MC68HC08AS32CAFN datasheet(71/296 Pages) FREESCALE

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MC68HC08AS32A — Rev. 1.1

Data Sheet

Freescale Semiconductor

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.

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.

MC68HC08AS32CAFN Datasheet(PDF) 71 Page - Freescale Semiconductor, Inc

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