SAM3X8E Atmel Corporation, SAM3X8E Datasheet - Page 1227

• Manufacturer Part Number: SAM3X8E
• Manufacturer: Atmel Corporation

40.8.3.2
11057A–ATARM–17-Feb-12
11057A–ATARM–17-Feb-12
Transmission Handling
A mailbox is in Transmit Mode once the MOT field in the CAN_MMRx register has been config-
ured. Message ID and Message Acceptance mask must be set before Receive Mode is enabled.
After Transmit Mode is enabled, the MRDY flag in the CAN_MSR register is automatically set
until the first command is sent. When the MRDY flag is set, the software application can prepare
a message to be sent by writing to the CAN_MDx registers. The message is sent once the soft-
ware asks for a transfer command setting the MTCR bit and the message data length in the
CAN_MCRx register.
The MRDY flag remains at zero as long as the message has not been sent or aborted. It is
important to note that no access to the mailbox data register is allowed while the MRDY flag is
cleared. An interrupt is pending for the mailbox while the MRDY flag is set. This interrupt can be
masked depending on the mailbox flag in the CAN_IMR global register.
It is also possible to send a remote frame setting the MRTR bit instead of setting the MDLC field.
The answer to the remote frame is handled by another reception mailbox. In this case, the
device acts as a consumer but with the help of two mailboxes. It is possible to handle the remote
frame emission and the answer reception using only one mailbox configured in Consumer Mode.
Refer to the section
Several messages can try to win the bus arbitration in the same time. The message with the
highest priority is sent first. Several transfer request commands can be generated at the same
time by setting MBx bits in the CAN_TCR register. The priority is set in the PRIOR field of the
CAN_MMRx register. Priority 0 is the highest priority, priority 15 is the lowest priority. Thus it is
possible to use a part of the message ID to set the PRIOR field. If two mailboxes have the same
priority, the message of the mailbox with the lowest number is sent first. Thus if mailbox 0 and
mailbox 5 have the same priority and have a message to send at the same time, then the mes-
sage of the mailbox 0 is sent first.
Setting the MACR bit in the CAN_MCRx register aborts the transmission. Transmission for sev-
eral mailboxes can be aborted by writing MBx fields in the CAN_MACR register. If the message
is being sent when the abort command is set, then the application is notified by the MRDY bit set
and not the MABT in the CAN_MSRx register. Otherwise, if the message has not been sent,
then the MRDY and the MABT are set in the CAN_MSR register.
When the bus arbitration is lost by a mailbox message, the CAN controller tries to win the next
bus arbitration with the same message if this one still has the highest priority. Messages to be
sent are re-tried automatically until they win the bus arbitration. This feature can be disabled by
setting the bit DRPT in the CAN_MR register. In this case if the message was not sent the first
time it was transmitted to the CAN transceiver, it is automatically aborted. The MABT flag is set
in the CAN_MSRx register until the next transfer command.
Figure 40-15
The first MBx message is sent, the second is aborted and the last one is trying to be aborted but
too late because it has already been transmitted to the CAN transceiver.
shows three MBx message attempts being made (MRDY of MBx set to 0).
“Remote Frame Handling” on page
1228.
SAM3X/A
SAM3X/A
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