mc68hc912dg128 Freescale Semiconductor, Inc, mc68hc912dg128 Datasheet - Page 317

• Manufacturer Part Number: mc68hc912dg128
• Description: M68hc12 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

17.4.3 Transmit Structures
MC68HC912DG128 — Rev 3.0
MOTOROLA
NOTE:
An overrun condition occurs when both the foreground and the
background receive message buffers are filled with correctly received
messages with accepted identifiers and another message is correctly
received from the bus with an accepted identifier. The latter message is
discarded and an error interrupt with overrun indication is generated if
enabled. The msCAN12 is still able to transmit messages with both
receive message buffers filled, but all incoming messages are
discarded.
The msCAN12 will receive its own messages into the background
receive buffer RxBG but will not overwrite RxFG and will not emit a
receive interrupt nor will it acknowledge (ACK) its own messages on the
CAN bus. The exception to this rule is that when in loop-back mode
msCAN12 will treat its own messages exactly like all other incoming
messages.
The msCAN12 has a triple transmit buffer scheme in order 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
All three buffers have a 13 byte data structure similar to the outline of the
receive buffers (see
additional transmit buffer priority register (TBPR) contains an 8-bit so
called local priority field (PRIO) (see
(TBPR)).
In order to transmit a message, the CPU12 has to identify an available
transmit buffer which is indicated by a set transmit buffer empty (TXE)
flag in the msCAN12 transmitter flag register (CTFLG) (see
Transmitter Flag Register
The CPU12 then stores the identifier, the control bits and the data
content into one of the transmit buffers. Finally, the buffer has to be
flagged as being ready for transmission by clearing the TXE flag.
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MSCAN Controller
Programmer’s Model of Message
(CTFLG)).
Transmit Buffer Priority Registers
Storage). An
MSCAN Controller
Message Storage
Technical Data
msCAN12
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