CAN bus is a message-based protocol allowing individual systems, devices, and controllers within a network to communicate. In general, a bus is a multi-node communication system that transfers data between components. A Controller Area Network allows for robust, low-latency, data transfer between sensors and compute units in a system. Without CAN Bus, wiring harnesses could contain miles of wire, with bundles of wires required to carry various signals to and from interconnected systems. In contrast, CAN bus utilizes a high-speed (25kbps - 1Mbps) twisted pair wiring system, greatly reducing the amount of wire necessary to allow system components to communicate.
The CAN protocol is a serial communication protocol that is used in the automotive industry for communicating between devices inside of a vehicle, the engine control unit and dashboard for example. Data is sent in frames and is done in such a way that if more than one device transmits at the same time the highest priority device is able to continue while the other devices back off. There are two CAN standards that are in use today, CAN 2.0 and CAN FD. CAN 2.0 is the older of the two protocols and has two parts; part A is for the standard format with an 11-bit identifier, commonly called CAN 2.0A, and part B if for the extended format with a 29-bit identifier, commonly called CAN 2.0B. Both parts can transmit data with bit rates up to 1MBit/s with up to 8 data bytes. CAN FD is a newer protocol that has flexible data-rate, an option for switching to a faster data rate, up to 5 Mbits/s, after the arbitration bits, which is limited to 1Mbits/s for compatibility with CAN 2.0, and it increases the max number of data bytes that can be transmitted in a frame to 64. CAN FD is compatible with existing CAN 2.0 networks so new CAN FD devices can coexist on the same network with existing CAN 2.0 devices.
There are several PIC® microcontrollers that have a built-in CAN 2.0 or CAN FD modules. For these devices, the CCS C Compiler comes with drivers for communicating with these protocols. There are separate drivers depending on the device being used. Additionally, the CCS C Compiler comes with several external CAN controllers. The following are a list of can drivers that are currently available in the CCS C Compiler:
- can-pic18f_ecan.c - PIC18 CAN 2.0
- can-pic24_dsPIC33.c - PIC24 and dsPIC33 CAN 2.0
- can-dspic30f.c - dsPIC30 CAN 2.0
- can-mcp2515.c - External MCP2515 controller CAN 2.0
- can-dspic33_fd.c - dsPIC33 CAN FD
- can-mcp2517.c - External MCP2517 controller CAN FD
J1939 is an upper level protocol that specifies how to send messages in a vehicle using the CAN 2.0 and CAN FD protocols. J1939 is maintained by SAE and the full J1939 specifications can be obtained from them. The J1939 is broken into several layers including, but not limited to, the Data Link Layer, Network Layer and Application Layer. These layers contain information about how to communicate on the network, how to claim an address, the format of messages, how often a message can be transmitted, etc. The CCS C Compiler comes with a J1939.c driver which is a library for the Data Link Layer running on a CAN 2.0 protocol network. The library has functions for claiming an address, responding to address claim messages, transmitting J1939 messages and receive J1939 messages.
Additionally, CCS also has several CAN development kits that can be used to aid in developing CAN Bus and J1939 projects. Each development kit has four nodes on it that can communicate with each other, as well as headers allowing the kit to be connected to an external Bus.
The first development kit CCS has is the CAN Bus development kit which has a PIC18F45K80 on the primary node and a PIC16F1938 on the secondary node. The primary node the PIC® MCU uses its built-in CAN peripheral for communicating on the Bus. The secondary node the PIC® MCU uses an external MCP2515 CAN controller for communicating on the Bus.
The second development kit CCS has is the CAN Bus 24 development kit which has a PIC24HJ256GP610 on the primary node and a dsPIC30F4012 on the secondary node. Like the previous kit, the primary node the PIC® MCU uses its built-in CAN peripheral for communicating on the Bus and the secondary node PIC uses an external MCP2515 CAN controller for communicating on the Bus.
The third development kit CCS has is CAN Bus FD which features a dsPIC33CH128MP506 on the primary node and a PIC16F18346 on the secondary node. The primary node the PIC® MCU uses its built-in CAN FD peripheral for communicating on the bus, and the secondary node the PIC® MCU uses an external MCP2517 CAN FD controller for communicating on the Bus.