TM4C1294NCPDT: CAN controller message objects understanding

Hi,

Abhijit Acharekar said:

1) If using just 1 message object for reception and before it is processed by application, a new message receives; will it override existing message in the message object?

The previous message will be lost. The incoming message will overwrite. Please see below in datasheet. 

The NEWDAT bit of the CANIFnMCTL register
is set to indicate that new data has been received. The CPU should clear this bit when it reads the
message object to indicate to the controller that the message has been received, and the buffer is
free to receive more messages. If the CAN controller receives a message and the NEWDAT bit is
already set, the MSGLST bit in the CANIFnMCTL register is set to indicate that the previous data
was lost.

Abhijit Acharekar said:

2) if using 2 message objects both with no filtering setting(accept all CAN messages), message in message object 1 is not yet processed by application, a new message receives; will it overwrite object 1 or copy to object 2.

It will have the same effect as the previous scenario. The reason is that the 2nd incoming message matches the acceptance filter of the first message object. But it didn't know that the first message object has not been read yet. What you can do is to configure for FIFO mode. In this case, the 2nd incoming message will go to the second message object. 

19.3.6 Accepting Received Message Objects
When the arbitration and control field (the ID and XTD bits in the CANIFnARB2 and the RMTEN and
DLC[3:0] bits of the CANIFnMCTL register) of an incoming message is completely shifted into
the CAN controller, the message handling capability of the controller starts scanning the message
RAM for a matching valid message object. To scan the message RAM for a matching message
object, the controller uses the acceptance filtering programmed through the mask bits in the
CANIFnMSKn register and enabled using the UMASK bit in the CANIFnMCTL register. Each valid
message object, starting with object 1, is compared with the incoming message to locate a matching
message object in the message RAM. If a match occurs, the scanning is stopped and the message
handler proceeds depending on whether it is a data frame or remote frame that was received.

If you configure message object 1 and message object 2 then see below description. Please refer to the datasheet more details and info. 

19.3.11.2 Reception of Messages with FIFO Buffers
Received messages with identifiers matching to a FIFO buffer are stored starting with the message
object with the lowest message number. When a message is stored into a message object of a
FIFO buffer, the NEWDAT of the CANIFnMCTL register bit of this message object is set. By setting
NEWDAT while EOB is clear, the message object is locked and cannot be written to by the message
handler until the CPU has cleared the NEWDAT bit. Messages are stored into a FIFO buffer until the
last message object of this FIFO buffer is reached. Until all of the preceding message objects have
been released by clearing the NEWDAT bit, all further messages for this FIFO buffer are written into
the last message object of the FIFO buffer and therefore overwrite previous messages.