Rainbow Electronics AT91CAP9S250A - Manual (Page 699)

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6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
39.7.3 CAN Controller Message Handling
39.7.3.1 Receive Handling
Two modes are available to configure a mailbox to receive messages. In Receive Mode, the
first message received is stored in the mailbox data register. In Receive with Overwrite
Mode, the last message received is stored in the mailbox.
Simple Receive Mailbox
A mailbox is in Receive Mode once the MOT field in the CAN_MMRx register has been config-
ured. Message ID and Message Acceptance Mask must be set before the 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 can be masked depending on the mailbox flag in the CAN_IMR global
register.
Message data are stored in the mailbox data register until the software application notifies that
data processing has ended. This is done by asking for a new transfer command, setting the
MTCR flag in the CAN_MCRx register. This automatically clears the MRDY signal.
The MMI flag in the CAN_MSRx register notifies the software that a message has been lost by
the mailbox. This flag is set when messages are received while MRDY is set in the
CAN_MSRx register. This flag is cleared by reading the CAN_MSRs register. A receive mail-
box prevents from overwriting the first message by new ones while MRDY flag is set in the
CAN_MSRx register. See
Figure 39-11 .
Figure 39-11. Receive Mailbox
Note: In the case of ARM architecture, CAN_MSRx, CAN_MDLx, CAN_MDHx can be read using an optimized ldm assembler
instruction.
Receive with Overwrite Mailbox
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