MC68HC908AS60ACFN FREESCALE [Freescale Semiconductor, Inc], MC68HC908AS60ACFN Datasheet - Page 349

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

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.
27.5 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.
Freescale Semiconductor
Freescale assumes that the reader is familiar with the J1850 specification
before this protocol handler description is read.
MC68HC908AZ60A • MC68HC908AS60A • MC68HC908AS60E Data Sheet, Rev. 6
Figure 27-12. BDLC Block Diagram
PHYSICAL INTERFACE
PROTOCOL HANDLER
MUX INTERFACE
CPU INTERFACE
TO J1850 BUS
NOTE
TO CPU
BDLC
BDLC Protocol Handler
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