134 www.xilinx.com Virtex-4 RocketIO MGT User GuideUG076 (v4.1) November 2, 2008Chapter 3: PCS Digital Design Considerations REnable serial loopback by setting LOOPBACK[1:0] to 11. The TXPOST_TAP_PD bit mustalso be set to 0. This can be set in the DRP at address 0x4C, bit 12 for MGTA, and at address0x4E, bit 12 for MGTB. The TXPOST_TAP_PD is also available as an attribute..Digital ReceiverThe MGT is equipped with a digital receiver that oversamples the incoming data for rates ator below 1.25 Gb/s. The upper limit, 1.25 Gb/s, is set by two times the VCO frequency of5 GHz divided by eight (the oversampling rate) — that is, 2 x 5 GHz / 8 = 1.25 Gb/s. Thereceiver also recovers the clock and delivers it to the fabric via the RXRECCLK1 output.Although incoming data rates up to 1.25 Gb/s can be received, the more common data rateof 0.622 Gb/s is used as an example in this section. In normal, non-oversampled mode, themaximum data rate is limited to 6.5 Gb/s.Figure 3-23 shows how a design running at 0.622 Gb/s uses the digital receiver and theresulting clocks. When using the digital receiver, the RX PLL must be locked to thereference clock, not to the incoming data. The deserializer (SIPO) runs eight times fasterthan the line rate, enabling the oversampler to capture eight samples for a single bit of data.The digital receiver then sends out parallel data synchronized with the 1X clock, a clockone-eighth of the parallel SIPO clock.Parameters for the example in Figure 3-23:• Line rate = 0.622 Gb/s• For an 8X mode, the SIPO sampling rate = 8 × 0.622 Gb/s = 4.976 Gb/s• VCO frequency = 4.976 Gb/s ÷ 2 = 2.488 GHz (SIPO uses both edges of the clock)• Effectively VCO frequency = line rate × 4• Parallel clock (PMA RXCLK0 frequency before digital receiver) = 4.976 Gb/s ÷ 40(parallel data width) = 124.4 MhzFigure 3-22: Loopback ModesTable 3-24: Loopback ModesLOOPBACK[1:0] Function00 Normal Operation (No Loopback)01 PCS Parallel10 Reserved11 Pre-driver Serial LoopbackTx PMARx PMATx PCSRx PCSFPGAI/ODigitalReceiverFabricLoopbackPCS ParallelLoopbackSerialLoopbackug076_ch_06_101205
