CAN modules UM0404
448/564 DocID13284 Rev 2
If more than one transmission request is pending, they are serviced according to the priority
of the corresponding Message Object.
21.9.3 Configuration of a transmit object
The next diagram shows how a Transmit Object should be initialized.
The Arbitration Registers (ID(28:0) and Xtd bit) are given by the application. They define the
identifier and type of the outgoing message. If an 11-bit Identifier (“Standard Frame”) is
used, it is programmed to ID(28:18), while ID(17:0) can then be disregarded.
If the TxIE bit is set, the IntPnd bit will be set after a successful transmission of the
Message Object.
If the RmtEn bit is set, a matching received Remote Frame will cause the TxRqst bit to be
set; the Remote Frame will autonomously be answered by a Data Frame.
The Data Registers (DLC(3:0) and Data(7:0)) are given by the application, TxRqst and
RmtEn may not be set before the data is valid.
The Mask Registers (Msk(28:0), UMask, MXtd, and MDir bits) may be used (UMask =‘1’)
to allow groups of Remote Frames with similar identifiers to set the TxRqst bit. For details
see Reception of remote frame on page 447, and handle with care. The Dir bit should not
be masked.
21.9.4 Updating a transmit object
The CPU may update the data bytes of a Transmit Object any time via the IFx Interface
registers, neither MsgVal nor TxRqst have to be reset before the update.
Even if only a part of the data bytes are to be updated, all four bytes of the corresponding
IFx Data A Register or IFx Data B Register have to be valid before the content of that
register is transferred to the Message Object. Either the CPU has to write all four bytes into
the IFx Data Register or the Message Object is transferred to the IFx Data Register before
the CPU writes the new data bytes.
When only the (eight) data bytes are updated, first 0x0087h is written to the Command
Mask Register and then the number of the Message Object is written to the Command
Request Register, concurrently updating the data bytes and setting TxRqst.
To prevent the reset of TxRqst at the end of a transmission that may already be in progress
while the data is updated, NewDat has to be set together with TxRqst. For details see
Transmission of messages on page 446.
When NewDat is set together with TxRqst, NewDat bit will be reset as soon as the new
transmission has started.
21.9.5 Configuration of a receive object
The next diagram shows how a Receive Object should be initialized.
MsgVal Arb Data Mask EoB Dir NewDat MsgLst RxIE TxIE IntPnd RmtEn TxRqst
1 appl appl. appl 1 1 0 0 0 appl 0 appl. 0
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