Virtex-5 RocketIO GTP Transceiver User Guide www.xilinx.com 101
UG196 (v1.3) May 25, 2007
Configurable 8B/10B Encoder
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. Table 6-5 shows how the TXCHARDISPMODE and
TXCHARDISPVAL ports are used to control outgoing disparity values.
Figure 6-9: 8B/10B Encoding
Transmitted
Last
a
0
b
0
c
0
d
0
e
0
i
0
f
0
g
0
h
0
j
0
a
0
b
0
c
0
d
0
e
0
i
0
f
0
g
0
h
0
j
0
a
1
b
1
c
1
d
1
e
1
i
1
f
1
g
1
h
1
j
1
UG196_c6_09_102306
TXDATA
TXDATAWIDTH = 1
8B/10B
1514131211109876543210
H
1
G
1
F
1
E
1
D
1
C
1
B
1
A
1
H
0
G
0
F
0
E
0
D
0
C
0
B
0
A
0
H
0
G
0
F
0
E
0
D
0
C
0
B
0
A
0
76543210
8B/10B
TXDATAWIDTH = 0
TXDATA
Transmitted
Last
Transmitted
First
Transmitted
First
Table 6-5: TXCHARDISPMODE and TXCHARDISPVAL vs Outgoing Disparity
TXCHARDISPMODE TXCHARDISPVAL Outgoing Disparity
00Calculated normally by the 8B/10B encoder
01
Inverts normal running disparity when
encoding TXDATA
10
Forces running disparity negative when
encoding TXDATA
11
Forces running disparity positive when
encoding TXDATA
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