Gigabit Transceivers

Access to 28 high-speed serial transceiver pairs (28 Rx and 28 Tx) corresponding to GTH quads 126, 127, 128, 224, 225, 226, and 227 on the FPGA are available on the expansion connectors. MGTREFCLK1 of quad 127, MGTREFCLK0 of quad 128, MGTREFCLK1 of quad 225 and MGTREFCLK0 of quad 226 are also routed to expansion connectors. MGTREFCLK0 of quad 127 and MGTREFCLK0 of quad 225 are connected to the on-board Si5338 programmable clock.


0.1μF AC-coupling capacitors are installed between the expansion connectors and the FPGA for all MGTREFCLK signals.

AC-coupling capacitors are not installed for any of the GTH transmit or receive pairs. If AC-coupling is desired or required for the serial application, they should be installed on the peripheral side (your board).

IBERT Configuration

Xilinx provides the IBERT tool to test and experiment with gigabit transceivers.  The settings below are compatible with the XEM8350 using Vivado 2019.1:

Protocol Custom 1  
Line Rate 12.5 Gbps  
Data Width 40  
Ref Clk 100.000 MHz  
Quad Count 7  
GTX Location Protocol Selected Refclk Selection
QUAD_126 Custom 1 / 12.5Gbps MGTREFCLK0_127
QUAD_127 Custom 1 / 12.5Gbps MGTREFCLK0_127
QUAD_128 Custom 1 / 12.5Gbps MGTREFCLK0_127
QUAD_224 Custom 1 / 12.5Gbps MGTREFCLK0_225
QUAD_225 Custom 1 / 12.5Gbps MGTREFCLK0_225
QUAD_226 Custom 1 / 12.5Gbps MGTREFCLK0_225
QUAD_227 Custom 1 / 12.5Gbps MGTREFCLK0_225
QUAD_228 None None
Add RXOUTCLK Probes Unchecked  
Clock Type System Clock  
Source External  
I/O Standard LVDS  
P Package Pin AM22  
Frequency 200 MHz  
Enable DIFF Term. Unchecked  

Gigabit Transceiver IBERT Performance

Xilinx’s IBERT tool enables an automated self-measurement of a GTH channel’s eye diagram when used in a loopback mode. Eye diagrams for two different speeds were captured using this tool with a simple loopback peripheral attached to the expansion headers. While results may vary, these are typical captures and actually represent the worst case capture over all channels for the respective rates.

Note that in loopback modes, it can be helpful to disable the GBT DFE (decision feedback equalizer) to avoid over-compensation. The TX Diff Swing must also be set so that it does not over saturate the receiver.

8 Gbps / XEM8350-KU060

Measurement taken with DFE disabled, TX Diff Swing set to 600mV.

12.5 Gbps / XEM8350-KU060

Measurement taken with DFE enabled, TX Diff Swing set to 320mV and RXTERM set to 260mV.