Datasheet

884
SAM9X35 [DATASHEET]
11055E–ATARM–10-Mar-2014
Figure 41-11.Receive Mailbox
Note: In the case of ARM architecture, CAN_MSRx, CAN_MDLx, CAN_MDHx can be read using an optimized ldm assem-
bler instruction.
Receive with Overwrite Mailbox
A mailbox is in Receive with Overwrite Mode once the MOT field in the CAN_MMRx register has been configured.
Message ID and Message Acceptance masks must be set before Receive Mode is enabled.
After Receive Mode is enabled, the MRDY flag in the CAN_MSR register is automatically cleared until the first message
is received. When the first message has been accepted by the mailbox, the MRDY flag is set. An interrupt is pending for
the mailbox while the MRDY flag is set. This interrupt is masked depending on the mailbox flag in the CAN_IMR global
register.
If a new message is received while the MRDY flag is set, this new message is stored in the mailbox data register,
overwriting the previous message. The MMI flag in the CAN_MSRx register notifies the software that a message has
been dropped by the mailbox. This flag is cleared when reading the CAN_MSRx register.
The CAN controller may store a new message in the CAN data registers while the application reads them. To check that
CAN_MDHx and CAN_MDLx do not belong to different messages, the application must check the MMI field in the
CAN_MSRx register before and after reading CAN_MDHx and CAN_MDLx. If the MMI flag is set again after the data
registers have been read, the software application has to re-read CAN_MDHx and CAN_MDLx (see Figure 41-12).
Message 1 Message 2 lost Message 3
Message 3
Message 1
Reading CAN_MSRx
Reading CAN_MDHx & CAN_MDLx
Writing CAN_MCRx
MMI
(CAN_MSRx)
MRDY
(CAN_MSRx)
CAN BUS
(CAN_MDLx
CAN_MDHx)
MTCR
(CAN_MCRx)
Message ID = CAN_MIDx