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eCAN Registers
829
SPRUI07–March 2020
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Controller Area Network (CAN)
13.8.19 Mailbox Layout
The following four 32-bit registers comprise each mailbox:
• MSGID − Stores the message ID
• MSGCTRL − Defines number of bytes, transmission priority and remote frames
• CANMDL − 4 bytes of data
• CANMDH − 4 bytes of data
13.8.19.1 Message Identifier Register (MSGID)
This register contains the message ID and other control bits for a given mailbox.
Figure 13-38. Message Identifier Register (MSGID) Register
31 30 29 28 0
IDE AME AAM ID[28:0]
R/W-x R/W-x R/W-x R/W-x
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset; x = indeterminate
Note: This register can be written only when mailbox n is disabled (CANME[n] (CANME.31-0) = 0). The reset-state of IDE, AME and AAM
bits are undefined. As part of module initialization, these bits must be initialized as appropriate. Otherwise, they may assume random values
and lead to improper operation of the mailboxes.
Table 13-34. Message Identifier Register (MSGID) Field Descriptions
Bit Field Value Description
31 IDE Identifier extension bit.
1 Receive Mailbox -> The received message has an extended identifier (29 bits). Transmit Mailbox->
The message to be sent has an extended identifier (29 bits).
0 Receive Mailbox -> The received message has a standard identifier (11 bits). Transmit Mailbox->
The message to be sent has a standard identifier (11 bits).
30 AME Acceptance mask enable bit. AME is only used for receive mailboxes. This bit is not modified by a
message reception.
1 The corresponding acceptance mask is used.
0 No acceptance mask is used, all identifier bits must match to receive the message
29 AAM Auto answer mode bit. This bit is only valid for message mailboxes configured as transmit. For
receive mailboxes, this bit has no effect: the mailbox is always configured for normal receive
operation.
This bit is not modified by a message reception.
1 Auto answer mode. If a matching remote request is received, the CAN module answers to the
remote request by sending the contents of the mailbox.
0 Normal transmit mode. The mailbox does not reply to remote requests. The reception of a remote
request frame has no effect on the message mailbox.
28:0 ID[28:0] Message identifier
1 In standard identifier mode, if the IDE bit (MSGID.31) = 0, the message identifier is stored in bits
ID.28:18. In this case, bits ID.17:0 have no meaning.
0 In extended identifier mode, if the IDE bit (MSGID.31) = 1, the message identifier is stored in bits
ID.28:0.
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