1 Introduction
With the gradual automation and modernization of industrial control systems, the field bus control system has received more and more attention and application. The CAN bus is a field bus that is currently simple to develop and has a high cost performance. Compared with other fieldbuses, CAN communication controllers have the most manufacturers, the most complete varieties, and the most extensive applications. Based on the intelligence and complexity of the fieldbus control system, as the core component microprocessor of the fieldbus, the traditional 51 chip and even ARM7 have gradually failed to meet the needs, and ARM9 has become a suitable choice. However, many ARM9s do not have an integrated CAN interface, and expanding the CAN interface has become a top priority. And ARM9 separates the data address bus, and the CAN controller multiplexes for the data address bus, so that the expansion of CAN cannot simply copy the way of the 51 processor. In the development of CAN module drivers, existing materials are often only introduced for driver development on Linux systems, and the real-time requirements of fieldbus control systems make Linux systems give way to more real-time operating systems.
This article introduces a control system based on the ARM920T field bus, details the two ways to expand the CAN bus on it, and gives specific CAN module driver development steps on the real-time operating system eCos, and finally expands the two A brief comparison was made.
2. Introduction to the fieldbus control system
The fieldbus control system uses EP9315 as the core processor. EP9315 is an industrial grade processor based on ARM920T developed by Cirrus Logic. The fieldbus control system has an expanded CAN bus interface, 64Mb SDRAM and 32MB FLASH, and has a PCMCIA interface, EPROM interface, 512kb SRAM, IDE interface, real-time clock, video display, color VGA TFT LCD touch screen, Support analog VGA connection, video decoding support compressed video output and S-VIDEO output, PS / 2 keyboard, three USB interfaces, three serial interfaces, audio interface, 1/10 / 100Mbps Ethernet interface, infrared receiving port; implementation It has realized the digitalization and intelligentization of the control system; it can realize multiple functions, localize the control function of the system, improve the reliability and real-time performance of the system; and simplify the structure of the system. The multi-interface system structure makes the expansion, modification, disassembly and assembly of the system more flexible and convenient.
3. Hardware system expansion
3.1 CAN bus module structure
This article uses SJA1000T as the CAN controller to expand the CAN module. SJA1000T is an independent CAN bus controller used in automobiles and general industrial environments. It has all the necessary features to complete the CAN high-performance communication protocol; SJA1000T with a simple bus connection can complete all functions of the physical layer and data link layer. Support CAN2.0 protocol.
The CAN bus transceiver TJA1040T is an interface chip between the CAN controller and the physical bus, which enhances the driving capability of the bus, thereby increasing the communication distance of the CAN bus and allowing more nodes to be hung on a bus.
In order to enhance the anti-interference ability of CAN bus nodes, TXO and RXO of SJA1000T are not directly connected to TXD and RXD of TJA1040, but are connected to TJA1040T through two optocoupler isolation chips 6N137. The electrical isolation between the transceiver and the controller is well implemented to protect the safe operation of the core circuit of the intelligent node; and the electrical isolation between the CAN nodes on the bus is realized. It should be emphasized that in order to achieve this electrical isolation, the DC power supplies on both sides of the optocoupler must be two DC power supplies with no direct electrical connection and isolated from each other. Therefore, it adopts two DC-DC isolated power supplies. To further enhance safety and anti-jamming capabilities, a current limiting resistor can be connected in series between the bus transceiver TJA1040T and the CAN bus to prevent the TJA1040T from being subjected to overcurrent impact. At the same time, a filter capacitor is connected in parallel between CANH and CANL and ground, which can filter out high-frequency interference on the bus and have certain anti-electromagnetic radiation capabilities. In addition, when the communication signal is transmitted on the line, the signal will be reflected when it is transmitted to the end of the wire, and the reflected signal will interfere with the transmission of the normal signal. To eliminate this effect, matching resistors can be connected in parallel at both ends of the CAN bus to play the dual role of matching the bus impedance and eliminating reflections. If these measures are ignored, the anti-interference and reliability of data communication will be greatly reduced, and even communication will not be possible.
3.2 Two connection methods of CAN module and ARM9
The SJA1000T data address bus is multiplexed, while the entire ARM9 series includes the ARM920T data and address bus separately. This makes it impossible to connect the ARM9 series with the traditional expansion connection for the 51 single-chip series. This article provides two expansion methods: all IO port connection and the least IO port data signal line connection.
3.2.1 IO interface connection
The general-purpose IO port of the microprocessor can provide a convenient method to control the SJA100T. The GP93 signal of EP9315 provides a great degree of flexibility to meet the time requirements of SJA1000T. Connect the SJA1000T data address multiplexing bus to the general-purpose IO interface of EP9315. Other signal lines WR, RD, ALE ... are also connected to the IO interface. Refer to Figure 2 for specific connection methods.
3.2.2 Data signal cable connection
In addition to all controlling the CAN module by the IO port, the data line and signal line of the ARM9 can also be used to realize the expansion of the CAN module. The connection circuit is shown in Figure 3. All signals other than SJA1000T are signal pins on the EP9315 chip. The SJA1000T data address multiplex bus is connected to the EP9315 data line. GPIOx, GPIOy, GPIOz can be any GPIO pin of EP9315, but choose The GPIO port of the same channel will make programming easier. Note that the INT of the SJA100T can be connected to the INT of the CPU or to the general-purpose IO with interrupts. In EP9315, GPIO channels A, B, F have interrupt function.
Figures 2 and 3 are schematic diagrams. Specifically, since the EP9315 signal high level is 3.3V and the SJA1000T high level is 5V, it is necessary to perform level matching through a level matching chip such as 74LVC245 before connecting.
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