Altera Corporation
2–41
October 2007
Stratix II GX Device Handbook, Volume 2
Stratix II GX Transceiver Architecture Overview
Figure 2–34. Transmitter Force Disparity Feature in Single-Width Mode
Figure 2–35
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 to maintain a neutral overall disparity.
The current running disparity at the end of time
n
+2 indicates that the
K28.5 at the low byte position in time
n
+4 should be encoded with a
positive disparity. Since the
tx_forcedisp
is high at time
n
+4, the
signal level of
tx_dispval
is used to convert the lower byte K28.5 to be
encoded as a negative disparity code word. As the upper bit of
tx_forcedisp
is low at
n
+4 the high byte K28.5 will take the current
running disparity dictated by the low byte and encode the upper byte
K28.5 with a positive disparity. If the upper bit of
tx_forcedisp
were
driven high in time
n
+4, the upper byte K28.5 in time
n
+4 will be encoded
with negative disparity.
Current Disparity
clock
tx_in[7:0]
tx_forcedisp
BC
BC
BC
BC
BC
BC
BC
tx_ctrlenable
BC
tx_out
17C
283
17C
283
283
283
17C
17C
RD-
RD+
RD+
RD-
RD+
RD-
RD+
RD-
n
n+1
n+2
n+3
n+4
n+5
n+6
n+7
tx_dispval