Datasheet

Byte Data Link Controller Module
Functional Description
MC9S12DP256 — Revision 1.1
Byte Data Link Controller Module
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 EOD symbols, but
including the CRC byte, are used to check the CRC. If the message is
error free, the remainder polynomial will equal X
7
+X
6
+X
2
($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 register.
EOD – End of Data
Symbol
The EOD symbol is a long 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 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.
EOF – End of
Frame Symbol
This symbol is a passive period on the J1850 bus, longer than an EOD
symbol, which signifies the end of a message. Since an EOF symbol is
longer than an EOD symbol, if no response is transmitted after an EOD
symbol, it becomes an EOF, and the message is assumed to be
completed.The EOF flag is set upon receiving the EOF symbol.
IFS – Inter-Frame
Separation
Symbol
The IFS symbol is a passive period on the J1850 bus which allows
proper synchronization between nodes during continuous message
transmission. The IFS symbol is transmitted by a node following the
completion of the EOF period.
When the last byte of a message has been transmitted onto the J1850
bus, and the EOF symbol time has expired, all nodes must then wait for
the IFS symbol time to expire before transmitting an SOF, marking the
beginning of another message.
However, if the BDLC is waiting for the IFS period to expire before
beginning a transmission and a rising edge is detected before the IFS
time has expired, it will internally synchronize to that edge.
Freescale Semiconductor, I
Freescale Semiconductor, Inc.
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