05-23-2018 07:58 AM
I'm trying to determine whether GTX receiver pins for unused GTX transceivers should be left floating or grounded. Table 5-7, "GTX/GTH Transceiver PCB Design Checklist", in "7 Series FPGAs GTX/GTH Transceivers User Guide, UG476 (v1.12) December 19, 2016", states that unused receivers that are biased should be left floating, whereas unused receivers that are unbiased should be grounded. If I bring up the transceiver wizard, the default setting for the RX termination on the "Comma Alignment and Equalization" tab indicates that the termination mode is "programmable", which is a biased receiver. Based on that, I would conclude that all unused receivers should be left floating. However, I'd like confirmation from more experienced users, so I don't fall victim to using logic to come to a false conclusion with confidence.
05-23-2018 09:20 AM
If you look on page 13 of the VC707 schematic, you'll see that the unused GTX TX & RX pins are not connected, just floating.
That's a bit of evidence to support the "let them float" argument.
05-23-2018 11:25 AM - edited 05-23-2018 01:06 PM
There is a common mode voltage associated with the receiver when it is powered. This is what is meant by biased. I tested the VC707 and the RX pins do bias when the chip is powered so if the power group for the transceiver in question is powered the RX pins should be left floating.
05-23-2018 01:32 PM
Wow, thank-you for doing that. I'm convinced at this point to leave the RX pins floating.