Virtex-6 FPGA GTX Transceivers User Guide www.xilinx.com 149UG366 (v2.5) January 17, 2011TX GearboxExternal Sequence Counter Operating ModeAs shown in Figure 3-14, the external sequence counter operating mode uses theTXSEQUENCE[6:0], TXDATA[31:0], and TXHEADER[2:0] inputs. A binary counter mustexist in the user logic to drive the TXSEQUENCE input port. For 64B/66B encoding, thecounter increments from 0 to 32 and repeats from 0. For 64B/67B encoding, the counterincrements from 0 to 66 and repeats from 0. When using 64B/66B encoding, tieTXSEQUENCE[6] to a logic 0 and tie the unused TXHEADER[2] to a logic 0. The sequencecounter increment ranges ({0 to 32}, {0 to 66}) are identical for both the 2-byte and 4-byteinterfaces. However, the counter must increment once every two TXUSRCLK2 cycles whenusing a 2-byte interface and every TXUSRCLK2 cycle when using a 4-byte interface.Due to the nature of the 64B/66B and 64B/67B encoding schemes, user data is held(paused) during various sequence counter values. Data is then paused for twoTXUSRCLK2 cycles in 2-byte mode and for one TXUSRCLK2 cycle in 4-byte mode. Validdata transfer is resumed on the next TXUSRCLK2 cycle. The data pause only applies toTXDATA and not to TXHEADER.• 64B/67B encoding: data is held (paused) for sequence counter values of 21, 44, and 65.• 64B/66B encoding, data is held (paused) at counter value 31.Figure 3-15 shows how a pause occurs at counter value 31 when using a 4-byte interface,external sequence counter mode, and 64B/66B encoding.X-Ref Target - Figure 3-14Figure 3-14: TX Gearbox in External Sequence Counter ModeData SourcePauseSequence Counter0-32 or 0-66TXSEQUENCE[6:0]TXHEADER[2:0]TXDATA[15:0] or TXDATA[31:0]TX Gearbox(in GTX Transceiver)Design in FPGA LogicUG366_c3_04_051509X-Ref Target - Figure 3-15Figure 3-15: Pause at Sequence Counter Value 31TXUSRCLK20124 25 26 27 28 29 30 31 32 0 11223 4 5 6--TXHEADER0TXSEQUENCE0TXDATA0UG366_c3_05_051509c1a9308dd38921414e658bf853365af5523cd4135714e976Pause for 1 USRCLK2 cycle. Data is ignored.120459d54e1fea877c5804988777acf1e38287118d14111a629a147087964daawww.BDTIC.com/XILINX
