XC912BC32CFU8 Motorola Semiconductor Products, XC912BC32CFU8 Datasheet - Page 236

• Manufacturer Part Number: XC912BC32CFU8
• Description: M68HC12B Family Data Sheet
• Manufacturer: Motorola Semiconductor Products

Byte Data Link Communications (BDLC)
15.7.2.4 EOD — End-of-Data Symbol
15.7.2.5 IFR — In-Frame Response Bytes
15.7.2.6 EOF — End-of-Frame Symbol
15.7.2.7 IFS — Interframe Separation Symbol
Data Sheet
236
CRC generation uses the divisor polynomial X
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 equals
X
calculated CRC does not equal $C4, the BDLC recognizes this as a CRC error and
sets the CRC error flag in the BSVR.
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.
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.
This symbol is a long 280-µs passive period on the J1850 bus and is longer than
an end-of-data (EOD) symbol, which signifies the end of a message. Since an EOF
symbol is longer than a 200-µs 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.
The IFS symbol is a 20-µs passive period on the J1850 bus which allows proper
synchronization between nodes during continuous message transmission. The IFS
symbol is transmitted by a node after the completion of the end-of-frame (EOF)
period and, therefore, is seen as a 300-µs passive period.
When the last byte of a message has been transmitted onto the J1850 bus and the
EOF symbol time has expired, all nodes then must wait for the IFS symbol time to
expire before transmitting a start-of-frame (SOF) symbol, marking the beginning of
another message.
7
+ X
6
+ X
2
= $C4, regardless of the data contained in the message. If the
Byte Data Link Communications (BDLC)
8
+ X
4
+ X
3
M68HC12B Family — Rev. 8.0
+ X
2
+ 1. The remainder
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