2–78
Altera Corporation
Stratix II GX Device Handbook, Volume 2
October 2007
Receiver Modules
Manual SONET/SDH Alignment Mode (Two Consecutive 8-bit
Characters (A1A2) or Four Consecutive 8-bit Characters (A1A1A2A2))
The word aligner can be configured to align to a 16-bit word boundary in
SONET/SDH protocol mode. In the SONET/SDH protocol mode, the
word aligner either aligns to two consecutive 8-bit characters (A1A2) or
four consecutive 8-bit characters (A1A1A2A2). The
rx_a1a2size
signal
can be used to differentiate between the two and four consecutive modes.
The word aligner aligns to the A1A2 pattern when the
rx_a1a2size
signal is held low “0,” or to the A1A1A2A2 when
rx_a1a2size
is high
“1.” The
rx_a1a2sizeout
port sends the state of the
rx_a1a2size
signal as seen by the word aligner back to the PLD logic array.
Word alignment is enabled or re-enabled by the
rx_enapatternalign
signal, but the behavior is different than that described for the 7-bit or
10-bit manual mode in single-width configuration. In the 7/10-bit mode
the byte boundary can be dynamically changed if the
rx_enapatternalign
signal is held high. However, in the
SONET/SDH mode the byte boundary is locked after the first alignment
pattern is detected and aligned after the rising edge of the
rx_enapatternalign
signal. If the byte boundary changes, the
rx_enapatternalign
signal must be deasserted and reasserted to
re-enable the alignment circuit. This feature is valuable in SONET/SDH
because the data is scrambled and not encoded. The alignment pattern
can potentially exist across byte boundaries and can trigger a false
realignment. In SONET/SDH the byte boundary should be aligned and
locked at the beginning of a SONET/SDH frame since the A1A2
alignment pattern resides in the framing section at the beginning of the
transport overhead.
Initially, the word aligner locks onto the first alignment pattern detected.
In this scenario the
rx_patterndetect
signal is asserted for one clock
cycle to signify that the alignment pattern has been aligned. The
rx_syncstatus
signal is asserted for a clock cycle to signify that the
word boundary has been synchronized. After the word boundary has
been locked, regardless of whether the
rx_enapatternalign
signal is
held high or low, the
rx_syncstatus
signal asserts for one clock cycle
whenever the alignment pattern is detected across a different byte
boundary. The
rx_syncstatus
signal operates in this
resynchronization state until a rising edge is detected on
rx_enapatternalign
.
Figure 2–62
shows an example of how the word aligner signals interact in
SONET/SDH alignment mode for an A1A2 pattern. For this example, a
SONET/SDH A1A2 framing pattern uses 16'hF628
(16'b1111011000101000) with the reverse bit ordering. This option reverses