EMAC CAN Interface
The CAN interface is a vehicle bus standard that allows communication between a controller and various sensors. EMAC produces several boards capable of communicating on a CAN bus. For more information about which products support the CAN bus, see the hardware page.
Contents
Background
CAN (controller area network) is a multi-master serial bus for communicating between "nodes" over a two-wire interface. The wires are generally a 120 ohm twisted pair. Each CAN node has the ability to communicate with every other node, one at a time.
EMAC CAN Interface
The CAN bus is able to be accessed from the command line, or using a C library on EMAC products (with CAN support).
CAN Bus Setup
Before using the CAN interface on an EMAC device, two commands must be run. These commands could be scripted to be run at startup, see this page for details.
| NOTE |
| The settings in the first command may need to be changed according to the application in question. |
root@ipac9x25:~# ip link set can0 type can bitrate 125000 triple-sampling on
root@ipac9x25:~# ip link set can0 up Command Line CAN
The command line tools included with EMAC OE 5 are part of the canutils package. The tools included in this package are:
canconfigcandumpcanechocansendcansequence
Of these five, the most easily used are cansend and candump.
cansend
cansend, as the name implies, is used for sending CAN messages over the bus. Up to eight bytes at a time can be sent using cansend, these bytes are formatted at a space separated list of hexadecimal octets e.g. "0xBE 0x42 0xEF 0x9A." The command for sending the previous hexadecimal digits on the can0 device is:
root@ipac9x25:~# cansend can0 0xBE 0x42 0xEF 0x9A The received message, on a board running candump, will look like the following:
<0x001> [4] be 42 ef 9a
cansend uses, by default, an identifier of 0x01. If an identifier other than default, the -i 0x<IDENT> option must be passed on the command line. In the following example, the identity 0x89 is used.
root@ipac9x25:~# cansend can0 -i 0x89 0xBE 0x42 0xEF 0x9A The received message, on a board running candump, will look like the following:
<0x089> [4] be 42 ef 9a
Using the standard identity, the length of the identity is limited to a maximum hexadecimal value of 0x7FF. If a larger identity is needed, the -e option can be passed, which allows an extended CAN frame with a maximum hexadecimal value of 0x1FFFFFFF. The following example shows a message with an extended frame.
root@ipac9x25:~# cansend can0 -e -i 0x8008 0xBE 0x42 0xEF 0x9A The received message, on a board running candump, will look like the following:
<0x00008008> [4] be 42 ef 9a
candump
candump is used to receive messages over CAN. These messages can be output to stdout or directly to a file. candump will run until it is killed, either with Ctrl-C or a kill command. The following command shows how to receive any message over CAN and output it to stdout. A command of cansend can0 0xBE 0x42 0xEF 0x9A was given on a connected board.
root@ipac9x25:~# candump can0 interface = can0, family = 29, type = 3, proto = 1
<0x001> [4] be 42 ef 9a
To receive any message over CAN and output it to a file, the following command is used. A command of cansend can0 0xBE 0x42 0xEF 0x9A was given on a connected board.
root@ipac9x25:~# candump can0 -o /tmp/candump.txt interface = can0, family = 29, type = 3, proto = 1
Since candump will continue running, a Ctrl-C should be given before viewing the contents of the file. To view the file, cat can be used.
root@ipac9x25:~# cat /tmp/candump.txt <0x001> [4] be 42 ef 9a
candump can also filter its output according to two values: filter and mask. The filter [can_id]:[can_mask] matches when [received_can_id] & [can_mask] == [can_id] & [mask]. For example, to only receive messages from the identity 0x088, use the following command.
root@ipac9x25:~# candump can0 --filter 0x088:0x7FFC Programming CAN
Conclusion
CAN is a useful protocol for communicating quickly with sensors and other controllers. EMAC boards with CAN should be able to quickly integrate with existing CAN networks with little trouble.
