COP884BC Datasheet, PDF (31/57 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 2k Memory, Comparators, and CAN Interface

Frame Formats (Continued)
FIGURE 24. CAN Frame Format
DS012067-59
START OF FRAME (SOF)
The Start of Frame indicates the beginning of data and re-
mote frames. It consists of a single “dominant” bit. A node is
only allowed to start transmission when the bus is idle. All
nodes have to synchronize to the leading edge (first edge af-
ter the bus was idle) caused by SOF of the node which starts
transmission first.
ARBITRATION FIELD
The arbitration field is composed of the identifier field and the
RTR (Remote Transmission Request) bit. The value of the
RTR bit is “dominant” in a data frame and “recessive” in a re-
mote frame.
ACK FIELD
The ACK field is two bits long and contains the ACK slot and
the ACK delimiter. The ACK slot is filled with a “recessive” bit
by the transmitter. This bit is overwritten with a “dominant” bit
by every receiver that has received a correct CRC se-
quence. The second bit of the ACK field is a “recessive” bit
called the acknowledge delimiter. As a consequence the ac-
knowledge flag of a valid frame is surrounded by two “reces-
sive” bits, the CRC-delimiter and the ACK delimiter.
EOF FIELD
The End of Frame Field closes a data and a remote frame. It
consists of seven “recessive” bits.
CONTROL FIELD
The control field consists of six bits. It starts with two bits re-
served for future expansion followed by the four-bit Data
Length Code. Receivers must accept all possible combina-
tions of the two reserved bits. Until the function of these re-
served bits is defined, the transmitter only sends “0” (domi-
nant) bits. The first reserved bit (IDE) is actually defined to
indicate an extended frame with 29 Identifier bits if set to “1”.
CAN chips must tolerate extended frames, even if they can
only understand standard frames, to prevent the destruction
of an extended frames on an existing network.
The Data Length Code indicates the number of bytes in the
data field. This Data Length Code consists of four bits. The
data field can be of length zero. The permissible number of
data bytes for a data frame ranges from 0 to 8.
DATA FIELD
The Data field consists of the data to be transferred within a
data frame. It can contain 0 to 8 bytes and each byte con-
tains 8 bits. A remote frame has no data field.
CRC FIELD
The CRC field consists of the CRC sequence followed by the
CRC delimiter. The CRC sequence is derived by the trans-
mitter from the modulo 2 division of the preceding bit fields,
starting with the SOF up to the end of the data field, exclud-
ing stuff-bits, by the generator polynomial:
χ¹⁵ + χ¹⁴ + χ¹⁰ + χ⁸ + χ⁷ + χ⁴ + χ³ + 1
The remainder of this division is the CRC sequence transmit-
ted over the bus. On the receiver side the module divides all
bit fields up to the CRC delimiter, excluding stuff-bits, and
checks if the result is zero. This will then be interpreted as a
valid CRC. After the CRC sequence a single “recessive” bit
is transmitted as the CRC delimiter.
INTERFRAME SPACE
Data and remote frames are separate from every preceding
frame (data, remote, error and overload frames) by the inter-
frame space see Figure 25 and Figure 26 for details. Error
and overload frames are not preceded by an interframe
space. They can be transmitted as soon as the condition oc-
curs. The interframe space consists of a minimum of three
bit fields depending on the error state of the node.
These bit fields are coded as follows:
The intermission has the fixed form of three “recessive” bits.
While this bit field is active, no node is allowed to start a
transmission of a data or a remote frame. The only action to
be taken is signaling an overload condition. This means that
an error in this bit field would be interpreted as an overload
condition. Suspend transmission has to be inserted by error-
passive nodes that were transmitter for the last message.
This bit field has the form of eight “recessive” bits. However,
it may be overwritten by a “dominant” start-bit from another
non error passive node which starts transmission. The bus
idle field consists of “recessive” bits. Its length is not speci-
fied and depends on the bus load.
ERROR FRAME
The Error Frame consists of two bit fields: the error flag and
the error delimiter. The error field is built up from the various
error flags of the different nodes. Therefore, its length may
vary from a minimum of six bits up to a maximum of twelve
bits depending on when a module detects the error. When-
ever a bit error, stuff error, form error, or acknowledgment er-
ror is detected by a node, this node starts transmission of the
error flag at the next bit. If a CRC error is detected, transmis-
sion of the error flag starts at the bit following the acknowl-
edge delimiter, unless an error flag for a previous error con-
dition has already been started. Figure 27 shows how a local
fault at one module (module 2) leads to a 12-bit error frame
on the bus.
The bus level may either be “dominant” for an error-active
node or “recessive” for an error-passive node. An error ac-
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