Freescale Semiconductor MPC5604B, Quick Start Manual

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1.2. Frame Description
CAN protocol uses variable length frames (depending on data size or standard/extended ID) as
data frames.
Figure 112 : CAN 2.0A standard frame format
The figure above shows different fields included in a frame. Here’s brief descriptions of some
fields:

SOF(Start of Frame): a dominant bit set by the transmitter. Other devices will not try to
transmit from now on.

Arbitration field: in standard 2.0A, this 12-bit field is made of a 11-bit ID and a RTR
(Remote Transmission Request) bit. ID is a way of identifying a context for the message
but it also sets the messages priority; priority is higher for lower IDs. RTR allows a
device to request data transmission from another by sending a frame with no data when
it is recessive. In standard 2.0B, this field is extended to 32-bit with a new id field of 18-
bits (so making a total ID of 29-bits) and with two recessive flag bits.

Command field: has two reserved bits (for future standards, one of them is used in CAN
2.0B to specify an extended ID or not) at dominant state and a 4-bit DLC (Data Length
Code) field which indicates the number of bytes contained within the frame (limited to
8).

Data field: is made of up to 8 bytes of data, depending on DLC’s value. MSB is transferred
first in each byte.

CRC (Cyclic Redundancy Code) field is a 15-bit error checking code followed by a
recessive delimiter bit.

ACK (Acknowledge) field has one ACK slot where transmitter sends recessive and
receiver has to send dominant to acknowledge. This bit is followed by another recessive
delimiter bit.

EOF (End of Frame) is made of 7 successive recessive bits.

INT(Interframe) is the separator between two successive frames, made of at least three
recessive bits.
There are also error frames that are made of 6-12 dominant bits which is superposition of
different devices error flags. These flags are followed by 8 recessive delimiter bits. Six
dominant bits point to an active error flag and six recessive bits is emitted on a passive error
flag.