50.7.2.1 Message Acceptance Procedure

If the MIDE field in the Message ID register (CAN_MIDx) is set, the mailbox can handle the extended format identifier; otherwise, the mailbox handles the standard format identifier. Once a new message is received, its ID is masked with the Message Acceptance Mask register (CAN_MAMx) value and compared with the CAN_MIDx value. If accepted, the message ID is copied to CAN_MIDx.

If a mailbox is dedicated to receiving several messages (a family of messages) with different IDs, the acceptance mask defined in CAN_MAMx must mask the variable part of the ID family. Once a message is received, the application must decode the masked bits in CAN_MIDx. To speed up the decoding, masked bits are grouped in the Message 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 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]();