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MC9S08DZ60 Datasheet, PDF (254/396 Pages) Freescale Semiconductor, Inc – Microcontrollers | |||
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Freescaleâs Controller Area Network (S08MSCANV1)
bit position in the ï¬lter register. Finally, registers CANIDAR0/1/2/3 determine the value of those bits
determined by CANIDMR0/1/2/3.
For instance in the case of the ï¬lter value of:
0001x1001x0
The CANIDMR0/1/2/3 register would be conï¬gured as:
00001000010
and so all message identiï¬er bits except bit 1 and bit 6 would be compared against the CANIDAR0/1/2/3
registers. These would be conï¬gured as:
00010100100
In this case bits 1 and 6 are set to â0â, but since they are ignored it is equally valid to set them to â1â.
12.5.3.1 Identiï¬er Acceptance Filters example
As described above, ï¬lters work by comparisons to individual bits in the CAN message identiï¬er ï¬eld. The
ï¬lter will check each one of the eleven bits of a standard CAN message identiï¬er. Suppose a ï¬lter value of
0001x1001x0. In this simple example, there are only three possible CAN messages.
Filter value: 0001x1001x0
Message 1: 00011100110
Message 2: 00110100110
Message 3: 00010100100
Message 2 will be rejected since its third most signiï¬cant bit is not â0â - 001. The ï¬lter is simply a
convenient way of deï¬ning the set of messages that the CPU must receive. For full 29-bits of an extended
CAN message identiï¬er, the ï¬lter identiï¬es two sets of messages: one set that it receives and one set that
it rejects. Alternatively, the ï¬lter may be split into two. This allows the MSCAN to examine only the ï¬rst
16 bits of a message identiï¬er, but allows two separate ï¬lters to perform the checking. See the example
below:
Filter value A: 0001x1001x0
Filter value B: 00x101x01x0
Message 1: 00011100110
Message 2: 00110100110
Message 3: 00010100100
MSCAN will accept all three messages. Filter A will accept messages 1 and 3 as before and ï¬lter B will
accept message 2. In practice, it is unimportant which ï¬lter accepts the message - messages accepted by
either will be placed in the input buffer. A message may be accepted by more than one ï¬lter.
MC9S08DZ60 Series Data Sheet, Rev. 1 Draft E
254
PRELIMINARY
Freescale Semiconductor
Subject to Change
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