Datasheet

Byte Data Link
MC9S12DP256 — Revision 1.1
Byte Data Link Controller Module
Figure 123 J1850 VPW Bitwise Arbitrations
During arbitration, or even throughout the transmitting message, when
an opposite bit is detected, transmission is immediately stopped unless
it occurs on the 8th bit of a byte. In this case the BDLC will automatically
append up to two extra 1 bits and then stop transmitting. These two extra
bits will be arbitrated normally and thus will not interfere with another
message.The second 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 ones will not corrupt the current message. However, if the BDLC
has lost arbitration due to noise on the bus, then the two extra ones will
ensure that the current message will be detected and ignored as a
noise-corrupted message.
Since a “0” dominates a “1”, the message with the lowest value will have
the highest priority, and will always win arbitration, i.e. 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.
Transmitter A
Transmitter B
J1850 Bus
SOF
Data
Bit 1
Data
Bit 4
Data
Bit 5
“0”
Transmitter A detects
an active state on
the bus, and stops
transmitting
Transmitter B wins
Passive
Active
Passive
Active
Passive
Active
“0”
“0”
“1”
“1”
“1”
Data
Bit 2
“1”
“1”
“1”
Data
Bit 3
“0”
“0”
“0”
“0”
“1”
arbitration and
continues
transmitting
Freescale Semiconductor, I
Freescale Semiconductor, Inc.
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