How to set up a CANBus interface on the TX2

How to set up CAN on the TX2

Linux, and SPEAR, rely on a set of kernal modules originally developed by Volkswagen Engineering called "SocketCAN" for interfacing with the CAN network. Refer to this Wikipedia page for info about it.. Traditionally, CAN has worked by just sending characters to a file like /dev/can0, but this had the issue of only letting one process use it at a time (bad for literally anything!). SocketCAN alleviates this by setting up a network interface and extending the default Linux socket implementation to let any amount of processes send CAN frames at the same time.

We specifically use UAVCAN, which defines a message protocol (similar to ROS's messages) so that you don't just have to manually send arbritrary bytes around and figure out which CAN frame belongs to which node. It's a good thing.

For debugging CAN, I (Johann) recommend using uavcan_gui_tool. It should already be installed. If it isn't, go yell at someone install it.

Using CAN without Docker

For testing without anything connected

When testing without anything connected, you set up a virtual can interface, like this:

$ sudo modprobe can
$ sudo modprobe can_raw
$ sudo modprobe vcan
$ sudo ip link add dev vcan0 type vcan
$ sudo ip link set up vcan0
# Check to make sure it works (should show something similar to this):
$ ip -d -s -h -c link show vcan0
3: vcan0: <NOARP,UP,LOWER_UP> mtu 16 qdisc noqueue state UNKNOWN 
    link/can

Or, use the setup-vcan.bash script on the desktop.

For using with physical CAN nodes hooked up

Pretty much the same as testing without anything connected, but you modprobe can_dev instead of modprobe vcan, and set a whole bunch of parameters:

$ sudo modprobe can_dev
$ sudo modprobe can
$ sudo modprobe can_raw
$ sudo modprobe mttcan
$ sudo ip link set can0 \
	type can \
	bitrate 500000 \
	sjw 4 \
	dbitrate 1000000 \
	dsjw 4 \
	berr-reporting on \
	fd on \
	restart-ms 1000
$ sudo ip link set up can0
# Check to make sure it works:
$ ip -d -s -h -c link show can0

Or, use the setup-can.bash script on the desktop.

Explanation of parameters:

• type can: Set type of interface to can
• bitrate 500000: Set bitrate to 500kbit, which all of our devices use
• sjw 4: Allow controller to adjust for frames arriving at slightly wrong baud rates
• dbitrate 1000000: Sets the bitrate for any nodes that might support CAN FD
• dsjw 4: Adjust for frames coming from CAN FD nodes
• berr-reporting on: Allows for bus error reporting. Useful for figuring out why things don't work.
• fd on: Enables CAN FD. Regular CAN nodes will still work with this though - CAN FD frames are backwards-compatible.
• restart-ms 1000: Allows the can0 interface to restart if everything is inactive. Helps for when the buffer is filled.

Using CAN with Docker

See this section in the README.md.

Installing uavcan gui tool

Go to the project page for up-to-date steps, but as of 2020/01/27 the steps are:

$ sudo apt-get install -y python3-pip python3-setuptools python3-wheel
$ sudo apt-get install -y python3-numpy python3-pyqt5 python3-pyqt5.qtsvg git-core
$ sudo pip3 install git+https://github.com/UAVCAN/gui_tool@master

Open it by running uavcan_gui_tool at the command line.

I (Johann) personally recommend:

1. Set the uavcan_gui_tool's node id by hitting the checkmark next to the "Set local node ID" box
2. Open the bus monitor by going Panels > Bus Monitor (or just hit Ctrl-Shift-B)
3. Click the bus monitor's "Start / stop capturing" button to show UAVCAN (and other CAN) messages
4. Have fun 😁

Generating a pdf file from this markdown document

$ pandoc "$HOME/Desktop/CANBus - How To.md" --from=markdown_github --latex-engine=xelatex -o "$HOME/Desktop/CANBus - How To.pdf"