Why is the frame rate incorrectly detected as 15 FPS (Frames Per Second) when the resolution is set to 4K and identified as a custom resolution?
This can happen if the VIC is set to 0, or to another number that is not in the cross reference. These resolution get identified as [Custom Mode] resolutions.
This issue affects all resolution that are above 3.2Gbps. This happens because for resolutions above 3.2Gbps, the TMDS clock ration is changed from 10:1 to 40:1, and the calculation for the frame rate for custom resolutions does not take this into account.
As a result the frame rate gets reported as 15 FPS instead of 60 FPS.
This issue will be fixed in the updated driver for the HDMI Receiver Subsystem v2.0 in Vivado 2016.4.
Note: Vivado 2016.3 users should see (Xilinx Answer 68153) for a patch.