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IDLE SOF
DATA
PRIORITY
(DATA0)
MESSAGE
ID (DATA1)
BDLC MUX Interface
DATA_N
CRC
E
OPTIONAL
ON
DB
IFR
I
F
EOF _S IDLE
. J1850 Bus Message Format (VPW)
SOF — Start-of-Frame Symbol
All messages transmitted onto the J1850 bus must begin with a long-active 200-μs period SOF symbol.
This indicates 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 ID byte (typically
the physical address of the responder), 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 eight
bits in length and is 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. It also performs CRC detection on any messages it
receives from the J1850 bus.
CRC generation uses the divisor polynomial X⁸ + X⁴ + X³ + X² + 1. The remainder polynomial initially
is set to all ones. Each byte in the message after the start of frame (SOF) symbol is processed serially
through the CRC generation circuitry. The one’s complement of the remainder then becomes the 8-bit
CRC byte, which is appended to the message after the data bytes in MSB-to-LSB order.
When receiving a message, the BDLC uses the same divisor polynomial. All data bytes, excluding the
SOF and end of data symbols (EOD) but including the CRC byte, are used to check the CRC. If the
message is error free, the remainder polynomial will equal X⁷ + X⁶ + X² = $C4, regardless of the data
contained in the message. If the calculated CRC does not equal $C4, the BDLC will recognize this as
a CRC error and set the CRC error flag in the BSVR.
EOD — End-of-Data Symbol
The EOD symbol is a long 200-μs passive period on the J1850 bus used to signify to any recipients of
a message that the transmission by the originator has completed. No flag is set upon reception of the
EOD symbol.
MC68HC908AZ60A • MC68HC908AS60A • MC68HC908AS60E Data Sheet, Rev. 6
Freescale Semiconductor
341