.

5 Conclusion

With embedded systems as the research direction, facing the industrial control field, and taking instruments as the application object, we built and developed a general platform for embedded industrial controllers based on ARM and CPLD, and completed the specific design of embedded industrial controllers based on ARM processors. The system test results are good, the performance is stable, and the expected design goals are achieved. The general platform for embedded industrial controllers based on ARM and CPLD has broad application prospects and can be widely used in control fields such as industrial and agricultural detection, intelligent control, etc., providing a useful reference for future system upgrades.