XRT7245
DS3 UNI FOR ATM
á
PRELIMINARY
REV. 1.03
256
The “Detection” State
When the “HEC Byte Error Detection/Correction”
algorithm is operating in the Detection mode, then
all errored cells (e.g., those cells with single-bit errors
and multi-bit errors) will be discarded, unless config-
ured otherwise by the user. The user can configure
the Receive Cell Processor to retain errored cells by
writing to bit 0 (HEC Error Ignore) of the Rx CP
Configuration register (Address = 4Ch), as described
above.
The “HEC Byte Error Correction/Detection” Algorithm
will transition back into the “Correction” state once the
Receive Cell Processor has detected “M” consecutive
cells with the correct HEC byte values. The user has
the option to use the following values for “M”: 0, 1, 3,
and 7. The user can configure the UNI to use any of
these values for M by writing the appropriate values
to the “RxCP Additional Configuration” Register
(Address = 4Dh), as depicted below.
The definition of the bits relevant to the “HEC Byte
Error Correction/Detection” algorithm follow:
Bit 1—Correction (Mode) Enable
This “Read/Write” bit field allows the user to enable/
disable the “Correction Mode” portion of the “HEC
Byte Error Correction/Detection” algorithm. If the user
writes a “0” to this bit-field, the “HEC Byte Error
Correction/Detection” algorithm will be disabled from
entry/operation in the “Correction” mode. Therefore,
the Receive Cell Processor will only operate in the
“Detection” mode. If the user writes a “1” to this bit
field then the “HEC Byte Error Correction/Detection”
algorithm will transition into and out of the “Correction”
as dictated by the “Correction Threshold”.
Bits 2 and 3—Correction Threshold [1, 0]
These “Read/Write” bit-fields allow the user to select
the “Correction” Threshold for the “HEC Byte Error
Correction/Detection” algorithm. The following table
relates the content of these bit-fields to the Correction
Threshold Value (M). Once again, M is the number of
consecutive “Error-Free” cells that the Receive Cell
Processor must detect before the “HEC Byte Correc-
tion/Detection” algorithm will allow a transition back
into the “Correction” Mode.
7.3.2.3
As mentioned earlier, the Receive Cell Processor will fil-
ter (e.g., discard) incoming cells based upon the follow-
ing criteria.
HEC Byte Errors (via the “HEC Byte Correction/
Detection” algorithm, as described in 7.3.2.2.)
Idle Cells
Header Byte Patterns—User Cells
Segment OAM Cells
Cell Filtering
Each of these cell filtering approaches are presented
below.
Filtering of Cells with HEC Byte Errors
Please see the “HEC Byte Correction/Detection”
algorithm in Section 7.3.2.2.
7.3.2.3.1
The user can configure the Receive Cell Processor
to either discard or retain Idle cells by writing to bit 4
(Idle Cell Discard) of the Rx CP Configuration Register,
as depicted below.
Idle Cell Filtering
RxCP Additional Configuration Register (Address = 4Dh)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
Unused
User Cell Filter
Discard
User Cell Filter
Enable
Correction Threshold [1, 0]
Correct
Enable
Unused
RO
RO
R/W
R/W
R/W
R/W
R/W
RO
T
ABLE
56: T
HE
R
ELATIONSHIP
BETWEEN
C
ORR
T
HRESHOLD
[1:0]
AND
THE
“C
ORRECTION
T
HRESHOLD
” V
ALUE
(M)
B
IT
3
B
IT
2
C
ORRECTION
T
HRESHOLD
V
ALUE
(M)
0
0
M = 0
0
1
M = 1
1
0
M = 3
1
1
M = 7