Altera Stratix GX Transceiver - Manual (Page 87)

Altera Corporation 4–5
January 2005 Stratix GX Transceiver User Guide
SONET Mode
In SONET mode, the byte boundary is locked after the first comma is
detected, and the boundary is aligned after the rising edge of the
rx_enacdet[]
signal. If the byte boundary changes the
rx_enacdet[] signal must be deasserted and reasserted to reset the
alignment circuit. This feature is valuable in SONET because the data is
scrambled and not encoded. The comma can exist across byte boundaries
and can trigger a false re-alignment. In SONET, the byte boundary must
be aligned and locked at the beginning of a SONET frame, because the
A1A2 comma resides in the framing section at the beginning of the
transport overhead.
Because the SONET frame is a set size, the occurrence of the A1A2
framing bytes is anticipated. The actual A1A2 framing bytes are checked
with a counter (A1A2 framing bytes occur every 125
µs based on an STS-
1 Frame and a rate of 51.84 Mbps).
As stated earlier, at the rising edge of the rx_enacdet[], the word
aligner locks onto the first comma detected. In this scenario, the
rx_patterndetect[]
is asserted for one clock cycle to signify that the
comma has been aligned. Also, 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_enacdet[] is held high or low, the rx_syncstatus[]
signal asserts itself for one clock cycle whenever the comma is detected
across a different byte boundary. The rx_syncstatus[] operates in this
re-synchronization state until a rising edge is detected on the
rx_enacdet[]
.
Figure 4–4 shows an example of how the word aligner signals interact in
SONET alignment mode for an A1A2 pattern. In this example, a SONET
A1A2 Framing pattern is used (16'b0001010001101111). In this case,
the A1 is represented by 8'b01101111, and A2 is represented by
8'b00010100
.