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STR720 - CONTROLLER AREA NETWORK (CAN)

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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 in the corresponding Message Object. This is done

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 application software should reset NewDat bit when the Message Object has been read. If

at the time of 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:

1) 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.

2) 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.

3) 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 register is stored

in the Message Object of the Message RAM and the NewDat bit of 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.

18.7.2.4 Receive/Transmit Priority

The receive/transmit priority for the Message Objects is attached to the message number.

Message Object 1 has the highest priority, while Message Object 32 has the lowest priority. If

more than one transmission request is pending, they are serviced due to the priority of the

corresponding Message Object.

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