HC4GX15 Datasheet, PDF (310/668 Pages) Altera Corporation – HardCopy IV Device Handbook

1–46
Chapter 1: HardCopy IV GX Transceiver Architecture
Transmitter Channel Datapath
A high value on the tx_forcedisp port is the control signal to the disparity value of
the output data. The disparity value (RD+ or RD–) is indicated by the value on the
tx_dispval port. If the tx_forcedisp port is low, tx_dispval is ignored and the
current running disparity is not altered. Forcing disparity can either maintain the
current running disparity calculations if the forced disparity value (on the
tx_dispval bit) happens to match the current running disparity, or flip the current
running disparity calculations if it does not. If the forced disparity flips the current
running disparity, the downstream 8B/10B decoder might detect a disparity error.
Table 1–8 shows the tx_forcedisp and tx_dispval port values.
Table 1–8. tx_forcedisp and tx_dispval Port Values
tx_forcedisp
0
1
1
tx_dispval
X
0
1
Disparity Value
Current running disparity has no change
Encoded data has positive disparity
Encoded data has negative disparity
Figure 1–33 shows the current running disparity being altered in Basic single-width
mode by forcing a positive disparity K28.5 when it was supposed to be a negative
disparity K28.5. In this example, a series of K28.5 code groups are continuously being
sent. The stream alternates between a positive running disparity (RD+) K28.5 and a
negative running disparity (RD–) K28.5 to maintain a neutral overall disparity. The
current running disparity at time n + 3 indicates that the K28.5 in time n + 4 should be
encoded with a negative disparity. Because tx_forcedisp is high at time n + 4, and
tx_dispval is also high, the K28.5 at time n + 4 is encoded as a positive disparity
code group.
Figure 1–33. 8B/10B Encoder Force Running Disparity Operation in Single-Width Mode
clock
n n+1 n+2 n+3 n+4 n+5 n+6 n+7
tx_in[7:0]
BC
BC
BC
BC
BC
BC BC
BC
tx_ctrlenable
tx_forcedisp
tx_dispval
Current Running
Disparity
dataout[9:0]
RD- RD+ RD-
17C 283 17C
RD+ RD+ RD- RD+ RD-
283 283 17C 283 17C
Figure 1–34 shows the current running disparity being altered in Basic double-width
mode by forcing a positive disparity on a negative disparity K28.5. In this example, a
series of K28.5 are continuously being sent. The stream alternates between a positive
ending running disparity (RD+) K28.5 and a negative ending running disparity (RD–)
K28.5 as governed by the 8B/10B encoder specification to maintain a neutral overall
disparity. The current running disparity at the end of time n + 2 indicates that the
HardCopy IV Device Handbook Volume 3
© June 2009 Altera Corporation