RM0319 datasheet(196/368 Pages) STMICROELECTRONICS

196 / 368 page

Controller area network ports (CAN)

RM0319

196/368

Doc ID 022640 Rev 3

will be set after a successful transmission. If the CAN has lost the arbitration or if an error

occurred during the transmission, the message will be retransmitted as soon as the CAN

bus is free again. If meanwhile the transmission of a message with higher priority has been

requested, the messages will be transmitted in the order of their priority.

Acceptance filtering of received messages

When the arbitration and control field (Identifier + IDE + RTR + DLC) of an incoming

message is completely shifted into the Rx/Tx Shift register of the CAN Core, the message

handler FSM starts the scanning of the message RAM for a matching valid message object.

To scan the message RAM for a matching message object, the acceptance filtering unit is

loaded with the arbitration bits from the CAN core shift register. Then, the arbitration and

mask fields (including MsgVal, UMask, NewDat, and EoB) of message object 1 are loaded

into the acceptance filtering unit and compared with the arbitration field from the shift

object is found or until the end of the message RAM is reached.

If a match occurs, the scanning is stopped and the message handler FSM proceeds

depending on the type of frame (data frame or remote frame) received.

Reception of data frame

The message handler FSM stores the message from the CAN core Shift register into the

respective message object in the message RAM. Not only the data bytes, but all arbitration

bits and the data length code are stored into the corresponding message object. This is

implemented to keep the data bytes connected with the identifier even if arbitration mask

registers are used.

The NewDat bit is set to indicate that new data (not yet seen by the CPU) has been

received. The CPU should reset NewDat when it reads the message object. If at the time of

the reception the NewDat bit was already set, MsgLst is set to indicate that the previous

data (supposedly not seen by the CPU) is lost. If the RxIE bit is set, the IntPnd bit is set,

causing the Interrupt register to point to this message object.

The TxRqst bit of this message object is reset to prevent the transmission of a remote

frame, while the requested data frame has just been received.

Reception of remote frame

When a remote frame is received, three different configurations of the matching message

object have to be considered:

Dir = ‘1’ (direction = transmit), RmtEn = ‘1’, UMask = ‘1’ or ’0’

At the reception of a matching remote frame, the TxRqst bit of this message object is

set. The rest of the message object remains unchanged.

Dir = ‘1’ (direction = transmit), RmtEn = ‘0’, UMask = ’0’

At the reception of a matching remote frame, the TxRqst bit of this message object

remains unchanged. The remote frame is ignored.

Dir = ‘1’ (direction = transmit), RmtEn = ‘0’, UMask = ’1’

At the reception of a matching remote frame, the TxRqst bit of this message object is

reset. The arbitration and control field (Identifier + IDE + RTR + DLC) from the shift

this message object is set. The data field of the message object remains unchanged;

the remote frame is treated similar to a received data frame.

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