AN2283 Freescale Semiconductor / Motorola, AN2283 Datasheet - Page 7

• Manufacturer Part Number: AN2283
• Description: Scalable Controller Area Network (MSCAN)
• Manufacturer: Freescale Semiconductor / Motorola

MOTOROLA
Motorola Scalable Controller Area Network (MSCAN) Interrupts
Index into table
becomes priority
value
PRIO7–PRIO0 — Local Priority
In summary, a message ready for transmission consists of an Identifier (ID),
Data Length Code (DLC), Data, and the Transmit Buffer Priority Register
(TBPR). A Transmit Header Table (see
consisting of the ID, the DLC, and a pointer to the data (*Data_Pointer) of each
particular message to be transmitted from this node (ordered from highest
priority to lowest in the table). The index value into the table would then be used
as a local priority for each message to be transmitted. This technique will
ensure that each message will have a unique “local priority”. Messages
transmitted from the same node should have different “local priorities” (TBPR)
assigned to them so messages will not lose their precedence over one another
when it comes time to transmit.
This field defines the local priority of the associated message buffer. The
local priority is used for the internal prioritization of the transmit process of
the MSCAN and is defined to be highest for the smallest binary value.
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All transmission buffers with cleared Transmitter Buffer Empty (TXE)
flags in the CAN Transmitter Flag Register (CTFLG) participate in
the prioritization immediately before the Start of Frame (SOF) is
sent.
The transmission buffer with the lowest local priority field wins
prioritization.
If more than one buffer has the same lowest priority value, the
message buffer with the lower index wins. Proper assignment of
priority levels should be done at design (referring to the message ID
strategy) to prevent this from possibly changing the intended priority
of messages transmitted.
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N
0
Table 4. Transmit Header Table
ID
DLC
*Data_Pointer
Table
4) can be implemented
Highest Priority Message
Lowest Priority Message
Transmit Structure Overview
AN2283/D
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