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2.19 Mailbox Layout
2.19.1 Message Identifier Register (MSGID)
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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
This register contains the message ID and other control bits for a given mailbox.
Figure 2-29. 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).
Table 2-26. Message Identifier Register (MSGID) Field Descriptions
Bit
Field
Value
Description
31
IDE
Identifier extension bit. The characteristics of the IDE bit changes according to the value of the AMI
bit.
When AMI = 1:
1.
The IDE bit of the receive mailbox is a "don't care." The IDE bit of the receive mailbox is
overwritten by the IDE bit of the transmitted message.
2.
The filtering criterion must be satisfied in order to receive a message.
3.
The number of bits to be compared is a function of the value of the IDE bit of the transmitted
message.
When AMI = 0:
1.
The IDE bit of the receive mailbox determines the number of bits to be compared.
2.
Filtering is not applicable. The MSGIDs must match bit-for-bit in order to receive a message.
When AMI = 1:
IDE = 1: The RECEIVED message had an extended identifier
IDE = 0: The RECEIVED message had a standard identifier
When AMI = 0:
IDE = 1: The message TO BE RECEIVED must have an extended identifier
IDE = 0: The message TO BE RECEIVED must have a standard identifier.
30
AME
Acceptance mask enable bit. AME is only used for receive mailboxes. It must not be set for
automatic reply (AAM[
n
]=1, CANMD[
n
]=0) mailboxes, otherwise the mailbox behavior is undefined.
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.
SPRU074F – May 2002 – Revised January 2009
eCAN Registers
61