STR720 datasheet(242/401 Pages) STMICROELECTRONICS

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STR720 - CONTROLLER AREA NETWORK (CAN)

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After a partial write of a Message Object, the Message Buffer Registers that are not selected

in the Command Mask Register will set the actual contents of the selected Message Object.

After a partial read of a Message Object, the Message Buffer Registers that are not selected

in the Command Mask Register will be left unchanged.

18.7.2.2 Message Transmission

If the shift register of the CAN Core cell is ready for loading and if there is no data transfer

between the IFn Registers and Message RAM, the MsgVal bits in the Message Valid Register

and TxRqst bits in the Transmission Request Register are evaluated. The valid Message

Object with the highest priority pending transmission request is loaded into the shift register

by the Message Handler and the transmission is started. The NewDat bit of the Message

Object is reset.

After a successful transmission and also if no new data was written to the Message Object

(NewDat = ‘0’) since the start of the transmission, the TxRqst bit of the Message Control

(CAN_IFn_MCR) is set, IntPnd bit of the Interrupt Identifier register (CAN_IDR) will be set

after a successful transmission. If the CAN Peripheral 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. Meanwhile, if the transmission of a message with higher priority has been

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

18.7.2.3 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. The arbitration and mask

fields (including MsgVal, UMask, NewDat, and EoB) of Message Object 1 are then loaded into

the Acceptance Filtering unit and compared with the arbitration field from the shift register.

This is repeated with each following Message Object until a matching Message Object is

found or until the end of the Message RAM is reached.

If a match occurs, the scan is stopped and the Message Handler FSM proceeds depending on

the type of frame (Data Frame or Remote Frame) received.

STR720 数据表(PDF) 242 Page - STMicroelectronics