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7-27
CAN SERIAL COMMUNICATIONS CONTROLLER
3:2 MSGLST or
CPUUPD
Message Lost (Receive)
For a receive message object, the CAN controller sets this bit-pair to
indicate that it stored a new message while the NEWDAT bit-pair was still
set, overwriting the previous message.
bit 3 bit 2
0 1 no overwrite occurred
1 0 a message was lost (overwritten)
CPU Updating (Transmit)
For a transmit message object, software should set this bit-pair to indicate
that it is in the process of updating the message contents. This prevents a
remote frame from triggering a transmission that would contain invalid
data.
bit 3 bit 2
0 1 the message is valid
1 0 software is updating data
1:0 NEWDAT New Data
This bit-pair indicates whether a message object is valid (configured and
ready for transmission).
bit 1 bit 2
0 1 not ready
1 0 message object is valid
For receive message objects, the CAN peripheral sets this bit-pair when it
stores new data into the message object.
For transmit message objects, set this bit-pair and clear the CPUUPD bit-
pair to indicate that the message contents have been updated. Clearing
CPUUPD prevents a remote frame from triggering a transmission that
would contain invalid data.
During initialization, clear this bit for any unused message objects.
CAN_MSG
x
CON1 (Continued)
x
= 1–15 (87C196CB)
Address:
Reset State:
1E
x
1H (
x
= 1–F)
Unchanged
The CAN message object
x
control 1 (CAN_MSG
x
CON1) register indicates whether a message
object has been updated, whether a message has been overwritten, whether the CPU is updating the
message, and whether a transmission or reception is pending.
This register consists of four bit-pairs — the most-significant bit of each pair is in true form and the
least-significant bit is in complement form. This format allows software to set or clear any bit with a
single write operation, without affecting the remaining bits.
7 0
87C196CB
RMTPND RMTPND TX_REQ TX_REQ
MSGLST
CPUUPD
MSGLST
CPUUPD
NEWDAT NEWDAT
Bit
Number
Bit
Mnemonic
Function
Figure 7-15. CAN Message Object
x
Control 1 (CAN_MSG
x
CON1) Register (Continued)