Altera Stratix GX User Manual page 167

Figure 6–13. Disparity Error
clock
rx_out[7:0 ]
rx_disperr
rx_errdetect
rx_ctrldetect
Expected RD code
RD code received
rx_in
Altera Corporation
January 2005
At time n + 4, because the current disparity is negative, a K28.5 from the
RD- column is expected, but a K28.5 code from the RD+ is received
instead. This disparity prompts the rx_disperr signal to go high
during time n + 4 to indicate that this particular K28.5 code contained a
disparity error. The current running disparity at the end of time n + 4 is
negative because a K28.5 code from the RD+ column was received. Based
on the current running disparity at the end of time n + 5, a positive
disparity K28.5 code (from the RD-) column is expected at time n + 5.
n n+1 n+2
BC BC BC
RD- RD+ RD-
RD- RD+ RD-
17C 283 17C
Control Detect
The 8B/10B decoder differentiates between data and control codes using
the rx_ctrldetect port. Although this port is optional, there is no way
of differentiating a Dx.y code group from a Kx.y code group if the port
is unused.
Figure 6–14 shows an example waveform demonstrating the receipt of a
K28.5 code (BC + ctrl). The rx_ctrldetect=1'b1 port is aligned with
8'hbc, indicating that it is a control code. The rest of the code received is
data.
n+3 n+4 n+5
BC xx BC
RD+ RD- RD-
RD+ RD+ RD-
283 283 17C
Stratix GX Transceiver User Guide
GigE Mode
n+6 n+7
BC BC
RD+ RD-
RD+ RD-
283 17C
6–13