From the Cyclone IV device handbook:
> When you use the VREF pin as a regular input or output, you can expect
> a reduced performance of toggle rate and tCO because of higher pin
> capacitance.
Previously, we had set all HDMI_TX pins to fast output, but doing so
produces some worrying timing violations which were masked over by
relaxation of the SDC constraints. With fast output enabled, actually
fixing the timing constraints would require substantial RTL
optimization.
Instead, by only setting fast output on the VREF pins, I'm able to avoid
the glitching that would occur without any fast output pins when
displaying high clock rate line3x output, while also allowing fitter
enough flexibility to avoid timing violations.
In addition, this commit restores the previously relaxed HDMI_TX timing
constraints to those documented in the IT6613 datasheet.
This change allows highly letterboxed content (e.g. the PSP's 480x272
picture in a 720x480 frame) to be "zoomed" to a full screen picture by
treating the letterbox as horizontal and vertical backporch.
Co-authored-by: Chris Lockfort <clockfort@gmail.com>
Russell Harmon