SAM7X128_14 Datasheet, PDF (485/662 Pages) ATMEL Corporation – ARM-based Flash MCU

Figure 36-3. Message Acceptance Procedure
CAN_MIDx
CAN_MAMx
Message Received
&
&
==
No
Yes
Message Accepted
Message Refused
CAN_MFIDx
If a mailbox is dedicated to receiving several messages (a family of messages) with different IDs, the acceptance mask
defined in the CAN_MAMx register must mask the variable part of the ID family. Once a message is received, the
application must decode the masked bits in the CAN_MIDx. To speed up the decoding, masked bits are grouped in the
family ID register (CAN_MFIDx).
For example, if the following message IDs are handled by the same mailbox:
ID0 101000100100010010000100 0 11 00b
ID1 101000100100010010000100 0 11 01b
ID2 101000100100010010000100 0 11 10b
ID3 101000100100010010000100 0 11 11b
ID4 101000100100010010000100 1 11 00b
ID5 101000100100010010000100 1 11 01b
ID6 101000100100010010000100 1 11 10b
ID7 101000100100010010000100 1 11 11b
The CAN_MIDx and CAN_MAMx of Mailbox x must be initialized to the corresponding values:
CAN_MIDx = 001 101000100100010010000100 x 11 xxb
CAN_MAMx = 001 111111111111111111111111 0 11 00b
If Mailbox x receives a message with ID6, then CAN_MIDx and CAN_MFIDx are set:
CAN_MIDx = 001 101000100100010010000100 1 11 10b
CAN_MFIDx = 00000000000000000000000000000110b
If the application associates a handler for each message ID, it may define an array of pointers to functions:
void (*pHandler[8])(void);
When a message is received, the corresponding handler can be invoked using CAN_MFIDx register and there is no need
to check masked bits:
unsigned int MFID0_register;
MFID0_register = Get_CAN_MFID0_Register();
// Get_CAN_MFID0_Register() returns the value of the CAN_MFID0 register
pHandler[MFID0_register]();
36.6.2.2 Receive Mailbox
When the CAN module receives a message, it looks for the first available mailbox with the lowest number and compares
the received message ID with the mailbox ID. If such a mailbox is found, then the message is stored in its data registers.
Depending on the configuration, the mailbox is disabled as long as the message has not been acknowledged by the
application (Receive only), or, if new messages with the same ID are received, then they overwrite the previous ones
(Receive with overwrite).
SAM7X Series [DATASHEET]
6120K–ATARM–11-Feb-14
485