MC68HC908AT32 Datasheet, PDF (335/378 Pages) Freescale Semiconductor, Inc – Microcontrollers

IDLE
SOF
DATA
PRIORITY
(Data0)
MESSAGE ID
(DATA1)
BDLC MUX Interface
DATA_n
E
OPTIONAL
CRC
ON
DB
IFR
I
F
EOF S IDLE
Figure 28-5. 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 (least significant bit).
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 1s. 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.
IFR — In-Frame Response Bytes
The IFR section of the J1850 message format is optional. Users desiring further definition of in-frame
response should review the SAE J1850 Class B Data Communications Network Interface
specification.
MC68HC908AT32 Data Sheet, Rev. 3.1
Freescale Semiconductor
335