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Chapter 6: GTP Transmitter (TX)
Configurable 8B/10B Encoder
Overview
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.
Table 6-3
are mapped to 10-bit data and control sequences in 8B/10B.
Table 6-3: 8B/10B Trade-Offs
The GTP 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.
98
outlines the benefits and costs of 8B/10B.
8B/10B Benefits
• DC Balanced: No increase in bit errors due
to line charging on AC-coupled channels.
• Limited Run Lengths: The maximum
number of bits without a transition is 5,
making it easy for receivers to achieve and
maintain lock.
• Error Detection: All single-bit errors and
many multibit errors can be detected using
disparity and out of table error checking.
• Control Characters: 8B/10B allows bytes to
be marked as control characters. This
feature is heavily used in many standard
protocols.
www.xilinx.com
Appendix C
shows how 8-bit values
8B/10B Costs
• Two-bit overhead per byte: Every byte
transmitted is mapped to a 10-bit
character. As a result, 20% of the channel
bandwidth is consumed for overhead.
• Both sides of the channel must use
8B/10B: 8B/10B data must be decoded
before it can be used.
Virtex-5 RocketIO GTP Transceiver User Guide
UG196 (v1.3) May 25, 2007
R
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