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18.7.5

Configuring a Receive Object

Table 55 shows how a Receive Object should be initialized.

The Arbitration Registers values (ID28-0 and Xtd bit) are provided by the application. They

define the identifier and type of accepted received messages. If an 11-bit Identifier (“Standard

Frame”) is used, it is programmed to ID28 - ID18. Then ID17 - ID0 can be disregarded. When

a Data Frame with an 11-bit Identifier is received, ID17 - ID0 will be set to ‘0’.

If the RxIE bit is set, the IntPnd bit will be set when a received Data Frame is accepted and

stored in the Message Object.

The Data Length Code (DLC3-0) is provided by the application. When the Message Handler

stores a Data Frame in the Message Object, it will store the received Data Length Code and

eight data bytes. If the Data Length Code is less than 8, the remaining bytes of the Message

Object will be overwritten by unspecified values.

The Mask Registers (Msk28-0, UMask, MXtd, and MDir bits) may be used (UMask=’1’) to

allow groups of Data Frames with similar identifiers to be accepted. The Dir bit should not be

masked in typical applications.

18.7.6

Handling Received Messages

The CPU may read a received message any time via the IFn Interface registers. The data

consistency is guaranteed by the Message Handler state machine.

Typically, the CPU will write first 0x007F to the Command Mask Register and then the number

of the Message Object to the Command Request Register. This combination will transfer the

whole received message from the Message RAM into the Message Buffer Register.

Additionally, the bits NewDat and IntPnd are cleared in the Message RAM (not in the Message

Buffer).

If the Message Object uses masks for acceptance filtering, the arbitration bits shows which of

the matching messages have been received.

The actual value of NewDat shows whether a new message has been received since the last

time this Message Object was read. The actual value of MsgLst shows whether more than

one message has been received since the last time this Message Object was read. MsgLst

will not be automatically reset.

Table 55. Initialization of a Receive Object

appl.

1000

appl.

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