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XRT74L74 Datasheet, PDF (162/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER | |||
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XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.1.1
PRELIMINARY
will not be made and bit-field â0â will be treated as a
in these comparison operations is more clearly de-
âdonât careâ. The role of these two read/write registers fined in Table 26 , on following page.
TABLE 26: ILLUSTRATION OF THE ROLE OF THE âRXCP USER CELL FILTER PATTERN HEADER BYTEâ REGISTER
AND THE âRXCP USER CELL FILTER MASK HEADER BYTEâ REGISTER.
Content of Header Byte-1 (of Incoming User Cell)
BIT 7
BIT 6
BIT 5
BIT 4
1
0
1
0
BIT 3
0
BIT 2
1
BIT 1
0
BIT 0
1
Content of âRxCP User Cell Filter Mask Header Byte-1 Register
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
1
1
1
1
0
BIT 2
0
BIT 1
0
BIT 0
0
Content of âRxCP User Cell Filter Pattern Header Byte-1 Register
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
1
1
1
1
0
BIT 2
0
BIT 1
0
BIT 0
0
Comments
BIT 7
BIT 6
BIT 5
BIT 4
Comparison is Forced (by the â1sâ in the RxCP User Cell Filter
Mask Header Byte-1 Register)
BIT 3
Donât Care
BIT 2
Donât Care
BIT 1
Donât Care
BIT 0
Donât Care
Resulting âUser Cell Fiilterâ Pattern for Header Byte-1
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
1
0
1
0
x
BIT 2
x
BIT 1
x
BIT 0
x
Based upon these register settings, any cell containing
values in the range of A0hâAFh are considered to be
matching, at the first byte. This cell will be subjected to
three (3) more tests (e.g., one for each of the remain-
ing header bytes.)
After all of these comparison tests have been
performed, a given User cell will be deemed either
âmatchingâ or ânot matchingâ the settings of the User
Cell Filter. Once the cell has been classified into one
of these two categories, its disposition (or fate) is
dependent upon the content of bit-field 5 (User Cell
Filter Discard) within the âRxCP Additional Configura-
tion Register (Address = 4Dh). If this bit-field is â0â,
then only-matching cells will be retained, and written
into the RxFIFO. All remaining User Cells will be
discarded. Conversely, it this bit-field is â1â, then only
ânon-matchingâ User Cells will be retained and written
to the RxFIFO. All âmatchingâ User Cells will be
discarded.
The bit-formats of the 8 registers that define the User
Cell Filtering criteria are presented below.
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