2–40
Altera Corporation
Stratix II GX Device Handbook, Volume 2
October 2007
Transmitter Modules
K24.1 (
tx_datain
= 8'h38 +
tx_ctrl
= 1'b1) can be encoded to
10'b0110001100 (0x18C), which is equivalent to a D24.6+ (8'hD8 from the
RD+ column). Altera recommends that you do not send invalid control
words.
Transmitter Force Disparity
Upon power on or reset, the 8B/10B encoder has a negative disparity and
will choose the 10-bit code from the RD- column. The Transmitter Force
Disparity feature allows altering the running disparity via the
tx_forcedisp
and
tx_dispval
ports.
Two optional ports namely
tx_forcedisp
and
tx_dispval
ports are
available in 8B/10B enabled Basic single-width and Basic double-width
modes. In double-width mode, both
tx_forcedisp
and
tx_dispval
signals have two bits. The low bit is associated to the LSByte and the high
bit is associated to the MSByte. A high value on the
tx_forcedisp
bit
will change the disparity value of the data to the value indicated by the
associated
tx_dispval
bit. If the
tx_forcedisp
bit is low, then
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 down stream 8B/10B decoder may
detect a disparity error which should be tolerated by the down stream
device.
Figure 2–34
shows the current running disparity being altered in Basic
single-width mode by forcing a positive disparity on 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 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 time
n
+3 indicates that the
K28.5 in time
n
+4 should be encoded with a negative disparity. Since the
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. As the
tx_forcedisp
is low at
n
+5 the K28.5 will take the current running
disparity of
n
+4 and encode the K28.5 in time
n
+5 with a negative
disparity. If the
tx_forcedisp
were driven high at time
n
+5, that K28.5
would also be encoded with positive disparity.