Datasheet

MSCAN
MC9S12DP256 — Revision 1.1
MSCAN
value of the PRIO field is defined to be the highest priority. The internal
scheduling process takes place whenever the MSCAN arbitrates for the
bus. This is also the case after the occurrence of a transmission error.
When a high priority message is scheduled by the application software,
it may become necessary to abort a lower priority message in one of the
three transmit buffers. As messages that are already in transmission
cannot be aborted, the user has to request the abort by setting the
corresponding Abort Request bit (ABTRQ) (see MSCAN Transmitter
Message Abort Control (CANTARQ)). The MSCAN then grants the
request, if possible, by: 1) setting the corresponding Abort Acknowledge
flag (ABTAK) in the CANTAAK register, 2) setting the associated TXE
flag to release the buffer, and 3) generating a transmit interrupt. The
transmit interrupt handler software can tell from the setting of the ABTAK
flag whether the message was aborted (ABTAK=1) or sent (ABTAK=0).
Receive Structures The received messages are stored in a four stage input FIFO. The four
message buffers are alternately mapped into a single memory area (see
User Model for Message Buffer Organization). While the background
receive buffer (RxBG) is exclusively associated with the MSCAN, the
foreground receive buffer (RxFG) is addressable by the CPU (see User
Model for Message Buffer Organization). This scheme simplifies the
handler software as only one address area is applicable for the receive
process.
All receive buffers have a size of 15 bytes to hold the CAN control bits,
the identifier (standard or extended), the data contents and a time
stamp, if enabled (for details see Programmer’s Model of Message
Storage)
1
.
The Receiver Full flag (RXF) (see MSCAN Receiver Flag Register
(CANRFLG)) signals the status of the foreground receive buffer. When
the buffer contains a correctly received message with a matching
identifier, this flag is set.
On reception, each message is checked to see if it passes the filter (see
Identifier Acceptance Filter) and in parallel, is written into the active
1. Reference the Bosch CAN 2.0A/B protocol specification dated September 1991 for details.
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