Figure 51:
Rise and Fall Times
80%
20%
T
RCLK
T
FCLK
X20933-053118
The following figure illustrates the internal details of the IBUFDS. The dedicated differential
reference clock input pair MGTREFCLKP/MGTREFCLKN is internally terminated with 100Ω
differential impedance. The common mode voltage of this differential reference clock input pair is
4/5 of MGTAVCC, or nominal 0.8V for UltraScale FPGAs. The common mode voltage for
Ult FPGAs is MGTAVCC, or nominal 0.9V. See the UltraScale and Ult device
data sheets (see
http://www.xilinx.com/documentation
) for exact specifications.
Figure 52:
GTM Transceiver Board Design Guidelines
MGTREFCLKP
50Ω
50Ω
Ult FPGAs:
MGTAVCC
MGTREFCLKN
REFCLK
to GTM Transceiver
Dedicated
Clock
Routing
X21023-060718
GTM Transceiver Reference Clock Checklist
These criteria must be met when choosing an oscillator for a design with GTM transceivers:
• Provide AC coupling between the oscillator output pins and the dedicated GTM transceiver
DUAL clock input pins.
• Ensure that the differential voltage swing of the reference clock is the range as specified in the
Ult device data sheets (see
http://www.xilinx.com/documentation
). The nominal range
is 250 mV–2000 mV and the nominal value is 1200 mV).
Chapter 5: Board Design Guidelines
UG581 (v1.0) January 4, 2019
Virtex Ult GTM Transceivers
122