Altera Stratix GX Transceiver - Manual (Page 57)

Altera Corporation 3–11
January 2005 Stratix GX Transceiver User Guide
Basic Mode
Disparity Error Detector
The 8B/10B decoder can detect disparity errors based on which 10-bit
code it received. The disparity error is indicated at the optional
rx_disperr
port. The current running disparity is based on the
disparity calculation of the last code it received. The disparity calculation
is described in
Appendix A, Data & Control Codes .
If negative disparity is calculated for the last 10-bit code, a neutral or
positive disparity 10-bit code is expected. If the decoder does not receive
a neutral or positive disparity 10-bit code as the next code word, the
rx_disperr
signal goes high, indicating that the code that was received
contained a disparity error.
If a positive disparity is calculated, the next code-group must be a neutral
or negative disparity 10-bit code. The rx_disperr signal goes high if the
code that is received is not as expected. When the rx_disperr signal
transitions high, rx_errdetect also transitions high.
Figure 3–9 shows a case where the disparity is violated. A K28.5 code has
an 8-bit value of 8'hbc and a 10-bit value (jhgfiedcba). The 10-bit value
is 10'b0011111010 (10'h17c) for RD- or 10'b1100000101 (10'h283) for RD+.
Assume that the running disparity at time n-1 is negative, so the expected
code at time n is taken from the RD- column. Because a K28.5 does not
have a balanced 10-bit code (equal number of 1’s and 0’s), the expected
RD code toggles back and forth between RD- and RD+. At time n+3, the
8B/10B decoder received a RD+ K28.5 code (10'h283), which would make
the current running disparity negative. 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 code prompts
rx_disperr
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 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.