Xilinx RocketIO User Manual

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Xilinx RocketIO User Manual Manual pdf 68 page image

68 www.xilinx.com RocketIO™ Transceiver User GuideUG024 (v3.0) February 22, 2007Chapter 2: Digital Design ConsiderationsRWhen ALIGN_COMMA_MSB is TRUE, the PCS places the comma into the mostsignificant byte (MSB) of RXDATA in the 2-byte mode. Because the PCS is optimized forthe 2-byte mode, some uncertainty exists in the 4-byte mode as to which byte will containthe comma, as shown in Table 2-12. See “Receive Data Path 32-bit Alignment” for moredetails on this case.ENPCOMMAALIGN,ENMCOMMAALIGNThese two alignment ports control how the PMA aligns incoming serial data. It can alignon a minus-comma (negative disparity), a plus-comma (positive disparity), both, orneither if comma alignment is not desired. These signals are latched inside the transceiverwith RXRECCLK.Care must be taken not to de-assert these signals at the improper time. Comma detectionmay be vulnerable to spurious realignment if RXRECCLK occurs at the wrong time. Toavoid this problem, ENPCOMMAALIGN and ENMCOMMAALIGN should be passedthrough a flip-flop that is clocked with RXRECCLK. These flip-flops should be locatednear the MGT, and RXRECCLK should use local interconnect (not global clock resources)to reduce skew. For both top and bottom edges, the best slices to use are in the CLBimmediately to the left of the transceiver, next to the bottom of the transceiver. For the topside of the chip, this is the fourth CLB row; for the bottom side, the bottom CLB row. Forexample, for the XC2VP7, here are the best slices to use for two of the transceivers:• For GT_X0Y1 (top edge), the best slices are SLICE_X15Y72 and SLICE_X15Y73.• For GT_X0Y0 (bottom edge), the best slices are SLICE_X14Y0 and SLICE_X14Y1.This must be done for each MGT. Figure 2-17 shows this recommendation.Table 2-12: Possible Locations of Comma CharacterALIGN_COMMA_MSB:Data Path Width:1 byte 2 bytes 4 bytes[7:0] [15:8] [7:0] [31:24] [23:16] [15:8] [7:0]TRUE √ √ √ √FALSE √ √ √ √ √ √ √Figure 2-17: Synchronizing Comma Align Signals to RXRECCLKGT_std_*ENPCOMMAALIGNENMCOMMAALIGNRXRECCLKDDQQPCOMMA_CONTROLMCOMMA_CONTROLUG024_39_013103Product Not Recommended for New Designs