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successfully, the transmit message buffer automatically becomes a receive message buffer,
with the same ID as the remote frame which was transmitted.
When a remote frame is received by the FlexCAN, the remote frame ID is compared to the
IDs of all transmit message buffers programmed with a code of 1010. If there is an exact
matching ID, the data frame in that message buffer is transmitted. If the RTR bit in the
matching transmit message buffer is set, the FlexCAN will transmit a remote frame as a
response.
A received remote frame is not stored in a receive message buffer. It is only used to trigger
the automatic transmission of a frame in response. The mask registers are not used in
remote frame ID matching. All ID bits (except RTR) of the incoming received frame must
match for the remote frame to trigger a response transmission.
25.4.5 Overload Frames
Overload frame transmissions are not initiated by the FlexCAN unless certain conditions
are detected on the CAN bus. These conditions include:
• Detection of a dominant bit in the first or second bit of intermission.
• Detection of a dominant bit in the seventh (last) bit of the end-of-frame (EOF) field
in receive frames.
• Detection of a dominant bit in the eighth (last) bit of the error frame delimiter or
overload frame delimiter.
25.4.6 Time Stamp
The value of the free-running 16-bit timer is sampled at the beginning of the identifier field
on the CAN bus. For a message being received, the time stamp will be stored in the time
stamp entry of the receive message buffer at the time the message is written into that buffer.
For a message being transmitted, the time stamp entry will be written into the transmit
message buffer once the transmission has completed successfully.
The free-running timer can optionally be reset upon the reception of a frame into message
buffer 0. This feature allows network time synchronization to be performed.
25.4.7 Listen-Only Mode
In listen-only mode, the FlexCAN module is able to receive messages without giving an
acknowledgment. Whenever the module enters this mode the status of the Error Counters
is frozen and the FlexCAN module operates like in error passive mode. Since the module
does not influence the CAN bus in this mode the host device is capable of functioning like
a monitor or for automatic bit-rate detection.
MOTOROLA
Chapter 25. FlexCAN
Functional Overview
25-13
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