STR720 datasheet(227/401 Pages) STMICROELECTRONICS

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STR720 - CONTROLLER AREA NETWORK (CAN)

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18.5.3.2 Message Object in the Message Memory

There are 32 Message Objects in the Message RAM. To avoid conflicts between CPU access

to the Message RAM and CAN message reception and transmission, the CPU cannot directly

access the Message Objects, these accesses are handled through the IFn Interface

Registers.

Table 51 provides an overview of the structures of a Message Object.

The Arbitration Registers ID28-0, Xtd, and Dir are used to define the identifier and type of

outgoing messages and are used (together with the mask registers Msk28-0, MXtd, and MDir)

for acceptance filtering of incoming messages. A received message is stored in the valid

Message Object with matching identifier and direction set to receive (Data Frame) or transmit

(Remote Frame). Extended frames can be stored only in Message Objects with Xtd set,

standard frames in Message Objects with Xtd clear. If a received message (Data Frame or

Remote Frame) matches more than one valid Message Object, it is stored into that with the

lowest message number. For details see Section “Acceptance Filtering of Received

Messages”.

Table 51. Structure of a Message Object in the Message Memory

Message Object

UMa

Msk

28-0

Dir

EoB

NewDat

gLs

RxI

TxI

Int

Pnd

Rmt

TxRq

Msg

Val

ID28-

Dir

DLC

3-0

Dat

a 0

Dat

a 1

Dat

a 2

Dat

a 3

Dat

a 4

Dat

a 5

Data

Data 7

MsgVal

Message Valid

1: The Message Object is configured and should be considered by the

Message Handler.

0: The Message Object is ignored by the Message Handler.

Note

The application software must reset the MsgVal bit of all unused

Messages Objects during the initialization before it resets bit Init

in the CAN Control Register. This bit must also be reset before the

identifier Id28-0, the control bits Xtd, Dir, or the Data Length Code

DLC3-0 are modified, or if the Messages Object is no longer

required.

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