Freescale Semiconductor, Inc.
MSCAN Controller
The msCAN12 will then schedule the message for transmission and will
signal the successful transmission of the buffer by setting the TXE flag.
A transmit interrupt will be emitted(1) when TXE is set and this can be
used to drive the application software to re-load the buffer.
If more than one buffer is scheduled for transmission when the CAN bus
becomes available for arbitration, the msCAN12 uses the local priority
setting of the three buffers for prioritisation. For this purpose every
transmit buffer has an 8-bit local priority field (PRIO). The application
software sets this field when the message is set up. The local priority
reflects the priority of this particular message relative to the set of
messages being emitted from this node. The lowest binary value of the
PRIO field is defined to be the highest priority.
The internal scheduling process takes places whenever the msCAN12
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 being set up
in one of the three transmit buffers. As messages that are already under
transmission cannot be aborted, the user has to request the abort by
setting the corresponding abort request flag (ABTRQ) in the
transmission control register (CTCR). The msCAN12 grants the request,
if possible, by setting the corresponding abort request acknowledge
(ABTAK) and the TXE flag in order to release the buffer and by
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 in the meantime (ABTAK=0).
17.5 Identifier Acceptance Filter
The identifier acceptance registers (CIDAR0–7) define the acceptable
patterns of the standard or extended identifier (ID10–ID0 or ID28–ID0).
Any of these bits can be marked don’t care in the identifier mask
registers (CIDMR0–7).
1. The transmit interrupt will occur only if not masked. A polling scheme can be applied on TXE
also.
Technical Data
MC68HC912DG128 — Rev 3.0
MSCAN Controller
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