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7-21
CAN SERIAL COMMUNICATIONS CONTROLLER
7.5.1 Specifying a Message Object’s Configuration
Each message object configuration register (Figure 7-12) specifies a message identifier type
(standard or extended), transfer direction (transmit or receive), and data length (in bytes).
Set the XTD bit for a message object with an extended identifier; clear it for a message with a
standard identifier. If you accidentally clear the XTD bit for a message that has an extended iden-
tifier, the CAN controller will clear the extended bits in the identification register. If you set the
XTD bit for a message object, that message object cannot receive message objects with standard
identifiers.
For a transmit message, set the DIR bit and write the number of programmed data bytes (0–8) to
the DLC field. For a receive message, clear the DIR bit. The CAN controller stores the data length
from the received message in the DLC field.
CAN_MSG
x
CFG
x
= 1–15 (87C196CB)
Address:
Reset State:
1E
x
6H (
x
= 1–F)
Unchanged
Program the CAN message object
x
configuration (CAN_MSG
x
CFG) register to specify a message
object’s data length, transfer direction, and identifier type.
7 0
87C196CB
DLC3 DLC2 DLC1 DLC0 DIR XTD — —
Bit
Number
Bit
Mnemonic
Function
7:4 DLC3:0 Data Length Code
Specify the number of data bytes this message object contains. Valid
values are 0–8. The CAN controller updates a receive message object’s
data length code after each reception to reflect the number of data bytes in
the current message.
3 DIR Direction
Specify whether this message object is to be transmitted or is to receive a
message object from a remote node.
0 = receive
1 = transmit
2 XTD Extended Identifier Used
Specify whether this message object’s identification registers contain an
extended (29-bit) or a standard (11-bit) identifier.
0 = standard identifier
1 = extended identifier
1:0 — Reserved; for compatibility with future devices, write zeros to these bits.
Figure 7-12. CAN Message Object
x
Configuration (CAN_MSGxCFG) Register