The UCAN Protocol

UCAN is the protocol used by the microcontroller-based USB-CAN adapter that is integrated on System-on-Modules from Theobroma Systems and that is also available as a standalone USB stick.

The UCAN protocol has been designed to be hardware-independent. It is modeled closely after how Linux represents CAN devices internally. All multi-byte integers are encoded as Little Endian.

All structures mentioned in this document are defined in drivers/net/can/usb/ucan.c.

USB Endpoints

UCAN devices use three USB endpoints:

CONTROL endpoint
The driver sends device management commands on this endpoint
IN endpoint
The device sends CAN data frames and CAN error frames
OUT endpoint
The driver sends CAN data frames on the out endpoint

CONTROL Messages

UCAN devices are configured using vendor requests on the control pipe.

To support multiple CAN interfaces in a single USB device all configuration commands target the corresponding interface in the USB descriptor.

The driver uses ucan_ctrl_command_in/out and ucan_device_request_in to deliver commands to the device.

Setup Packet

bmRequestType Direction | Vendor | (Interface or Device)
bRequest Command Number
wValue Subcommand Number (16 Bit) or 0 if not used
wIndex USB Interface Index (0 for device commands)
wLength
  • Host to Device - Number of bytes to transmit
  • Device to Host - Maximum Number of bytes to receive. If the device send less. Commom ZLP semantics are used.

Error Handling

The device indicates failed control commands by stalling the pipe.

Device Commands

UCAN_DEVICE_GET_FW_STRING

Dev2Host; optional

Request the device firmware string.

Interface Commands

UCAN_COMMAND_START

Host2Dev; mandatory

Bring the CAN interface up.

Payload Format
ucan_ctl_payload_t.cmd_start
mode or mask of UCAN_MODE_*

UCAN_COMMAND_STOP

Host2Dev; mandatory

Stop the CAN interface

Payload Format
empty

UCAN_COMMAND_RESET

Host2Dev; mandatory

Reset the CAN controller (including error counters)

Payload Format
empty

UCAN_COMMAND_GET

Host2Dev; mandatory

Get Information from the Device

Subcommands
UCAN_COMMAND_GET_INFO

Request the device information structure ucan_ctl_payload_t.device_info.

See the device_info field for details, and uapi/linux/can/netlink.h for an explanation of the can_bittiming fields.

Payload Format
ucan_ctl_payload_t.device_info

UCAN_COMMAND_GET_PROTOCOL_VERSION

Request the device protocol version ucan_ctl_payload_t.protocol_version. The current protocol version is 3.

Payload Format
ucan_ctl_payload_t.protocol_version

Note

Devices that do not implement this command use the old protocol version 1

UCAN_COMMAND_SET_BITTIMING

Host2Dev; mandatory

Setup bittiming by sending the structure ucan_ctl_payload_t.cmd_set_bittiming (see struct bittiming for details)

Payload Format
ucan_ctl_payload_t.cmd_set_bittiming.

UCAN_SLEEP/WAKE

Host2Dev; optional

Configure sleep and wake modes. Not yet supported by the driver.

UCAN_FILTER

Host2Dev; optional

Setup hardware CAN filters. Not yet supported by the driver.

Allowed interface commands

Legal Device State Command New Device State
stopped SET_BITTIMING stopped
stopped START started
started STOP or RESET stopped
stopped STOP or RESET stopped
started RESTART started
any GET no change

IN Message Format

A data packet on the USB IN endpoint contains one or more ucan_message_in values. If multiple messages are batched in a USB data packet, the len field can be used to jump to the next ucan_message_in value (take care to sanity-check the len value against the actual data size).

len field

Each ucan_message_in must be aligned to a 4-byte boundary (relative to the start of the start of the data buffer). That means that there may be padding bytes between multiple ucan_message_in values:

+----------------------------+ < 0
|                            |
|   struct ucan_message_in   |
|                            |
+----------------------------+ < len
          [padding]
+----------------------------+ < round_up(len, 4)
|                            |
|   struct ucan_message_in   |
|                            |
+----------------------------+
            [...]

type field

The type field specifies the type of the message.

UCAN_IN_RX

subtype
zero

Data received from the CAN bus (ID + payload).

UCAN_IN_TX_COMPLETE

subtype
zero

The CAN device has sent a message to the CAN bus. It answers with a list of tuples <echo-ids, flags>.

The echo-id identifies the frame from (echos the id from a previous UCAN_OUT_TX message). The flag indicates the result of the transmission. Whereas a set Bit 0 indicates success. All other bits are reserved and set to zero.

Flow Control

When receiving CAN messages there is no flow control on the USB buffer. The driver has to handle inbound message quickly enough to avoid drops. I case the device buffer overflow the condition is reported by sending corresponding error frames (see CAN Error Handling)

OUT Message Format

A data packet on the USB OUT endpoint contains one or more struct ucan_message_out values. If multiple messages are batched into one data packet, the device uses the len field to jump to the next ucan_message_out value. Each ucan_message_out must be aligned to 4 bytes (relative to the start of the data buffer). The mechanism is same as described in len field.

+----------------------------+ < 0
|                            |
|   struct ucan_message_out  |
|                            |
+----------------------------+ < len
          [padding]
+----------------------------+ < round_up(len, 4)
|                            |
|   struct ucan_message_out  |
|                            |
+----------------------------+
            [...]

type field

In protocol version 3 only UCAN_OUT_TX is defined, others are used only by legacy devices (protocol version 1).

UCAN_OUT_TX

subtype
echo id to be replied within a CAN_IN_TX_COMPLETE message

Transmit a CAN frame. (parameters: id, data)

Flow Control

When the device outbound buffers are full it starts sending NAKs on the OUT pipe until more buffers are available. The driver stops the queue when a certain threshold of out packets are incomplete.

CAN Error Handling

If error reporting is turned on the device encodes errors into CAN error frames (see uapi/linux/can/error.h) and sends it using the IN endpoint. The driver updates its error statistics and forwards it.

Although UCAN devices can suppress error frames completely, in Linux the driver is always interested. Hence, the device is always started with the UCAN_MODE_BERR_REPORT set. Filtering those messages for the user space is done by the driver.

Bus OFF

  • The device does not recover from bus of automatically.
  • Bus OFF is indicated by an error frame (see uapi/linux/can/error.h)
  • Bus OFF recovery is started by UCAN_COMMAND_RESTART
  • Once Bus OFF recover is completed the device sends an error frame indicating that it is on ERROR-ACTIVE state.
  • During Bus OFF no frames are sent by the device.
  • During Bus OFF transmission requests from the host are completed immediately with the success bit left unset.

Example Conversation

  1. Device is connected to USB
  2. Host sends command UCAN_COMMAND_RESET, subcmd 0
  3. Host sends command UCAN_COMMAND_GET, subcmd UCAN_COMMAND_GET_INFO
  4. Device sends UCAN_IN_DEVICE_INFO
  5. Host sends command UCAN_OUT_SET_BITTIMING
  6. Host sends command UCAN_COMMAND_START, subcmd 0, mode UCAN_MODE_BERR_REPORT