mc68hc08qa24 Freescale Semiconductor, Inc, mc68hc08qa24 Datasheet - Page 346

• Manufacturer Part Number: mc68hc08qa24
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

CAN Controller
22.5.3 Transmit Structures
Technical Data
344
NOTE:
An overrun condition occurs when both the foreground and the
background receive message buffers that are filled with correctly
received messages and another message is being received from the
bus. The latter message will be discarded and an error interrupt with
overrun indication will occur if enabled. The over-writing of the
background buffer is independent of the identifier filter function. In the
overrun situation, the MSCAN08 will stay synchronized to the CAN bus.
While it is able to transmit messages, all incoming messages will be
discarded.
MSCAN08 will receive its own messages into the background receive
buffer RxBG but will not overwrite RxFG and will NOT emit a receive
interrupt. It also will not acknowledge (ACK) its own messages on the
CAN bus. The only exception to this rule is in loop-back mode when
MSCAN08 will treat its own messages exactly like all other incoming
messages.
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
All three buffers have a 13-byte data structure similar to the outline of the
receive buffers (see
Storage). An additional transmit buffer priority register (TBPR) contains
an 8-bit “local priority” field (PRIO) (see
Priority
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
Transmitter Flag
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.
Registers).
CAN Controller
Register).
22.13 Programmer’s Model of Message
22.13.5 Transmit Buffer
22.14.7 MSCAN08
Freescale Semiconductor
MC68HC08QA24
Figure
22-2.