Receive And Transmit Priorities; Message Acceptance Filtering - Intel 8XC196K Series User Manual

8XC196K x , J x , CA USER'S MANUAL
12.3.2.1

Receive and Transmit Priorities

The lowest-numbered message object always has the highest priority, regardless of the message
identifier. When multiple messages are ready to transmit, the CAN controller transmits the mes-
sage from the lowest-numbered message object first. When multiple message objects are capable
of receiving the same message, the lowest-numbered message object receives it. For example, if
all identifier bits are masked, message object 1 receives all messages.
12.3.2.2

Message Acceptance Filtering

The mask registers provide a method for developing an acceptance filtering strategy for a specific
system. Software can program the mask registers to require an exact match on specific identifier
bits while masking ("don't care") the remaining bits. Without a masking strategy, a message ob-
ject could accept only those messages with an identical message identifier. With a masking strat-
egy in place, a message object can accept messages whose identifiers are not identical.
The CAN controller filters messages by comparing an incoming message's identifier with that of
an enabled internal message object. The standard global mask register applies to messages with
standard (11-bit) identifiers, while the extended global mask register applies to those with extend-
ed (29-bit) identifiers. The CAN controller applies the appropriate global mask to each incoming
message identifier and checks for an acceptance match in message objects 1–14. If no match ex-
ists, it then applies the message 15 mask and checks for a match on message object 15. The mes-
sage 15 mask is ANDed with the global mask, so any bit that is masked by the global mask is
automatically masked for message 15.
The CAN controller accepts an incoming data message if the message's identifier matches that of
any enabled receive message object. It accepts an incoming remote message (request for data
transmission) if the message's identifier matches that of any enabled transmit message object.
The remote message's identifier is stored in the transmit message object, overwriting any masked
bits. Table 12-5 shows an example.
12-6
Table 12-4. Message Object Structure
†
Hex Address
1E x 7–1E x E Data Bytes 0–7
1E x 6 Message Configuration
1E x 2–1E x 5 Message Identifier 0–3
1E x 0–1E x 1 Message Control 0–1
†
x = message object number, in hexadecimal
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