XC886/888CLM
Controller Area Network (MultiCAN) Controller
User’s Manual
15-22
V1.3, 2010-02
MultiCAN, V1.0
15.1.6.2
Transmit Acceptance Filtering
A message is requested for transmission by setting a transmit request in the message
object that holds the message. If more than one message object have a valid transmit
request for the same CAN node, one of these message objects is chosen for
transmission, because only a single message object can be transmitted at one time on
a CAN bus.
A message object is qualified for transmission on a CAN node if the following four
conditions are are fulfilled.
1. The message object is allocated to the message object list of the CAN node.
2. Bit MOSTATn.MSGVAL is set.
3. Bit MOSTATn.TXRQ is set.
4. Bit MOSTATn.TXEN0 and MOSTATn.TXEN1 are set.
A priority scheme determines which of all qualifying message objects is transmitted first.
The following assumption is made: message object a (MOa) and message object b
(MOb) are two message objects qualified for transmission. MOb is a list successor of
MOa. This means, MOb can be reached by means of successively stepping forward in
the list, starting from a.
If both message objects belong to a different priority class (different value of bit field
MOARn.PRI), then the message object with lower MOAR.PRI value has higher transmit
priority and will be transmitted first.
If both message objects belong to the same priority class (identical PRI bit field in register
MOARn), MOa has a higher transmit priority than MOb if one of the following conditions
is fulfilled.
•
PRI = 10
B
and CAN message MOa has higher or equal priority than CAN message
MOb with respect to CAN arbitration rules (see
•
PRI = 01
B
or PRI = 11
B
(priority by list order).
The message object that is qualified for transmission and has highest transmit priority
wins the transmit acceptance filtering, and will be transmitted first. All other message
objects lose the current transmit acceptance filtering round. They get a new chance in
subsequent acceptance filtering rounds.
The priority rules are valid for normal CAN operation.
*
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