Intel 8XC196NT 87C196CB SUPPLEMENT -4. Message Object Structure, Receive and Transmit Priorities

87C196CB SUPPLEMENT

Table 7-4. Message Object Structure

Hex Address†	Contents
1Ex7–1ExE	Data Bytes 0–7
1Ex6	Message Configuration
1Ex2–1Ex5	Message Identifier 0–3
1Ex0–1Ex1	Message Control 0–1

†x = message object number, in hexadecimal

7.3.2.1Receive 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.

7.3.2.2Message 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 7-5 shows an example.

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