CAN bus to RS232 design based on ATmega16 (1)

CAN (Controller Area Network) bus is a serial data communication protocol developed by German BOSCH Company to solve the data exchange between many control and test in modern automobile. It is a multi-master serial communication bus. CAN bus is an open, digital, multi-point communication control system LAN, featuring high communication speed, short transmission time, long transmission distance, strong error correction capability, simple control, strong expansion capability and high cost performance. One of the most widely used fieldbuses. However, the current PC is generally equipped with RS232 serial communication interface, and there is no CAN bus interface. In practice, many devices have RS-232 interface without CAN bus interface. In order to facilitate the data communication between the device with RS-232 interface and CAN bus, this paper completed the circuit design of CAN bus and RS-232 converter based on single chip ATMega16 control. The software design realizes the data transmission of CAN bus data and RS-232 interface device.

First, a brief description of CAN bus and RS232 protocol

As a multi-master serial communication bus, CAN's basic design specification requires high bit rate and high anti-electromagnetic interference performance, and it can detect any errors generated on the communication bus. When the signal transmission distance is up to 10km, CAN can still provide data transmission rate of up to 5kbit/s. Table 1 shows the maximum transmission distance between any two nodes on the CAN bus.

Correspondence with its bit rate.

It can be seen that the CAN bus is an excellent serial bus in terms of real-time, adaptability, flexibility, and reliability. When the two serial devices are far apart and cannot be directly connected by RS-232, RS-232 can be converted to CAN, and the serial port device network interconnection can be realized through the CAN bus.

Table 1 Maximum distance between any two sections of the CAN bus system

Both RS-232 and CAN are very different in level and frame format. The specific performance is as follows: RS-232 standard level ± (3V ~ 15V). The CAN signal is transmitted using differential voltage. The two signal lines are called “CAN_H” and “CAM_L”. The static time is about 2.5V. The frame format of the RS-232 serial port is: one start bit, eight data bits. , one programmable ninth bit, one stop bit. The data frame format of CAN is: frame information + ID + data.

Second, CAN bus to RS232 overall design

CAN bus to 2 3 2 interface hardware circuit, mainly selected MAX232, ATMega16, SJAl000 and MCP2551 design.

Its principle block diagram is shown in Figure 1.

Third, the converter hardware design

3.1 main controller design

The main controller selects ATMEL16's ATMega16 as the main processor. The chip is a low-voltage, high-performance microcontroller with EEPROM and random access data memory (RAM), compatible with standard MCS-51 command system. .

ATMega16 has 44 pins, 32 external bidirectional I/0 ports, and includes 3 external interrupt ports, 2 8-bit and 1 16-bit programmable timing counters, and 1 full-duplex serial communication port. ATMega16 It can be programmed in the usual way or online. ATMega16's 4-pin reset circuit, active low; 9,10 pin connected to MAX232 data receiving/transmitting terminal; PD port connected to SJAl000 control terminal; 7,8 pin connected to crystal oscillator circuit; PB port connected to SJAl000 Use the address / data bus.

3.2 MAx232 circuit design

The MAX232 chip was chosen as the level shifter for the RS232 and CAN buses. The MAX232 is a level-shifting driver that provides TTL and RS232 level conversion. Its 13 and 14 feet are connected to the 2, 3 pins of the 232 interface; the 11 and 12 pins are connected to the receiving and transmitting ends of the MCU.

[1] [2]