Xilinx RocketIO User Manual

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Xilinx RocketIO User Manual Manual pdf 61 page image

RocketIO™ Transceiver User Guide www.xilinx.com 61UG024 (v2.3.2) June 24, 2004 1-800-255-77788B/10B Encoding/Decoding R8B/10B Encoding/DecodingOverviewThe RocketIO transceiver has the ability to encode eight bits into a 10-bit serial stream usingstandard 8B/10B encoding. This guarantees a DC-balanced, edge-rich serial stream, facilitating DC-or AC-coupling and clock recovery. Table 2-10, page 63, shows the significance of 8B/10B portsthat change purpose, depending on whether 8B/10B is bypassed or enabled.8B/10B EncoderA bypassable 8B/10B encoder is included in the transmitter. The encoder uses the same 256 datacharacters and 12 control characters (shown in Appendix B, “8B/10B Valid Characters”) that areused for Gigabit Ethernet, XAUI, Fibre Channel, and InfiniBand.The encoder accepts 8 bits of data along with a K-character signal for a total of 9 bits per characterapplied. If the K-character signal is High, the data is encoded into one of the twelve possibleK-characters available in the 8B/10B code. (See Table B-2, page 141.) If the K-character input isLow, the 8 bits are encoded as standard data. If the K-character input is High and a user applies otherthan one of the twelve possible combinations, TXKERR indicates the error.8B/10B DecoderAn optional 8B/10B decoder is included in the receiver. A programmable option allows the decoderto be bypassed. When it is bypassed, the 10-bit character order is as shown in Figure 2-14, page 66.The decoder uses the same table that is used for Gigabit Ethernet, Fibre Channel, and InfiniBand.The decoder separately detects both “disparity errors” and “out-of-band” errors. A disparity erroroccurs when a 10-bit character is received that exists within the 8B/10B table (Table B-1, page 133),but has an incorrect disparity. An out-of-band error occurs when a 10-bit character is received thatdoes not exist within the 8B/10B table. It is possible to obtain an out-of-band error without havinga disparity error. The proper disparity is always computed for both legal and illegal characters. Thecurrent running disparity is available at the RXRUNDISP signal.The 8B/10B decoder performs a unique operation if out-of-band data is detected. Should this occur,the decoder signals the error, passes the illegal 10 bits through, and places them on the outputs. Thiscan be used for debugging purposes if desired.The decoder also signals reception of one of the twelve valid K-characters (Table B-2, page 141) byway of the RXCHARISK port.In addition, a programmable comma detect is included. The comma detect signal RXCOMMADETregisters a comma on the receipt of any plus-comma, minus-comma, or both. Since the comma isdefined as a 7-bit character, this includes several out-of-band characters. RXCHARISCOMMAallows the decoder to detect only the three defined commas (K28.1, K28.5, and K28.7) as plus-comma, minus-comma, or both. In total, there are six possible options, three for valid commas andthree for “any comma.”Note that all bytes (1, 2, or 4) at the RX FPGA interface each have their own individual 8B/10Bindicators (K-character, disparity error, out-of-band error, current running disparity, and commadetect).