AN1212 Freescale Semiconductor / Motorola, AN1212 Datasheet - Page 14

• Manufacturer Part Number: AN1212
• Description: J1850 Multiplex Bus Communication Using the MC68HC705C8 and the SC371016 J1850 Communications Interface (JCI)
• Manufacturer: Freescale Semiconductor / Motorola

AN1212/D
Error Detection
14
J1850 Multiplex Bus Communication Using the MC68HC705C8
and the SC371016 J1850 Communications Interface (JCI)
the remainder of the frame is ignored. Any bits in the target address byte
corresponding to bits in the AMR which are not set are not compared.
Figure 7
Message filter is not currently available on the JCI in either handshake interface
mode. However, it may be available in the future as a factory mask option.
For more information the JCI’s message filtering capabilities, refer to the J1850
Communications Interface Specification, Chapter 4: MCU Interface.
The JCI uses a variety of methods to ensure the data transmitted onto or
received from the MUX bus is error-free. These include a digital input filter,
CRC generation and checking, and a constant monitoring of bit and symbol
timing, as well as message framing.
All data received from the MUX bus passes through a digital filter. This filter
removes short noise pulses from the input signal, which could otherwise
corrupt the data being received. The "cleaned up" signal is then passed to the
symbol decoder, which decodes the data stream, determining what each bit or
symbol is, whether it is of the proper length, and that the message is framed
properly.
The CRC byte is calculated by the JCI as it transmits a frame onto the MUX bus
and is then appended to the message following the data portion of the frame.
The CRC of any frame the JCI receives, including its own, is checked, and if it
is not correct, the frame is discarded.
Any frame in which any type of error is detected is discarded by the JCI. If the
JCI detects an error while it is transmitting a frame onto the MUX bus, it will
immediately halt transmission, wait for an idle bus, and attempt to retransmit
the frame. Following the detection of a transmission error, the JCI will attempt
to transmit a message up to two more times. Following the third attempt, the
Freescale Semiconductor, Inc.
For More Information On This Product,
illustrates this procedure.
X = MASKED BIT
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0
1
0
1
1
1
1
0
X
Figure 7. JCI Message Filtering
COMPARATOR
0
1
0
0
0
X
1
0
X
1
1
1
1
0
X
FRAME TARGET I.D. BYTE
ACCEPTANCE MASK REGISTER
COMPARE OUTPUT
(TRUE, FOR THIS EXAMPLE)
ACCEPTANCE CODE REGISTER
MOTOROLA