Decoder; Figure 2-6: 10-Bit Rx Data Map With 8B/10B Bypassed - Xilinx RocketIO X User Manual

R

Decoder

An optional 8B/10B decoder is included in the receiver. A programmable option allows the
decoder to be bypassed. When the 8B/10B decoder is bypassed, the 10-bit character order
is shown in
The decoder uses the same table (see
for Gigabit Ethernet, Fibre Channel, and InfiniBand. In addition to decoding all data and
K-characters, the decoder has several extra features. The decoder separately detects both
"disparity errors" and "out-of-band" errors. A disparity error occurs when a 10-bit character
is received that exists within the 8B/10B table, but has an incorrect disparity. An out-of-band
error occurs when a 10-bit character is received that does not exist within the 8B/10B table.
It is possible to obtain an out-of-band error without having a disparity error, or more
commonly, a disparity error is possible without an out-of-band error. The proper disparity
is always computed for both legal and illegal characters. The current running disparity is
available at the RXRUNDISP signal.
The 8B/10B decoder performs a unique operation if out-of-band data is detected. If out-of-
band data is detected, the decoder signals the error and passes the illegal 10-bits through
and places them on the outputs. This can be used for debugging purposes if desired.
The decoder also signals reception of one of the 12 valid K-characters. In addition, a
programmable comma detect is included. The comma detect signal registers a comma on
the receipt of any comma+, comma–, or both. Since the comma is defined as a 7-bit
character, this includes several out-of-band characters. Another option allows the decoder
to detect only the three defined commas (K28.1, K28.5, and K28.7) as comma+, comma–, or
both. In total, there are six possible options, three for valid commas and three for "any
comma."
Note that all bytes (1, 2, 4 or 8) at the RX FPGA interface each have their own individual
8B/10B indicators (K-character, disparity error, out-of-band error, current running
disparity, and comma detect).
During receive, while 8B/10B decoding is enabled, the running disparity of the serial
transmission can be read by the transceiver from the RXRUNDISP port, while the
RXCHARISK port indicates presence of a K-character. When 8B/10B decoding is
bypassed, these bits remain as Bits "b" and "a," respectively, of the 10-bit encoded data that
the transceiver passes on to the user logic.
8B/10B bypass.
50
Figure 2-6 for a graphical representation of the received 10-bit character.
RXCHARISK[0]
RXRUNDISP[0]
RXDATA[7] . . .
j h g
0 1 2 3
Last received

Figure 2-6: 10-Bit RX Data Map with 8B/10B Bypassed

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Chapter 2: Digital Design Considerations
. . . RXDATA[0]
f i e d c
4 5 6 7 8
First received
UG024_10b_080404
Appendix B, "8B/10B Valid
Table 2-7 illustrates the RX data map during
RocketIO™ X Transceiver User Guide
UG035 (v1.5) November 22, 2004
b a
9
Characters") that is used