Fdcanusb

mjcanfd-usb-1x / fdcanusb

The mjcanfd-usb-1x and fdcanusb provide access to a CAN-FD bus using a USB 2.0 full speed connection. They present two possible interfaces:

1. A gs_usb interface compatible with the Linux kernel gs_usb socketcan driver
2. serial-like interface over the USB port using the CDC ACM class, i.e. a virtual COM port.

The designs and firmware are licensed under the Apache 2.0 License.

Pre-assembled hardware can be purchased at https://mjbots.com/products/fdcanusb

Usage with moteus tools and libraries

To use a device with moteus tools or libraries, first install the relevant udev rule following the instructions in the top of the file:

• 70-fdcanusb.rules

Second, just use the application. moteus utilities like moteus_tool, tview, and the python and C++ libraries will automatically discover any connected devices and use the CDC protocol.

USB CDC Protocol

Each command or response is a single newline terminated line, composed of 7-bit ASCII encoded characters. Every command sent from the client to the device has a response which consists of an optional command specific response followed by a final line consisting solely of OK or ERR <details>.

States

The device can be in one of two states.

1. Bus Off: This is the initial state of the device. In this state, no frames may be sent or received, but the device may be configured.

2. Bus On: In this state frames may be sent and received, but configuration is locked out.

Commands

Sample commands are prefixed with > to show the data sent from the client to the device and < to show the values sent from the device to the client. Those characters are for illustrative purposes only, and are not present in actual communications.

conf

The "conf" command is used to read and set configuration values. It has several subcommands.

conf enumerate

This emits all current configuration values one after another.

conf get

This queries the value of one specific configuration item.

>conf get sample.value
<31235
<OK

conf set

This sets the value of one specific configuration itme.

>conf set sample.value 1234
<OK

conf load

This loads all persistent configuration from the persistent storage.

>conf load
<OK

conf write

This stores all configuration values to persistent storage. They will then be the default when the device is powered on the in the future.

>conf write
<OK

conf default

Reset all configuration values to their "default" state.

>conf default
<OK

can

can on

Enter the "Bus On" state. All configurable values are validated and put into place. This results in an error if the device is already in the "Bus On" state.

can off

Enter the "Bus Off" state. Transmission and reception of CAN messages is halted, and configurable values may be changed.

can std

Send a standard CAN frame. The following format is used.

can std <HEXID> <HEXDATA> <options>

options may be zero or more of the following optionally separated by spaces.

1. B/b require/disable bitrate switching
2. F/f require/disable FDCAN format
3. R/r require/disable remote frame

can ext

Send an extended CAN frame. The following format is used.

can ext <HEXID> <HEXDATA> <options>

The allowable options are the same as for can std.

can send

Just like std or ext, but auto-detect the ID type.

can status

Report the current mode and status of various flags.

>can status
<mode:BUSON
<OK

Receiving CAN Frames

When in the "Bus On" state, the device may spontaneously emit the following lines upon receipt of valid CAN frames or other errors.

rcv

A CAN frame has been received.

<rcv <HEXID> <HEXDATA> <flags>

flags may be zero of more of the following separated by spaces:

1. E/e frame was received with extended/classic ID
2. B/b frame was received with/without bitrate switching
3. F/f frame was received in fdcan/classic mode
4. R/r frame was remote/data frame
5. tNNNNN timestamp of receipt measured in microseconds
6. fNN integer ID of which filter matched this frame

Configurable Values

The following items may be configured.

• can.bitrate - The bitrate used for the CAN frame (or header only if bitrate switching is used.

• can.fd_bitrate - The bitrate used for the data and CRC field when bitrate switching is used.

• can.sample_point - The sample point for the nominal part of the CAN frame. moteus requires 0.666, some other systems default to 0.75, 0.8, or 0.87.

• can.fd_sample_sample - The sample point for the data (BRS) part of the CAN frame. moteus requires 0.666, other systems may require alternate settings.

• can.automatic_retransmission - When true/non-zero, frames will be retried until acknowledged.

• can.fdcan_frame - By default, send frames in FDCAN mode

• can.bitrate_switch - By default, switch to the can.fd_bitrate for the data and CRC fields.

• can.restricted_mode - Only transmit acknowledgements.

• can.bus_monitor - Transmit nothing.

• can.termination - Enable the onboard terminator

• can.autostart - Enter BusOn immediately at power-on

• can.autorecover - Automatically recover from bus off events

• can.global.std_action

• can.global.ext_action

• can.global.remote_std_action

• can.global.remote_ext_action

  • 0/1 - accept
  • 2 - reject

• can.filter.N.id1 - first ID

• can.filter.N.id2 - second ID or bitmask

• can.filter.N.mode

  • 0 - range
  • 1 - exact match
  • 2 - bitmask

• can.filter.N.type

  • 0 - standard
  • 1 - extended

• can.filter.N.action

  • 0 - disable
  • 1 - accept
  • 2 - reject

gs_usb interface

To use the device with a socketcan driver on Linux, the normal socketcan utilities can be used. For instance, to bring up an interface compatible with moteus (not usually required because moteus tools and libraries will by default use the CDC interface), you would do:

ip link set can0 up type can \
  bitrate 1000000 dbitrate 5000000 \
  sjw 10 dsjw 5 \
  sample-point 0.666 dsample-point 0.666 \
  fd on

Then, all messages received can be displayed with:

candump can0

Building firmware

To build the firmware on an Ubuntu 22.04 or newer system.

sudo apt install curl openocd
./tools/bazel build //...

To flash:

./flash.py