COP87L88EB Datasheet, PDF (29/72 Pages) National Semiconductor (TI) – 8-Bit CMOS OTP Microcontrollers with 16k or 32k Memory, CAN Interface, 8-Bit A/D, and USART

Frame Formats (Continued)
DS100044-26
FIGURE 21. CAN Frame Format
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.
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 22 and Figure 23 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 24 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-
tive node detecting an error, starts transmitting an active er-
ror flag consisting of six “dominant” bits. This causes the de-
struction of the actual frame on the bus. The other nodes
detect the error flag as either a violation of the rule of bit-
stuffing or the value of a fixed bit field is destroyed. As a con-
sequence all other nodes start transmission of their own er-
ror flag. This means, that the error sequence which can be
monitored on the bus as a maximum length of twelve bits. If
an error passive node detects an error it transmits six “reces-
sive” bits on the bus. This sequence does not destroy a mes-
sage sent by another node and is not detected by other
nodes. However, if the node detecting an error was the
transmitter of the frame the other modules will get an error
condition by a violation of the fixed bit or stuff rule. Figure 24
shows how an error passive transmitter transmits a passive
error frame and when it is detected by the receivers.
After any module has transmitted its active or passive error
flag it waits for the error delimiter which consists of eight “re-
cessive” bits before continuing.
DS100044-27
FIGURE 22. Interframe Space for Nodes Which Are Not
Error Passive or Have Been Receiver for the Last Frame
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