The UCAN Protocol

UCAN is the protocol used by the microcontroller-based USB-CANadapter that is integrated on System-on-Modules from Theobroma Systemsand 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 devicesinternally. All multi-byte integers are encoded as Little Endian.

All structures mentioned in this document are defined indrivers/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 allconfiguration commands target the corresponding interface in the USBdescriptor.

The driver usesucan_ctrl_command_in/out anducan_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 toreceive. If the device send less. Common ZLPsemantics are used.

Error Handling

The device indicates failed control commands by stalling thepipe.

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 ofUCAN_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 structureucan_ctl_payload_t.device_info.

See thedevice_info field for details, anduapi/linux/can/netlink.h for an explanation of thecan_bittimingfields.

Payload Format

ucan_ctl_payload_t.device_info

UCAN_COMMAND_GET_PROTOCOL_VERSION

Request the device protocol versionucan_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 oldprotocol version 1

UCAN_COMMAND_SET_BITTIMING

Host2Dev; mandatory

Setup bittiming by sending the structureucan_ctl_payload_t.cmd_set_bittiming (seestructbittiming fordetails)

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 moreucan_message_in values. If multiple messages are batched in a USBdata packet, thelen field can be used to jump to the nextucan_message_in value (take care to sanity-check thelen valueagainst the actual data size).

len field

Eachucan_message_in must be aligned to a 4-byte boundary (relativeto the start of the start of the data buffer). That means that theremay be padding bytes between multipleucan_message_in values:

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

type field

Thetype 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 alist of tuples <echo-ids, flags>.

The echo-id identifies the frame from (echos the id from a previousUCAN_OUT_TX message). The flag indicates the result of thetransmission. Whereas a set Bit 0 indicates success. All other bitsare reserved and set to zero.

Flow Control

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

OUT Message Format

A data packet on the USB OUT endpoint contains one or morestructucan_message_out values. If multiple messages are batched into onedata packet, the device uses thelen field to jump to the nextucan_message_out value. Each ucan_message_out must be aligned to 4bytes (relative to the start of the data buffer). The mechanism issame as described inlen field.

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

type field

In protocol version 3 onlyUCAN_OUT_TX is defined, others are usedonly 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 sendingNAKs ontheOUT pipe until more buffers are available. The driver stops thequeue when a certain threshold of out packets are incomplete.

CAN Error Handling

If error reporting is turned on the device encodes errors into CANerror frames (seeuapi/linux/can/error.h) and sends it using theIN endpoint. The driver updates its error statistics and forwardsit.

Although UCAN devices can suppress error frames completely, in Linuxthe driver is always interested. Hence, the device is always started withtheUCAN_MODE_BERR_REPORT set. Filtering those messages for theuser 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 (seeuapi/linux/can/error.h)

  • Bus OFF recovery is started byUCAN_COMMAND_RESTART

  • Once Bus OFF recover is completed the device sends an error frameindicating 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 completedimmediately with the success bit left unset.

Example Conversation

  1. Device is connected to USB

  2. Host sends commandUCAN_COMMAND_RESET, subcmd 0

  3. Host sends commandUCAN_COMMAND_GET, subcmdUCAN_COMMAND_GET_INFO

  4. Device sendsUCAN_IN_DEVICE_INFO

  5. Host sends commandUCAN_OUT_SET_BITTIMING

  6. Host sends commandUCAN_COMMAND_START, subcmd 0, modeUCAN_MODE_BERR_REPORT