AR# 53500


Serial Transceiver - Why is REFCLK recommended over GREFCLK for any GT?


The GREFCLK input is reserved for internal testing purposes only.

It is strongly recommended not to use the GREFCLK input, but instead always use the dedicated REFCLK input pins.

The dedicated clock inputs and routing will result in a much higher quality clock, which is crucial for any GT based design.

This answer record explains some of the reasons for this recommendation.


The generation of a reference clock for any GT using the Clocking Wizard will involve the following fabric resources:

  1. MMCM powered by VCCAUX
  2. BUFG powered by VCCINT
  3. Routing resources

VCCINT powers most of the fabric resources and VCCAUX will power some I/O resources (for example, differential input buffers) along with MMCM/PLL.

Noise on these two power supplies will be design dependent.

As a result, the jitter levels on REFCLK generated via this method will be design dependent and are not possible to know deterministically.

Xilinx recommends using a clock generated on board (oscillator, clock synthesizer, etc.) as the REFCLK input to any GT.

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AR# 53500
Date 09/21/2015
Status Active
Type General Article
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