MC68HC08AZ32A Datasheet, PDF (329/456 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
MSCAN08 Controller (MSCAN08)
Message Storage
20.5.3 Transmit Structures
The MSCAN08 has a triple transmit buffer scheme to allow multiple
messages to be set up in advance and to achieve an optimized real-time
performance. The three buffers are arranged as shown in Figure 20-2.
All three buffers have a 13-byte data structure similar to the outline of the
receive buffers (see Programmer’s Model of Message Storage on
page 346). An additional transmit buffer priority register (TBPR) contains
an 8-bit “local priority” field (PRIO) (see Transmit Buffer Priority
Registers on page 352).
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 MSCAN08
Transmitter Flag Register on page 365).
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 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 as the highest priority.
1. The transmit interrupt will occur only if not masked. A polling scheme can be applied on TXE
also.
MC68HC08AZ32A — Rev 1.0
MOTOROLA
MSCAN08 Controller (MSCAN08)
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Technical Data
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