100 www.xilinx.com Virtex-5 RocketIO GTP Transceiver User Guide
UG196 (v1.3) May 25, 2007
Chapter 6: GTP Transmitter (TX)
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Description
Enabling 8B/10B Encoding
To disable the 8B/10B encoder on a given GTP transceiver, TXENC8B10BUSE must be
driven Low. To enable the 8B/10B encoder, TXENC8B10BUSE must be driven High. When
the encoder is turned off, the operation of the TXDATA port is as described in “FPGA TX
Interface.”
8B/10B Bit and Byte Ordering
The order of the bits after the 8B/10B encoder is the opposite of the order shown in
Appendix C, “8B/10B Valid Characters,” because 8B/10B encoding requires bit a0 to be
transmitted first, and the GTP transceiver always transmits the right-most bit first. To
match with 8B/10B, the 8B/10B encoder in the GTP transceiver automatically reverses the
bit order.
For the same reason, when a two-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]. This
placement ensures that the byte 0 bits are all sent before the byte 1 bits, as required by
8B/10B encoding.
TXENC8B10BUSE0
TXENC8B10BUSE1
In TXUSRCLK2
TXENC8B10BUSE is set High to enable the 8B/10B encoder.
INTDATAWIDTH must also be High.
0: 8B/10B encoder bypassed. This option reduces latency.
1: 8B/10B encoder enabled. INTDATAWIDTH must be 1.
TXKERR0[1:0]
TXKERR1[1:0]
Out TXUSRCLK2
TXKERR indicates an invalid code for a K character was
specified.
TXKERR[1] corresponds to TXDATA[15:8], and TXKERR[0]
corresponds to TXDATA[7:0].
TXRUNDISP0[1:0]
TXRUNDISP1[1:0]
Out TXUSRCLK2
TXRUNDISP indicates the current running disparity of the
8B/10B encoder. This disparity corresponds to TXDATA clocked
in several cycles earlier.
TXRUNDISP[1] corresponds to previous TXDATA[15:8] data,
and TXRUNDISP[0] corresponds to previous TXDATA[7:0] data.
Table 6-4: TX Encoder Ports (Continued)
Port Dir Clock Domain Description
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