COP888EB Datasheet, PDF (32/75 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 8k Memory, CAN Interface, 8-Bit A/D, and USART

Frame Formats (Continued)
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 25
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.
DS012837-27
FIGURE 22. Interframe Space for Nodes Which Are Not
Error Passive or Have Been Receiver for the Last Frame
DS012837-28
FIGURE 23. Interframe Space for Nodes Which Are Error Passive
and Have Been Transmitter for the Last Frame
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