Unused Reference Clocks; Reference Clock Power - Xilinx Virtex-6 FPGA User Manual

Chapter 5: Board Design Guidelines

Unused Reference Clocks

It is recommended to connect the unused differential input pin clock pair (both
MGTREFCLKP and MGTREFCLKN) to ground or leave them floating.

Reference Clock Power

The GTH transceiver reference clock input circuit is powered by MGTHAVCCPLL and
MGTHAVCCRX. Excessive noise on these supplies has a negative impact on the
performance of any GTH Quad that uses the reference clock from this circuit.
Power Supplies and Filtering
This section discusses the GTH transceiver power supplies, the two main types of power
supply regulators (linear and switching), and performance degradation due to crosstalk.
Overview
The Virtex-6 FPGA GTH Quad requires four analog power supplies:
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Noise on the GTH transceiver analog power supplies can cause degradation in the
performance of the transceivers. The most likely form of degradation is an increase in jitter
at the output of the GTH transmitter and reduced jitter tolerance in the receiver. Sources of
power supply noise are:
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Each noise source must be considered in the design and implementation of the GTH
transceiver analog power supplies. The total peak-to-peak noise as measured at the input
pin of the FPGA should not exceed 10 mV
Power Supply Regulators
Normally, the GTH transceiver analog voltage supplies have local power supply
regulators that provide a final stage of voltage regulation. Preferably these regulators are
placed as close as is feasible to the GTH transceiver power supply pins. Minimizing the
distance between the analog voltage regulators and the GTH transceiver power supply
pins reduces the opportunity for noise coupling into the supply after the regulator. It also
reduces the opportunity for noise generated by current transients caused by load
dynamics.
Linear vs. Switching Regulators
The 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 must
provide adequate power to the GTH transceiver with a minimum amount of noise while
meeting the overall system thermal and efficiency requirements. There are two major types
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MGTHAVCC, at a nominal voltage level of 1.1 V
MGTHAVCCRX, at a nominal voltage level of 1.1 V
MGTHAVTT, at a nominal voltage level of 1.2 V
MGTHAVCCPLL, at a nominal voltage of 1.8 V
Power supply regulator noise
The power distribution network
Coupling from other circuits
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Virtex-6 FPGA GTH Transceivers User Guide
UG371 (v2.0) February 16, 2010