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AR# 53107

7 Series GTX/GTH Transceivers - Recommended usage in Near End PCS loopback mode

This answer record provides guidelines to use 7 series GTX/GTH transceivers in PCS loopback mode.

In PCS loopback mode, if there is no connection from TXP/TXN to RXP/RXN, then RXRECCLK output from CDR is not expected to be good.

This is due to a lack of data transitions at the input to lock to.

If a design requires RXRECCLK for its fabric logic, the design can fail.

For these designs, it is recommended to use CDR in "lock to reference" mode.

Follow the steps below to make sure that the CDR is in "lock to reference" mode:

Step1: Drive the following signals

Set RXOUTCLKSEL to select RXOUTCLKPCS,

(OR)

Set RXCDRHOLD to 1'b1 and RXCDROVRDEN to 1'b0

Step2: Perform a full RX reset of the transceiver

Pulse GTRXRESET

If a design does not use RXRECCLK for fabric logic, only Step 2 is required.

Revision History

08/04/2014 - Updated settings required in Step1

07/10/2014 - Added GTH setting

12/18/2012 - Initial release

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