COP884BC Datasheet, PDF (32/57 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 2k Memory, Comparators, and CAN Interface

Frame Formats (Continued)
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 26
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.
DS012067-60
FIGURE 25. Interframe Space for Nodes Which Are Not
Error Passive or Have Been Receiver for the Last Frame
DS012067-61
FIGURE 26. Interframe Space for Nodes Which Are Error Passive
and Have Been Transmitter for the Last Frame
OVERLOAD FRAME
Like an error frame, an overload frame consists of two bit
fields: the overload flag and the overload delimiter. The bit
fields have the same length as the error frame field: six bits
for the overload flag and eight bits for the delimiter. The over-
load frame can only be sent after the end of frame (EOF)
field and in they way destroys the fixed form of the intermis-
sion field.
ORDER OF BIT TRANSMISSION
A frame is transmitted starting with the Start of Frame, se-
quentially followed by the remaining bit fields. In every bit
field the MSB is transmitted first.
FRAME VALIDATION
Frames have a different validation point for transmitters and
receivers. A frame is valid for the transmitter of a message, if
there is no error until the end of the last bit of the End of
Frame field. A frame is valid for a receiver, if there is no error
until and including the end of the penultimate bit of the End
of Frame.
FRAME ARBITRATION AND PRIORITY
Except for an error passive node which transmitted the last
frame, all nodes are allowed to start transmission of a frame
after the intermission, which can lead to two or more nodes
starting transmission at the same time. To prevent a node
from destroying another node’s frame, it monitors the bus
during transmission of the identifier field and the RTR-bit. As
soon as it detects a “dominant” bit while transmitting a “re-
cessive” bit it releases the bus, immediately stops transmis-
sion and starts receiving the frame. This causes no data or
remote frame to be destroyed by another. Therefore the
highest priority message with the identifier 0x000 out of
0x7EF (including the remote data request (RTR) bit) always
gets the bus. This is only valid for standard CAN frame for-
mat. Note that while the CAN specification allows valid stan-
dard identifiers only in the range 0x000 to 0x7EF, the device
will allow identifiers to 0x7FF.
There are three more items that should be taken into consid-
eration to avoid unrecoverable collisions on the bus:
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