MC68HC908GZ8 Datasheet, PDF (166/344 Pages) Motorola, Inc – Microcontrollers

MSCAN08 Controller (MSCAN08)
To transmit a message, the CPU08 has to identify an available transmit buffer
which is indicated by a set transmit buffer empty (TXE) flag in the MSCAN08
transmitter flag register (CTFLG) (see 16.13.7 MSCAN08 Transmitter Flag
Register).
The CPU08 then stores the identifier, the control bits and the data content into one
of the transmit buffers. Finally, the buffer has to be flagged ready for transmission
by clearing the TXE flag.
The MSCAN08 then will schedule the message for transmission and will signal the
successful transmission of the buffer by setting the TXE flag. A transmit interrupt is
generated⁽¹⁾ when TXE is set and can be used to drive the application software to
re-load the buffer.
In case more than one buffer is scheduled for transmission when the CAN bus
becomes available for arbitration, the MSCAN08 uses the local priority setting of
the three buffers for prioritization. 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 as the highest priority.
The internal scheduling process takes place whenever the MSCAN08 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 MSCAN08
will then grant 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 actually aborted
(ABTAK = 1) or sent (ABTAK = 0).
16.5 Identifier Acceptance Filter
The identifier acceptance registers (CIDAR0–CIDAR3) define the acceptance
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–CIDMR3).
A filter hit is indicated to the application on software by a set RXF (receive buffer
full flag, see 16.13.5 MSCAN08 Receiver Flag Register (CRFLG)) and two bits in
the identifier acceptance control register (see 16.13.9 MSCAN08 Identifier
Data Sheet
166
1. The transmit interrupt will occur only if not masked. A polling scheme can be applied on TXE also.
MSCAN08 Controller (MSCAN08)
MC68HC908GZ8
Freescale Semiconductor