Table 16-33. 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.
16.9.19.2 CPU Mailbox Access
Write accesses to the identifier can only be accomplished when the mailbox is disabled (CANME[
n
]
(CANME.31-0) = 0). During access to the data field, it is critical that the data does not change while the CAN
module is reading it. Hence, a write access to the data field is disabled for a receive mailbox.
For send mailboxes, an access is usually denied if the TRS (TRS.31-0) or the TRR (TRR.31-0) flag is set. In
these cases, an interrupt can be asserted. A way to access those mailboxes is to set CDR (CANMC.8) before
accessing the mailbox data.
After the CPU access is finished, the CPU must clear the CDR flag by writing a 0 to it. The CAN module checks
for that flag before and after reading the mailbox. If the CDR flag is set during those checks, the CAN module
does not transmit the message but continues to look for other transmit requests. The setting of the CDR flag also
stops the write-denied interrupt (WDI) from being asserted.
Controller Area Network (CAN)
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
TMS320x2806x Microcontrollers
1051
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
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