HC05V7GRSSLASHD datasheet(128/170 Pages) NXP

128 / 170 page

SECTION 15: MESSAGE DATA LINK CONTROLLER

Page 114

MC68HC05V7 Specification Rev. 1.0

15.5.2

J1850 FRAME FORMAT

All messages transmitted on the J1850 bus are structured using the format:

Each message has a maximum length of 12 bytes (excluding SOF, EOD, NB and EOF).

15.5.2.1

SOF - Start of Frame Symbol

All messages transmitted onto the J1850 bus must begin with an 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.

15.5.2.2

Data - In Message Data Bytes

The data bytes contained in the message include the message header bytes and any actual

data being transmitted to the receiving node. The MDLC can be used to transmit messages

using any of the three header formats outlined in the SAE J1850 document. However, the

MDLC does not utilize the IFR, Header type or functional/physical addressing bits defined

in the 1st byte of the 3-byte consolidated header message format. See

SAE J1850 - Class

B Data Communications Network Interface, for more information about 1- and 3-byte

headers.

Messages transmitted by the MDLC 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.

15.5.2.3

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 MDLC 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 X8+X4+X3+X2+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 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 MDLC 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 X7+X6+X2 ($C4),

regardless of the data contained in the message. If the calculated CRC does not equal $C4,

the MDLC will recognize this as a CRC error, and will discard the message, not informing

the CPU of the failure.

SOF

EOF

Datan

CRC

IFR

Idle

Figure 15-10:

J1850 Bus Message Format (VPW)

Optional

Data1

Data0

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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