MC68HC908QL4 Datasheet, PDF (154/226 Pages) Motorola, Inc – Microcontrollers

Slave LIN Interface Controller (SLIC) Module
14.9.7.1 LIN Message Headers
All LIN message frame headers are comprised of three components:
• The first is the SYNCHRONIZATION BREAK (SYNCH BREAK) symbol, which is a dominant (low)
pulse at least 13 or more bit times long, followed by a recessive (high) synchronization delimiter of
at least one bit time. In LIN 2.0, this is allowed to be 10 or more bit times in length.
• The second part is called the SYNCHRONIZATION FIELD (SYNCH FIELD) and is a single byte
with value 0x55. This value was chosen as it is the only one which provides a series of five falling
(recessive to dominant) transitions on the bus.
• The third section of the message frame header is the IDENTIFIER FIELD (ID). The identifier is
covered more in 14.9.8 Handling Command Message Frames and 14.9.9 Handling Request LIN
Message Frames.
The SLIC automatically reads the incoming pattern of the SYNCHRONIZATION BREAK and FIELD and
determines the bit rate of the LIN data frame, as well as checking for errors in form and discerning
between a genuine BREAK/FIELD combination and a similar byte pattern somewhere in the data stream.
After the header has been verified to be valid and has been processed, the SLIC module updates the
SLIC bit time register (SLCBT) with the value obtained from the SYNCH FIELD and begins to receive the
ID.
If there are errors in the SYNCH BREAK/FIELD pattern, then an interrupt is generated. If unmasked, it
will trigger an MCU interrupt request and the resulting code in the SLIC state vector register (SLCSV) will
be an “Inconsistent-Synch-Field-Error,” based on the LIN protocol specification.
After the ID for the message frame has been received, an interrupt is generated by the SLIC and will
trigger an MCU interrupt request if unmasked. At this point, it might be possible that the ID was received
with errors such as a parity error (based on the LIN specification) or a byte framing error. If the ID did not
have any errors, it will be copied into the SLCD for the software to read. The SLCSV will indicate the type
error or that the ID was received correctly.
In a LIN system, the meaning and function of all messages, and therefore all message identifiers, is
pre-defined by the system designer. This information can be collected and stored in a standardized format
file, called a Configuration Language Description (CLD) file. In using the SLIC module, it is the
responsibility of the user software to determine the nature of the incoming message, and therefore how
to further handle that message.
The simplest case is when the SLIC receives a message which the user software determines is of no
interest to the application. In other words, the slave node does not need to receive or transmit any data
for this message frame. This might also apply to messages with zero data bytes (which is allowed by the
LIN specification). At this point, the user can set the IMSG control bit, and exit the interrupt service routine
by clearing the SLCIF flag. Because there is no data to be sent or received, the SLIC will not generate
another interrupt until the next message frame header or bus goes idle long enough to trigger a
“No-Bus-Activity” error according to the LIN specification.
NOTE
IMSG will prevent another interrupt from occurring for the current message
frame; however, if data bytes are appearing on the bus they may be
received and copied into the message buffer. This will delete any previous
data which might have been present in the buffer, even though no interrupt
is triggered to indicate the arrival of this data.
MC68HC908QL4 Data Sheet, Rev. 7
154
Freescale Semiconductor