Functional Overview

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

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Motorola MCF5281, MCF5282 user manual Overload Frames, Time Stamp, Listen-Only Mode