`esw.can`

CAN bus access and DBC handling. Every tool that talks to a board goes through this module.

`esw.can.canbus`

CANBus

A context manager wrapping python-can and a cantools database. Opens a SocketCAN interface in FD mode with bit rate switching enabled.

CANBus(dbc: Database, channel: str, on_recv: Callable = lambda arg: None)

Method	Signature
`send`	`send(message_name: str, signals: dict[str, Any], src_id: int = 0, dest_id: int = 0) -> None`
`recv`	`recv(block: bool = True, timeout: float \\| None = None) -> Tuple[str, dict, int, int] \\| None`
`get_dbc`	`get_dbc() -> Database`

recv returns (message_name, signals, src_id, dest_id).

from esw.can.canbus import CANBus
from esw.can.dbc import get_dbc

with CANBus(get_dbc(dbc_name="MRoverCAN"), "can0") as bus:
    bus.send("BMCResetCmd", {"reset": 1, "clear_faults": 1}, dest_id=49)
    print(bus.recv(timeout=1.0))

Arbitration ID Layout

Messages carry a source and destination node ID packed into the arbitration ID alongside the DBC frame ID:

arbitration_id = frame_id + (src_id << 8) + dest_id

Constant	Value
`_CAN_DEST_ID_MASK`	`0x00FF`, offset 0
`_CAN_SRC_ID_MASK`	`0xFF00`, offset 8
`_MJBOTS_CAN_PREFIX`	`0x0000`

The same constants are emitted into the generated C++ header, so both sides agree. Frames with a base ID of 0x0000 are moteus traffic and are logged but not decoded.

`float2bits`

float2bits(value: float)

Reinterprets a float as its IEEE-754 bit pattern. Needed when sending a float through an integer DBC signal, which is what the configuration interface does. Raises TypeError on non-floats.

`esw.can.dbc`

`get_dbc`

get_dbc(filepath: Path | None = None, dbc_name: str | None = None) -> Database

Loads a DBC into a cantools database. With dbc_name, resolves <repo root>/dbc/<name>.dbc; with filepath, loads that path directly. One of the two is required. Cached, so repeated calls with the same argument are free.

Header Generation

generate_can_header(ctx: Path, dest: Path, files: list[str]) -> None

Renders <ctx>/templates/dbc_header.hpp.j2 once per DBC file into <dest>/<stem>.hpp. Invoked from lib/dbc/CMakeLists.txt during a firmware build, not usually by hand. See Generated Libraries.

Warning

generate_can_header catches every exception and prints it rather than raising. A malformed DBC produces a passing build with a missing or stale header, and the real failure surfaces later as a C++ compile error.

Two helpers back the template context:

get_c_type(signal) -> str maps a DBC signal to a C type, choosing width by bit length and float/double at the 32-bit boundary.
prepare_context(dbc_db, dbc_name) builds the full Jinja context: messages keyed by frame ID, per-signal types and lengths, and the sorted message name list.

DBC File

dbc/MRoverCAN.dbc is the single source of truth for the CAN protocol. It is consumed twice: by this module at runtime, and by the firmware build to generate C++ message classes.

Per-board message tables are on the board pages.

A tagged release packages the generated headers as mrover_can.tar.gz for the ros2 build, which needs the protocol without the Python toolchain. See Continuous Integration.