Rainbow Electronics AT91CAP9S250A - Manual (Page 700)

700
6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
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 reg-
ister 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 39-12 ).
Figure 39-12. 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.
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