HC05V7GRSSLASHD datasheet(124/170 Pages) NXP

124 / 170 page

SECTION 15: MESSAGE DATA LINK CONTROLLER

Page 110

MC68HC05V7 Specification Rev. 1.0

If the MDLC is transmitting a message and the message received contains invalid bits, or

framing symbols on non-byte boundaries, this constitutes a transmission error. When a

transmission error is detected, the MDLC will immediately cease transmitting.

If the MDLC is receiving a message and it detects an error during the message, the

message is discarded. No message with an error contained in it is passed to the CPU and

no error status information is available.

15.4.3.5.1

CRC Error

A CRC error is detected when the data bytes and CRC byte of a received message are

processed, and the CRC calculation result is not equal to $C4. The CRC code should

detect any single and 2 bit errors, as well as all 8 bit burst errors, and almost all other types

of errors.

15.4.3.5.2

Bit Format Error

A Bit Format error is detected when an abnormal (invalid) bit is detected in a message

being received from the J1850 bus. However, if the MDLC is transmitting when this

happens, it will be treated as a loss of arbitration rather than a transmitter error and the

automatic retry mechanism will apply.

15.4.3.5.3

Message Format Error

A Message Format error is detected if an EOD or EOF symbol is detected on a non-byte

boundary from the J1850 bus.

15.4.3.5.4

Bus Fault

If a bus fault occurs, the response of the MDLC will depend upon the type of bus fault.

If the bus is shorted to Vbatt, the MDLC will wait for the bus to fall to a passive state before

it will attempt to transmit a message. As long as the short remains, the MDLC will never

attempt to transmit a message onto the J1850 bus.

If the bus is shorted to ground, the MDLC will see an idle bus, begin to transmit the

message, and then detect a transmission error, since the short to ground would not allow

the bus to be driven to the dominant state. The MDLC will abort that transmission and wait

for the next CPU command to transmit.

In any case, if the bus fault is temporary, as soon as the fault is cleared, the MDLC will

resume normal operation. If the bus fault is permanent, it may result in permanent loss of

communication on the J1850 bus.

15.4.3.5.5

Message Under Length Error

Any message received by the MDLC must be at least two bytes long. Since the MDLC does

not have message filtering, these two bytes could just be one data byte and a CRC byte.

Any messages received with less than the minimum number of bytes will be discarded by

the MDLC.

15.4.3.5.6

Message Over Length Error

A Message Over Length error is detected when a message being received exceeds the

maximum length allowed for a message on the J1850 bus. If the Receive Block mode

(RXBM) bit is not set, the maximum received length from SOF to the EOD is 12 bytes

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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