CAN/Linux Basics

Why This Talk?

  • Everything is very complicated ((C) Fred Sinowatz) … if things are not kept apart

  • CAN bus is not usually associated with simplicity

  • ⟶ Linux makes it simple by keeping matters apart

    • Network programming ⟶ common knowledge

    • SocketCAN: donated by Volkswagen

    • Application is a network application

      • not bound to hardware

      • CAN controller independent ⟶ controllers can be exchanged

      • Programming language independent (Python is best of course 😘)

  • Special goodie: virtual CAN bus

    • ⟶ development and testing on the PC, no hardware at all needed

Network Programming: Datagram Exchange

Packet traffic (no connection) ⟶ datagrams

  • On the Internet: User Datagram Protocol (UDP)

  • Not a point to point connection like TCP

  • Broadcasts possible

  • Packet boundaries (as opposed to octet streams in TCP)

 ../../../../../../_images/datagram-socket.svg

And CAN?

  • Very small packets (max. 8 bytes payload)

  • Bus arbitration, packet prioritization, and “application” done by packet IDs

  • No addresses, only broadcasts

  • ⟶ looks like network, is network

../../../../../../_images/can-topo.svg

One CAN Network Packet …

  • Defined as a C struct

  • In host byteorder

  • Python: struct.pack(), struct.unpack()

#include <linux/can.h>

struct can_frame {
    canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
    __u8    can_dlc; /* frame payload length */
    __u8    data[8] __attribute__((aligned(8)));
};