CAN

CAN (Controller Area Network) is a common real-time communication protocol used for high-speed, reliable data communication in automotive, industrial automation, robotics, and other fields.

Key concepts regarding CAN:

Multi-master, Multi-slave Architecture: CAN communication follows a multi-master, multi-slave architecture where multiple devices can simultaneously transmit and receive data, enabling efficient distributed communication.
Frame Format: CAN communication uses a frame format to transmit data. A basic frame consists of an identifier, data, control bits, and CRC (Cyclic Redundancy Check).
Identifier: Each CAN frame has a unique identifier used to specify the type and priority of the message. The identifier is either 11 bits or 29 bits in length, which determines the message priority.
Data Rate: CAN supports various data transmission rates, such as 125 Kbps, 500 Kbps, and 1 Mbps. You can choose the rate according to your needs.
Collision Detection: CAN uses the CSMA/CR (Carrier Sense Multiple Access / Collision Resolution) mechanism to detect and resolve data collisions.
Error Detection and Correction: CAN employs CRC to detect errors and has some error correction capabilities to ensure reliable data transmission.
Broadcast and Unicast: CAN supports broadcast messages, allowing all devices to receive the message. It also supports unicast through identifiers, where only specific devices will receive the message.
Application Areas: CAN communication is widely used in automotive internal networks, industrial automation, aerospace, robotics, and medical devices, among other fields.

CAN has been abstracted as a network device in Linux, so programming for the CAN bus is referred to as SocketCAN.

In Linux, to use CAN, you first need to set the communication baud rate for CAN, and then enable the CAN bus.

# View CAN interface.
sudo ip link list

# Set the CAN interface speed to 500000.
sudo ip link set can0 type can bitrate 500000

# Activate the CAN interface.
sudo ip link set up can0

# Disable the CAN interface.
sudo ip link set down can0

To demonstrate CAN programming, create a virtual CAN bus.

sudo modprobe can
sudo modprobe can_raw
sudo modprobe vcan
sudo ip link add dev vcan0 type vcan
sudo ip link set up vcan0
sudo ip link show vcan0

Then read and write the CAN bus in a C program.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#include <net/if.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>

#include <linux/can.h>
#include <linux/can/raw.h>

int main(void)
{
    int s;
    int nbytes;
    struct sockaddr_can addr;
    struct can_frame frame;
    struct ifreq ifr;

    const char *ifname = "vcan0";

    if ((s = socket(PF_CAN, SOCK_RAW, CAN_RAW)) == -1)
    {
        perror("Error while opening socket");
        return -1;
    }

    strcpy(ifr.ifr_name, ifname);
    ioctl(s, SIOCGIFINDEX, &ifr);

    addr.can_family = AF_CAN;
    addr.can_ifindex = ifr.ifr_ifindex;

    printf("%s at index %d\n", ifname, ifr.ifr_ifindex);

    if (bind(s, (struct sockaddr *)&addr, sizeof(addr)) == -1)
    {
        perror("Error in socket bind");
        return -2;
    }

    frame.can_id = 0x123;
    frame.can_dlc = 2;
    frame.data[0] = 0x11;
    frame.data[1] = 0x22;

    nbytes = write(s, &frame, sizeof(struct can_frame));

    printf("Wrote %d bytes\n", nbytes);

    return 0;
}

You can analyze packets on the vcan0 interface using the candump utility, which is part of the SocketCAN can-utils package. If can-utils is not installed, you can use the following command to install it.

sudo apt install can-utils

First, run the command in a terminal:

candump vcan0

Then, start another terminal to compile and run the above program, and you will be able to see in the terminal where the candump command is running:

  vcan0  123   [2]  11 22

After viewing the information, you can press ctrl + c to terminate the candump program.

When compiling the above program, you can compile and run it in the examples/linux_can directory of M5Stack_Linux_Libs.

# Clone repository
git clone https://github.com/M5STACK/M5Stack_Linux_Libs.git

# Enter directory
cd M5Stack_Linux_Libs/examples/linux_can

# Compile
scons 


#Quote：
#    https://zhuanlan.zhihu.com/p/470681140?utm_id=0

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