FazCAN MPC2515 CAN Driver
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FazCAN MPC2515 CAN Driver Documentation

The FazCAN driver is a C/C++/Arduino driver for the MCP2515 CAN controller module. It allows the user to create a two-wire communication network between two or more CAN modules or to interface with other CAN networks.

To use the driver in the Arduino build environment, copy the CAN directory into the "libraries" subdirectory of your Arduino sketchbook directory.

To use the driver in a normal C/C++ build environment, open "my_spi.h" and delete the line that reads:

#include <Arduino.h>

For straight C code, rename "mcp2515.cpp" to "mcp2515.c." Both C and C++ will need to directly interface with mcp2515.c, as the C++ interface is currently dependent on the Arduino environment. The rest of this documentation describes using the Arduino interface.

To use the CAN module, you will need to include both the SPI.h and CAN.h headers in your sketch:

#include <SPI.h>
#include <CAN.h>

Before using the CAN module, you must call CAN.begin(), specifying the bus speed as an argument. Several standard speeds are defined in CAN.h, such as CAN_SPEED_125000, which indicates a speed for 125000 bits per second:

CAN.begin (CAN_SPEED_125000);

Alternatively, you can directly specify the length of each bit (called the bit period) in nanoseconds. If you want a bit period of 25 microseconds (equivalent to a bus speed of 40000 bits per second), you can specify it this way:

CAN.begin (25000);

Next you must set the CAN mode. The modes are defined in CAN.h. You will probably want to use CAN_MODE_NORMAL or CAN_MODE_LISTEN_ONLY.

CAN_MODE_NORMAL will allow the MCP2515 to transmit and receive on the CAN bus normally. CAN_MODE_LISTEN_ONLY only allows the MCP2515 to receive, preventing it from interacting on the bus. This ensures that the module does not interfere with regular network activity. (A CAN network requires at least two transmitting CAN nodes to function correctly. If you are creating your own network, you cannot have only one NORMAL and one LISTEN node, even if the NORMAL node is the only node sending messages. This is due to a technical detail of the CAN communication protocol. As long as there are at least two NORMAL nodes, everything can work fine.)

To determine if you have received a CanMessage, call CAN.available (). If this function returns true, there is a message to be retrieved. Retrieve the message by calling CAN.getMessage (). You must have declared a CanMessage variable to receive the message into.

CanMessage message;
if (CAN.available ()) {
message = CAN.getMessage ();

A CanMessage has an identifier, which is a number that indicates what kind of data that message has. There is no universal set of CAN identifiers; instead the identifier is specific to the application. If you are creating your own CAN network, you can pick the CAN identifiers. If you are interfacing with another network, you need to use the identifiers defined by that network protocol.

The CAN identifier can be either 11 bits or 29 bits. If the CAN message uses an 11-bit identifier, the "extended" flag of the CAN message will be false. If the message uses a 29-bit identifier, the "extended" flag will be true. The CAN ID and extended flags can be written and read directly from the message variable:

id = message.id;
if (message.extended) {
// Handle extended message
} else {
// Handle non-extended message

You only need to think about the extended flag if you are interfacing with an existing CAN network. If you are creating your own network, you can ignore this and use identifier numbers that are less than 2048 (hex: 0x7FF).

A CanMessage also has from zero to eight data bytes. The message identifier indicates what type of data is included in the message. If you are creating a CAN network of sensors that detect temperature and GPS position, you might use one identifier for a CanMessage whose data contains the temperature, and another identifier for a CanMessage whose data contains the GPS coordinates. When a receiver receives a message, it can use the identifier to determine how to use the data in the message. The extended flag does not change anything about the message data. Both extended and non-extended messages can have zero to eight bytes of data.

The data can be retrieved from the CanMessage in two ways. The first is to read the bytes of data directly from the message array:

byte0 = message.data[0];
byte1 = message.data[1];

The length field tells you how many bytes of data are in the message:

for (i = 0; i < message.len; i++) {
// Process data bytes

The second way is specific to this library, and may not work when interfacing with existing CAN networks. The functions getByteFromData, getIntFromData, and getLongFromData allow you to read one variable from the message. The reading begins at the beginning of the data array, and each read moves the reading position further along the message data. To correctly retrieve the data, you must read out the variables in the order they were set. Use the correct function for the length of the data you are trying to read:

long time;
byte pulses;
int temperature;
time = message.getLongFromData ();
pulses = message.getByteFromData ();
temperature = message.getIntFromData ();

Sending messages is very similar to receiving messages. Declare a CanMessage variable and set the identifier. (If you are using extended messages, remember to set the extended flag too).

CanMessage message;
message.id = 0x501;

Next set the data. You can do this in two ways. The first is by writing directly into the data array:

message.data[0] = my_val1;
message.data[1] = my_val2;
message.len = 2;

Note that the data array is an array of bytes, so you cannot store an int or long type in one array element:

int my_val = 12345;
message.data[0] = my_val; // WRONG

The second way is to use the set functions:

message.clear (); // Clears all previous data,
// 8 bytes available
message.setLongData (time); // Uses 4 bytes, 4 left
message.setByteData (pulses); // Uses 1 byte, 3 left
message.setIntData (temperature); // Uses 2 bytes, 1 left

The set functions store a variable in the data array and set the data length for you. You can use more than one set function to fill the message data with more than one variable. Remember that the message is only 8 bytes, so you cannot set more than two longs, four ints, eight bytes, or a combination adding up to eight. (See Ardunio documentation: byte, int, long)

After setting the identifier and data, a message is ready to be sent. You must wait for the CAN module to be ready before sending. When it is ready, the message can be sent by calling the send method:

while (CAN.ready () == false) {
// Wait for CAN to be ready
message.send ();

A CanMessage variable can be reused. After sending you can simply call send again to send the same message again. You can also clear the message by calling the clear function, then use the set functions to create a different message.

For more information, see the examples included in the FazCAN library.