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R
K Characters
The 8B/10B table includes special characters (K characters) that are often used for control
functions. To transmit TXDATA as a K character instead of regular data, the TXCHARISK
port must be driven High. If TXDATA is not a valid K character, the encoder drives
TXKERR High.
Running Disparity
8B/10B uses running disparity to balance the number of ones and zeros transmitted.
Whenever a character is transmitted, the encoder recalculates the running disparity. The
current TX running disparity can be read from the TXCHARDISP port. This running
disparity is calculated several cycles after the TXDATA is clocked into the FPGA TX
interface, so it cannot be used to decide the next value to send, as required in some
protocols.
Normally, running disparity is used to determine whether a positive or negative 10-bit
code should be transmitted next. The encoder allows the next disparity value to be
controlled directly as well, to accommodate protocols that use disparity to send control
information. For example, an Idle character sent with reversed disparity might be used to
trigger clock correction.
TXCHARDISPVAL ports are used to control outgoing disparity values.
Table 6-5: TXCHARDISPMODE and TXCHARDISPVAL vs Outgoing Disparity
TXCHARDISPMODE
Virtex-5 RocketIO GTP Transceiver User Guide
UG196 (v1.3) May 25, 2007
TXDATAWIDTH = 1
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
H
G
F
E
D
C
B
TXDATA
1
1
1
1
1
1
1
8B/10B
j
h
g
f
i
e
d
c
b
1
1
1
1
1
1
1
1
1
Transmitted
Last
Table 6-5
TXCHARDISPVAL
0
0
1
1
www.xilinx.com
A
H
G
F
E
D
C
B
A
1
0
0
0
0
0
0
0
0
a
j
h
g
f
i
e
d
c
b
a
1
0
0
0
0
0
0
0
0
0
0
Transmitted
First
Figure 6-9: 8B/10B Encoding
shows how the TXCHARDISPMODE and
Calculated normally by the 8B/10B encoder
0
Inverts normal running disparity when
1
encoding TXDATA
Forces running disparity negative when
0
encoding TXDATA
Forces running disparity positive when
1
encoding TXDATA
Configurable 8B/10B Encoder
TXDATAWIDTH = 0
7 6 5 4 3 2 1 0
H
G
F
E
D
C
B
TXDATA
0
0
0
0
0
0
0
8B/10B
j
h
g
f
i
e
d
c
0
0
0
0
0
0
0
0
Transmitted
Transmitted
Last
UG196_c6_09_102306
Outgoing Disparity
A
0
b
a
0
0
First
101
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