MC68HC58 Motorola, MC68HC58 Datasheet - Page 44

• Manufacturer Part Number: MC68HC58
• Description: Data Link Controller
• Manufacturer: Motorola

During arbitration, or even when the frame is being transmitted, if an opposite bit (a
dominant bit when a recessive bit is transmitted, or a recessive bit when a dominant
bit is transmitted) is detected, transmission is immediately stopped unless it occurs on
the eighth bit of the fourth or later bytes. In this case, the DLC automatically appends
up to two extra 1 bits, then stops transmitting. These two extra 1 bits are arbitrated nor-
mally, and do not interfere with another frame. The second 1 bit is not sent if the first
loses arbitration. If the DLC loses arbitration to another valid frame, the two extra 1 bits
do not corrupt the current frame. However, if the DLC has lost arbitration due to noise
on the J1850 bus, the two extra 1 bits ensure that the current frame is detected and
ignored as a noise-corrupted frame.
Since a zero dominates a one, the frame with the lowest value has the highest priority,
and always wins arbitration (a frame with priority 000 wins arbitration over a frame with
priority 001). This method of arbitration works no matter how many bits of priority en-
coding are contained in the frame.
If the DLC loses arbitration during the transmission of a frame, it attempts to retransmit
as soon as a valid IFS is detected on the J1850 bus. The DLC attempts to retransmit
the frame indefinitely, as long as frames with higher priorities continue to win arbitra-
tion. The host MCU can terminate the transmission by transmitting the “terminate auto
retry” command to the DLC.
MOTOROLA
J1850 FRAME FORMAT
MC68HC58
3-10
TECHNICAL DATA