Virtex-4 RocketIO MGT User Guide www.xilinx.com 109UG076 (v4.1) November 2, 20088B/10B Encoding/DecodingREncoderA bypassable 8B/10B encoder is included in the transmitter. The encoder uses the same256 data characters and 12 control characters (shown in Appendix B, “8B/10B ValidCharacters”) that are used 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 percharacter applied. If the K-character signal is set to a logic 1, the data is encoded into one ofthe 12 possible K-characters available in the 8B/10B code. If the K-character input is set toa logic 0, the 8 bits are encoded as standard data.There are two ports that enable the 8B/10B encoding in the transceiver:• TXENC8B10BUSE controls whether the 8B/10B encoding block is used or not. Whenset to logic 1, the 8B/10B encoding block is used. When set to logic 0, the 8B/10Bencoding block is not used, allowing complete encoding bypass. See Table 3-4.Applications wanting a direct 10-bit interface to the MGT supply the extra bits on theTXCHARDISPMODE and TXCHARDISPVAL buses. Refer toFigure 3-10.• TXBYPASS8B10B is a byte-mapped port that is 1, 2, 4 or 8 bits wide depending on thedata width of the transceiver primitive being used. These bits correlate to each byte ofthe data path. This signal allows the data to bypass the 8B/10B encoding of thetransmitter on a clock-by-clock basis. When set to a logic 1, the 8B/10B encoding isbypassed. In this mode, the extra bits are fed through the TXCHARDISPMODE andTXCHARDISPVAL buses. Otherwise, the normal 8-, 16-, 32-, or 64-bit fabric interfacesare used. Note that running disparity is not synchronized with the encoder when thisbypass is asserted. This feature should be used only by applications that are notinterested in tracking disparity errors.
