Datasheet

SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
1202
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 40-12).
Figure 40-12. Receive with Overwrite Mailbox
Chaining Mailboxes
Several mailboxes may be used to receive a buffer split into several messages with the same ID. In this case, the
mailbox with the lowest number is serviced first. In the receive and receive with overwrite modes, the field PRIOR
in the CAN_MMRx register has no effect. If Mailbox 0 and Mailbox 5 accept messages with the same ID, the first
message is received by Mailbox 0 and the second message is received by Mailbox 5. Mailbox 0 must be
configured in Receive Mode (i.e., the first message received is considered) and Mailbox 5 must be configured in
Receive with Overwrite Mode. Mailbox 0 cannot be configured in Receive with Overwrite Mode; otherwise, all
messages are accepted by this mailbox and Mailbox 5 is never serviced.
If several mailboxes are chained to receive a buffer split into several messages, all mailboxes except the last one
(with the highest number) must be configured in Receive Mode. The first message received is handled by the first
mailbox, the second one is refused by the first mailbox and accepted by the second mailbox, the last message is
accepted by the last mailbox and refused by previous ones (see Figure 40-13).
Message 1 Message 2 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
Message 4
Message 2 Message 4