Motorola MCF5281, MCF5282 Serial Message Buffers SMBs, Transmit Message Buffer Deactivation

Functional Overview

25.4.3.1 Serial Message Buffers (SMBs)

To allow double buffering of messages, the FlexCAN has two shadow buffers called serial message buffers. These two buffers are used by the FlexCAN for buffering both received messages and messages to be transmitted. Only one SMB is active at a time, and its function depends upon the operation of the FlexCAN at that time. At no time does the user have access to or visibility of these two buffers.

25.4.3.2 Transmit Message Buffer Deactivation

Any write access to the control/status word of a transmit message buffer during the process of selecting a message buffer for transmission immediately deactivates that message buffer, removing it from the transmission process.

If the user deactivates the transmit MB while a message is being transferred from a transmit message buffer to a SMB the message will not be transmitted.

If the user deactivates the transmit message buffer after the message is transferred to the SMB, the message will be transmitted, but no interrupt will be requested and the transmit code will not be updated.

If a message buffer containing the lowest ID is deactivated while that message is undergoing the internal arbitration process to determine which message should be sent, then that message may not be transmitted.

25.4.3.3 Receive Message Buffer Deactivation

Any write access to the control/status word of a receive message buffer during the process of selecting a message buffer for reception immediately deactivates that message buffer, removing it from the reception process.

If a receive message buffer is deactivated while a message is being transferred into it, the transfer is halted and no interrupt is requested. If this occurs, that receive message buffer may contain mixed data from two different frames.

Data should never be written into a receive message buffer. If this is done while a message is being transferred from an SMB, the control/status word will reflect a full or overrun condition, but no interrupt will be requested.

MOTOROLA Chapter 25. FlexCAN 25-11