RM0319 datasheet(197/368 Pages) STMICROELECTRONICS

197 / 368 page

RM0319

Controller area network ports (CAN)

Doc ID 022640 Rev 3

197/368

Receive/Transmit Priority

The receive/transmit priority of the message object 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 requests are pending, they are serviced due to the priority of

the corresponding message object.

15.4

Operation

15.4.1

Initializing the software

The software initialization is started by setting the bit Init in the CAN Control register, either

by software or by a hardware reset, or by going Bus_Off.

While Init is set, any message transfer from and to the CAN bus is stopped, the status of the

CAN bus output CAN_TX is recessive (HIGH). The counters of the EML are unchanged.

Setting Init does not change any configuration register.

To initialize the CAN Controller, the CPU has to set up the Bit Timing register and each

message object. If a message object is not needed, it is sufficient to set its MsgVal bit to “not

valid”. Otherwise,the whole message object has to be initialized.

To enable access to the Bit Timing register and to the BRP Extension register for the

configuration of the bit timing, you must set both Init and CCE bits in the CAN Control

Resetting Init (by CPU only) finishes the software initialization. Afterwards, the Bit Stream

Processor BSP synchronizes itself to the data transfer on the CAN bus by waiting for the

occurence of a sequence of 11 consecutive recessive bits (= Bus Idle) before it can take

part in bus activities, and then starts the message transfer.

The initialization of the message objects is independent of the Init and can be done on the

fly, but the message objects should all be configured to particular identifiers or set to “not

valid” before the BSP starts the message transfer.

To change the configuration of a message object during normal operation, the CPU has to

start by setting MsgVal to “not valid”. When the configuration is completed, MsgVal is set to

valid again.

15.4.2

Transferring messages

Once the CAN controller is initialized and Init is reset to zero, the CAN core synchronizes

itself to the CAN bus and starts the message transfer.

Received messages are stored into their appropriate message objects if they pass the

message handler’s acceptance filtering. The whole message including all arbitration bits,

DLC and eight data bytes is stored into the message object. If the identifier mask is used,

the arbitration bits which are masked to “don’t care” may be overwritten in the message

object.

The CPU may read or write each message any time via the interface registers, the message

handler guarantees data consistency in case of concurrent accesses.

Messages to be transmitted are updated by the CPU. If a permanent message object

(arbitration and control bits set up during configuration) exists for the message, only the data

bytes are updated and then TxRqst bit with NewDat bit are set to start the transmission. If

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