228 www.xilinx.com Virtex-6 FPGA GTX Transceivers User GuideUG366 (v2.5) January 17, 2011Chapter 4: ReceiverRX 8B/10B DecoderFunctional DescriptionMany protocols require receivers to decode 8B/10B data. 8B/10B is an industry standardencoding scheme that trades two bits of overhead per byte for improved performance.The GTX transceiver includes an 8B/10B decoder to decode RX data without consumingFPGA resources. The decoder includes status signals to indicate errors and incomingcontrol sequences. If decoding is not needed, the block can be disabled to minimize latency.8B/10B Decoder Bit and Byte Order8B/10B requires bit a0 to be received first, but the GTX transceiver always receives theright-most bit first. Consequently, the 8B/10B decoder is designed to automatically reversethe bit order of received data before decoding it. Similarly, because the GTX transceiverreceives the right-most bit first, when a 2-byte interface is used, the first byte received (byte0) is presented on RXDATA[7:0], and the second byte is presented on RXDATA[15:8].When a 4-byte interface is used, the first received byte is presented on RXDATA[23:16], andthe fourth byte is presented on RXDATA[31:24]. Figure 4-29 shows how the decoder maps10-bit data to 8-bit values.X-Ref Target - Figure 4-29Figure 4-29: RX Interface with 8B/10B Decoding15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0H1 G 1 F 1 E1 D 1 C1 B1 A 1 H0 G 0 F 0 E 0 D0 C0 B0 A0 H0 G0 F 0 E0 D0 C0 B0 A07 6 5 4 3 2 1 0RX_DATA_WIDTH = 20RXDATARX_DATA_WIDTH = 10UG366_c4_26_051509ReceivedLastReceivedLastReceivedFirstReceivedFirst8B/10Bg0h 0j 0 f 0 i 0 e0 d 0 c 0 b0 a0g 0h 0j 0 f 0 i 0 e0 d0 c0 b0 a0g1h1j1 f 1 i 1 e1 d 1 c1 b1 a115 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0H1 G1 F1 E1 D1 C1 B1 A1 H0 G0 F0 E0 D0 C0 B0 A0RX_DATA_WIDTH = 40ReceivedLastReceivedFirst8B/10Bg 0h 0j 0 f0 i 0 e 0 d 0 c 0 b0 a0g1h1j1 f 1 i 1 e1 d 1 c1 b1 a131 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16H3 G3 F3 E3 D3 C3 B3 A3 H2 G2 F2 E2 D2 C2 B2 A2RXDATAg2h2j2 f 2 i 2 e2 d2 c2 b2 a2g3h3j3 f3 i3 e3 d3 c3 b3 a 38B/10Bwww.BDTIC.com/XILINX
