Arduino MCP2515 Library

  • Arduino CANBUS Library

    Wikipedia defines the CANBUS as ... a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle without a host computer. CAN bus is a message-based protocol, designed specifically for automotive applications but now also used in other areas such as industrial automation and medical equipment.

    I have recently designed some hardware that is based off of the Arduino platform that is used to interface with my vehicles network. Over the past year, many people have been interested in how I implemented the CANBUS libraries to the arduino system. Because there weren't too many resources out at the time to help me out, I figured it would be nice to help out the coding community by providing my MCP2515 library. Hope this helps!

  • Refereces

  • Initializing The Bus

    If you examine the code that I have hosted on GitHub, you will see one of the first methods in the MCP2515 class is the initCAN method. This is one of the more important methods in the class since it is going to initialize the controller and help us set the baud rate. The method requires two parameters, a baud rate, and the slave select pin number.

    * Initialize the MCP2515 library
    * @param	baudConst	The baud rate of the MCP2515 object
    * @param	ss			The slave select pin
    * @return				True if MCP2515 configured properly
    boolean MCP2515::initCAN(const int& baudConst, const int& ss)

    By allowing the developer to supply the slave select pin, we make this code more portable than some other solutions that actually have the slave select as a variable or a preprocessor definition. This worked great for me since in my application I have two different CANBUSes which each operate using a different slave select. For reference, the Arduino 128 and 328 use the following SPI pins.

    SPI Pin Arduino ATMEGA 128/328
    SS/CS 10
    MOSI 11
    MISO 12
    SCK 13

    One thing that is worth noting about this init method, is that it will leave the controller in CONFIGURATION mode. You must explicitly set the mode back to NORMAL after you call the init mode unless you have more configuration to do. The modes of operation are defined in the header file as an enumeration to make it easier to set a mode.

    * Enumeration that defines the operation mode for
    * the MCP chip
    enum CanMode {NORMAL,		 // Normal Mode
    		  CONFIGURATION, // Configuration Mode
    		  LISTEN};       // Listen only mode
  • Baud Rate Setting

    Setting the baud rate could be complicated if you were to do it manually. A method has been provided called setBaud to help the developer specify a baud rate for the controller

    * Set the CAN Baud Rate
    * @param	baudConst	The baud rate to set
    * @return				True if baud rate set
    boolean MCP2515::setCANBaud(const int& baudConst)

    The baudConst input is an integer based indicator of the baud rate you want to set. If you take a look at the MCP2515 header file, you will notice a few preprocessor defintions that represent the baud rate.

    // Data rate selection constants
    #define CAN_BAUD_10K 1
    #define CAN_BAUD_50K 2
    #define CAN_BAUD_100K 3
    #define CAN_BAUD_125K 4
    #define CAN_BAUD_250K 5
    #define CAN_BAUD_500K 6

    The first part of setting the baud is to calculate the Baud Rate Prescaler (BRP). The easiest way to do this is to download the CAN Timing Calculator referenced at the top of this page. Once you install the utility, you can enter the oscillator frequency (in my case, 16Mhz) and the expected baud rate.

    Once you enter the information, and click on the right arrow at the top you will be presented with some information about selecting the BRP and the time quantum. For my case, I pick the TQ of 16, therefore the BRP for a 500K baud is 0. Follow suit for the rest of the baud rates that you would like to configure in your library.

    byte brp;
    case CAN_BAUD_500K: brp = 0; break;
    case CAN_BAUD_250K: brp = 1; break;
    case CAN_BAUD_125K: brp = 3; break;
    case CAN_BAUD_100K: brp = 4; break;
    default: Serial.println("Non-Supported Baud Requested"); return false;