STR720 datasheet(204/401 Pages) STMICROELECTRONICS

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STR720 - CONTROLLER AREA NETWORK (CAN)

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18.4 Functional Description

18.4.1

Software Initialization

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

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

While the Init bit is set, all message transfers to and from the CAN bus are stopped and the

status of the CAN_TX output pin is recessive (HIGH). The Error Management Logic (EML)

counters are unchanged. Setting the Init bit does not change any configuration register.

To initialize the CAN Controller, software has to set up the Bit Timing Register and each

Message Object. If a Message Object is not required, the corresponding MsgVal bit should be

cleared. Otherwise, the entire Message Object has to be initialized.

Access to the Bit Timing Register and to the BAud Rate Prescaler (BRP) Extension Register

for configuring bit timing is enabled when the Init and Configuration Change Enable (CCE) bits

in the CAN Control Register are both set.

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

Processor (BSP) (see Section 18.7.10:

Configuring the Bit Timing on page 249)

synchronizes itself to the data transfer on the CAN bus by waiting for the occurrence of a

sequence of 11 consecutive recessive bits (

≡ Bus Idle) before it can take part in bus activities

and start the message transfer.

The initialization of the Message Objects is independent of 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, software has to

start by resetting the corresponding MsgVal bit. When the configuration is completed, MsgVal

is set again.

18.4.2

CAN Message Transfer

Once the CAN Peripheral is initialized and Init bit is cleared, the CAN Peripheral Core

synchronizes itself to the CAN bus and starts the message transfer.

Received messages are stored in 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 are stored in 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.

Software can read or write each message any time through the Interface Registers and the

Message Handler guarantees data consistency in case of concurrent accesses.

Messages to be transmitted are updated by the application software. If a permanent Message

Object (arbitration and control bits are set during configuration) exists for the message, only

the data bytes are updated and the TxRqst bit with NewDat bit are set to start the

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