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MSCAN Controller
General Release Specification
MC68HC908AT32
—
Rev. 2.0
400
MSCAN Controller
MOTOROLA
Under all circumstances, at least three transmit buffers are required to
meet the first of the above requirements. The MSCAN08 has three
transmit buffers.
The second requirement calls for some sort of internal priorisation which
the MSCAN08 implements with the “l(fā)ocal priority” concept described in
23.5.2 Receive Structures
.
23.5.2 Receive Structures
The received messages are stored in a 2-stage input first in first out
(FIFO). The two message buffers are mapped using a Ping Pong
arrangement into a single memory area (see
Figure 23-2
). While the
background receive buffer (RxBG) is exclusively associated to the
MSCAN08, the foreground receive buffer (RxFG) is addressable by the
CPU08. This scheme simplifies the handler software, because only one
address area is applicable for the receive process.
Each buffer has 13 bytes to store the CAN control bits, the identifier
(standard or extended), and the data content (for details, see See
23.13
Programmer’s Model of Message Storage
.).
The receiver full flag (RXF) in the MSCAN08 receiver flag register
(CRFLG) (see
23.14.5 MSCAN08 Receiver Flag Register
) signals the
status of the foreground receive buffer. When the buffer contains a
correctly received message with matching identifier, this flag is set.
After the MSCAN08 successfully receives a message into the
background buffer, it copies the content of RxBG into RxFG
1
, sets the
RXF flag, and emits a receive interrupt to the CPU
2
. A new message,
which may follow immediately after the IFS field of the CAN frame, will
be received into RxBG.
The user’s receive handler has to read the received message from
RxFG and to reset the RXF flag to acknowledge the interrupt and to
release the foreground buffer.
1. Only if the RXF flag is not set.
2. The receive interrupt will occur only if not masked. A polling scheme can be applied on RXF
also.
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
.