MC68HC08AS32A Datasheet, PDF (79/296 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Byte Data Link Controller-Digital (BDLC-D)
BDLC Protocol Handler
ACTIVE
TRANSMITTER A
PASSIVE
ACTIVE
TRANSMITTER B
PASSIVE
ACTIVE
J1850 BUS
PASSIVE
0
1
1
1
TRANSMITTER A DETECTS
AN ACTIVE STATE ON
THE BUS AND STOPS
TRANSMITTING
0
1
1
0
0
0
1
1
0
0
TRANSMITTER B WINS
ARBITRATION AND
CONTINUES
TRANSMITTING
DATA DATA DATA
DATA DATA
SOF
BIT 1 BIT 2 BIT 3
BIT 4 BIT 5
Figure 4-13. J1850 VPW Bitwise Arbitrations
Since a logic 0 dominates a logic 1, the message with the lowest value will have
the highest priority and will always win arbitration. For instance, a message with
priority 000 will win arbitration over a message with priority 011.
This method of arbitration will work no matter how many bits of priority encoding
are contained in the message.
During arbitration, or even throughout the transmitting message, when an opposite
bit is detected, transmission is stopped immediately unless it occurs on the 8th bit
of a byte. In this case, the BDLC automatically will append up to two extra logic 1
bits and then stop transmitting. These two extra bits will be arbitrated normally and
thus will not interfere with another message. The second logic 1 bit will not be sent
if the first loses arbitration. If the BDLC has lost arbitration to another valid
message, then the two extra logic 1s will not corrupt the current message.
However, if the BDLC has lost arbitration due to noise on the bus, then the two
extra logic 1s will ensure that the current message will be detected and ignored as
a noise-corrupted message.
4.5 BDLC Protocol Handler
The protocol handler is responsible for framing, arbitration, CRC
generation/checking, and error detection. The protocol handler conforms to SAE
J1850 — Class B Data Communications Network Interface.
NOTE: Motorola assumes that the reader is familiar with the J1850 specification before
this protocol handler description is read.
MC68HC08AS32A — Rev. 1
MOTOROLA
Byte Data Link Controller-Digital (BDLC-D)
For More Information On This Product,
Go to: www.freescale.com
Data Sheet
79