MC9S08DZ16 FREESCALE [Freescale Semiconductor, Inc], MC9S08DZ16 Datasheet - Page 258

• Manufacturer Part Number: MC9S08DZ16
• Description: 8-Bit HCS08 Central Processor Unit (CPU)
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

Chapter 12 Freescale’s Controller Area Network (S08MSCANV1)
bit position in the filter 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 filter value of:
0001x1001x0
The CANIDMR0/1/2/3 register would be configured as:
00001000010
and so all message identifier bits except bit 1 and bit 6 would be compared against the CANIDAR0/1/2/3
registers. These would be configured as:
00010100100
In this case bits 1 and 6 are set to ‘0’, but since they are ignored it is equally valid to set them to ‘1’.
12.5.3.1
Identifier Acceptance Filters example
As described above, filters work by comparisons to individual bits in the CAN message identifier field. The
filter will check each one of the eleven bits of a standard CAN message identifier. Suppose a filter 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 significant bit is not ‘0’ - 001. The filter is simply a
convenient way of defining the set of messages that the CPU must receive. For full 29-bits of an extended
CAN message identifier, the filter identifies two sets of messages: one set that it receives and one set that
it rejects. Alternatively, the filter may be split into two. This allows the MSCAN to examine only the first
16 bits of a message identifier, but allows two separate filters 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 filter B will
accept message 2. In practice, it is unimportant which filter accepts the message - messages accepted by
either will be placed in the input buffer. A message may be accepted by more than one filter.
MC9S08DZ60 Series Data Sheet, Rev. 4
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Freescale Semiconductor