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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
347 of 571
NXP Semiconductors
UM10316
Chapter 21: LPC29xx CAN 0/1
Prior to writing the first data byte into a message object, the Acceptance Filter will write
the FrameInfo byte into the according buffer location with SEM[1:0] = 01.
After having written the last data byte into the message object, the Acceptance Filter will
update the semaphore bits by setting SEM[1:0] = 11.
Before reading a message object, the CPU should read SEM[1:0] to determine the current
state of the Acceptance Filter activity therein. If SEM[1:0] = 01, then the Acceptance Filter
is currently active in this message object. If SEM[1:0] = 11, then the message object is
available to be read.
Before the CPU begins reading from the message object, it should clear SEM[1:0] = 00.
When the CPU is finished reading, it can check SEM[1:0] again. At the time of this final
check, if SEM[1:0] = 01 or 11, then the Acceptance Filter has updated the message object
during the time when the CPU reads were taking place, and the CPU should discard the
data. If, on the other hand, SEM[1:0] = 00 as expected, then valid data has been
successfully read by the CPU.
shows how software should use the SEM field to ensure that all three words
read from the message are all from the same received message.
Table 299. FullCAN semaphore operation
SEM1
SEM0
activity
0
1
Acceptance Filter is updating the content
1
1
Acceptance Filter has finished updating the content
0
0
CPU is in process of reading from the Acceptance Filter