Tx 8B/10B Encoder; Functional Description; 8B/10B Bit And Byte Ordering - Xilinx Virtex-6 FPGA User Manual
Gtx transceivers
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TX 8B/10B Encoder
Functional Description
Many protocols use 8B/10B encoding on outgoing data. 8B/10B is an industry-standard
encoding scheme that trades two bits of overhead per byte for improved performance. The
GTX transceiver includes an 8B/10B encoder to encode TX data without consuming FPGA
resources. If encoding is not needed, the block can be disabled to minimize latency.
8B/10B Bit and Byte Ordering
8B/10B encoding requires bit a0 to be transmitted first, and the GTX transceiver always
transmits the right-most bit first. To match with 8B/10B, the 8B/10B encoder in the GTX
transceiver automatically reverses the bit order
For the same reason, when a 2-byte interface is used, the first byte to be transmitted (byte 0)
must be placed on TXDATA[7:0], and the second placed on TXDATA[15:8]. When a 4-byte
interface is used, byte 0 must be placed on TXDATA[7:0], byte 1 must be placed on
TXDATA[15:8], byte 2 must be placed on TXDATA[23:16], and byte 3 must be placed on
TXDATA[31:24]. This placement ensures that the byte 0 bits are all sent before the byte 1
bits, as required by 8B/10B encoding.
X-Ref Target - Figure 3-12
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
H
G
TXDATA
1
j
h
g
1
1
1
Transmitted
Last
31 30 29 28 27 26 25 24
H
G
F
TXDATA
3
3
j
h
g
f
i
e
d
3
3
3
3
3
3
Transmitted
Last
www.BDTIC.com/XILINX
Virtex-6 FPGA GTX Transceivers User Guide
UG366 (v2.5) January 17, 2011
TX_DATA_WIDTH = 20
F
E
D
C
B
A
H
G
F
E
D
1
1
1
1
1
1
1
0
0
0
0
8B/10B
f
i
e
d
c
b
a
j
h
g
f
1
1
1
1
1
1
1
0
0
0
0
TX_DATA_WIDTH = 40
23 22 21 20 19 18 17 16
E
D
C
B
A
H
G
F
E
D
C
3
3
3
3
3
3
2
2
2
2
2
c
b
a
j
h
g
f
i
e
d
c
3
3
3
3
2
2
2
2
2
2
2
Figure 3-12: 8B/10B Encoding
www.xilinx.com
(Figure
TX_DATA_WIDTH = 10
C
B
A
TXDATA
0
0
0
0
i
e
d
c
b
a
j
0
0
0
0
0
0
0
Transmitted
Transmitted
First
Last
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
B
A
H
G
F
E
D
C
B
A
H
2
2
2
1
1
1
1
1
1
1
1
8B/10B
b
a
j
h
g
f
i
e
d
c
b
2
2
2
1
1
1
1
1
1
1
1
TX 8B/10B Encoder
3-12).
7 6 5 4 3 2 1 0
H
G
F
E
D
C
B
A
0
0
0
0
0
0
0
0
8B/10B
h
g
f
i
e
d
c
b
a
0
0
0
0
0
0
0
0
0
Transmitted
First
G
F
E
D
C
B
A
0
0
0
0
0
0
0
0
a
j
h
g
f
i
e
d
c
b
a
1
1
0
0
0
0
0
0
0
0
0
Transmitted
First
UG366_c3_02_051509
0
143
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