Datasheet

Byte Data Link
MC9S12DP256 — Revision 1.1
Byte Data Link Controller Module
Figure 117 J1850 Bus Message Format (VPW)
SAE J1850 states that each message has a maximum length of 101 bit
times or 12 bytes (excluding SOF, EOD, NB and EOF).
SOF – Start of
Frame Symbol
All messages transmitted onto the J1850 bus must begin with an long
active SOF symbol. This indicates to any listeners on the J1850 bus the
start of a new message transmission. The SOF symbol is not used in the
CRC calculation.
Data – In Message
Data Bytes
The data bytes contained in the message include the message
priority/type, message I.D. byte, and any actual data being transmitted
to the receiving node. The message format used by the BDLC is similar
to the 3 Byte Consolidated Header message format outlined by the SAE
J1850 document. See SAE J1850 – Class B Data Communications
Network Interface, for more information about 1 and 3 Byte Headers.
Messages transmitted by the BDLC onto the J1850 bus must contain at
least one data byte, and therefore can be as short as one data byte and
one CRC byte. Each data byte in the message is 8 bits in length,
transmitted MSB to LSB.
CRC – Cyclical
Redundancy
Check Byte
This byte is used by the receiver(s) of each message to determine if any
errors have occurred during the transmission of the message. The BDLC
calculates the CRC byte and appends it onto any messages transmitted
onto the J1850 bus, and also performs CRC detection on any messages
it receives from the J1850 bus.
CRC generation uses the divisor polynomial X
8
+X
4
+X
3
+X
2
+1. The
remainder polynomial is initially set to all ones, and then each byte in the
message after the SOF symbol is serially processed through the CRC
generation circuitry. The one’s complement of the remainder then
SOF
EOD
EOF
Priority
Message
Data
n
CRC IFR
IFS
Idle
Idle
ID (Data1)
(Data0)
Optional
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Freescale Semiconductor, Inc.
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