MC68HC908AS60ACFN FREESCALE [Freescale Semiconductor, Inc], MC68HC908AS60ACFN Datasheet - Page 348

• Manufacturer Part Number: MC68HC908AS60ACFN
• Description: M68HC08 Microcontrollers
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

Byte Data Link Controller (BDLC)
27.4.5 Message Arbitration
Message arbitration on the J1850 bus is accomplished in a non-destructive manner, allowing the
message with the highest priority to be transmitted, while any transmitters which lose arbitration simply
stop transmitting and wait for an idle bus to begin transmitting again.
If the BDLC wants to transmit onto the J1850 bus, but detects that another message is in progress, it waits
until the bus is idle. However, if multiple nodes begin to transmit in the same synchronization window,
message arbitration will occur beginning with the first bit after the SOF symbol and will continue with each
bit thereafter.
The variable pulse width modulation (VPW) symbols and J1850 bus electrical characteristics are chosen
carefully so that a logic 0 (active or passive type) will always dominate over a logic 1 (active or passive
type) that is simultaneously transmitted. Hence, logic 0s are said to be dominant and logic 1s are said to
be recessive.
Whenever a node detects a dominant bit on BDRxD when it transmitted a recessive bit, the node loses
arbitration and immediately stops transmitting. This is known as bitwise arbitration.
348
TRANSMITTER A
TRANSMITTER B
J1850 BUS
PASSIVE
PASSIVE
PASSIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
PASSIVE
MC68HC908AZ60A • MC68HC908AS60A • MC68HC908AS60E Data Sheet, Rev. 6
Figure 27-10. J1850 VPW Received BREAK Symbol Times
Figure 27-11. J1850 VPW Bitwise Arbitrations
SOF
240 μs
DATA
BIT 1
0
0
0
DATA
BIT 2
1
1
1
DATA
BIT 3
1
1
1
e
1
DATA
BIT 4
0
0
DATA
BIT 5
0
0
(2) VALID BREAK SYMBOL
TRANSMITTER A DETECTS
AN ACTIVE STATE ON
THE BUS AND STOPS
TRANSMITTING
TRANSMITTER B WINS
ARBITRATION AND
TRANSMITTING
Freescale Semiconductor
CONTINUES