Xilinx Virtex-6 FPGA User Manual

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Xilinx Virtex-6 FPGA User Manual Manual pdf 144 page image

144 www.xilinx.com Virtex-6 FPGA GTH Transceivers User GuideUG371 (v2.0) February 16, 2010Chapter 5: Board Design GuidelinesUnused Reference ClocksIt is recommended to connect the unused differential input pin clock pair (bothMGTREFCLKP and MGTREFCLKN) to ground or leave them floating.Reference Clock PowerThe GTH transceiver reference clock input circuit is powered by MGTHAVCCPLL andMGTHAVCCRX. Excessive noise on these supplies has a negative impact on theperformance of any GTH Quad that uses the reference clock from this circuit.Power Supplies and FilteringThis section discusses the GTH transceiver power supplies, the two main types of powersupply regulators (linear and switching), and performance degradation due to crosstalk.OverviewThe Virtex-6 FPGA GTH Quad requires four analog power supplies:• MGTHAVCC, at a nominal voltage level of 1.1 VDC• MGTHAVCCRX, at a nominal voltage level of 1.1 VDC• MGTHAVTT, at a nominal voltage level of 1.2 V DC• MGTHAVCCPLL, at a nominal voltage of 1.8 VDCNoise on the GTH transceiver analog power supplies can cause degradation in theperformance of the transceivers. The most likely form of degradation is an increase in jitterat the output of the GTH transmitter and reduced jitter tolerance in the receiver. Sources ofpower supply noise are:• Power supply regulator noise• The power distribution network• Coupling from other circuitsEach noise source must be considered in the design and implementation of the GTHtransceiver analog power supplies. The total peak-to-peak noise as measured at the inputpin of the FPGA should not exceed 10 mV PK-PK.Power Supply RegulatorsNormally, the GTH transceiver analog voltage supplies have local power supplyregulators that provide a final stage of voltage regulation. Preferably these regulators areplaced as close as is feasible to the GTH transceiver power supply pins. Minimizing thedistance between the analog voltage regulators and the GTH transceiver power supplypins reduces the opportunity for noise coupling into the supply after the regulator. It alsoreduces the opportunity for noise generated by current transients caused by loaddynamics.Linear vs. Switching RegulatorsThe type of power supply regulator used can have a significant impact on the complexity,cost, and performance of the power supply circuit. A power supply regulator mustprovide adequate power to the GTH transceiver with a minimum amount of noise whilemeeting the overall system thermal and efficiency requirements. There are two major types