MC9S08DZ96F2CLL datasheet(292/458 Pages) FREESCALE

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Chapter 12 Freescale’s Controller Area Network (S08MSCANV1)

MC9S08DZ128 Series Data Sheet, Rev. 1

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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.

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