Cross-talk square-wave?

[David Bridgham]

> Don't trust ANYTHING!  Recent Xilinx FPGAs have permanent "weak
> keepers" on all pins, they can not be turned off.
> What this is is a non-inverting receiver on the pad, that is driving
> back to the pad with about a 50K Ohm resistor.
> Plays hob with analog stuff like crystal oscillators.  The weak keeper
> would PERFECTLY explain your square wave!
> When it gets a narrow pulse to high, it holds the line high.  When it
> gets a narrow pulse to low, it will switch to holding the line low. 
> So, if you are using a Xilinx FPGA of recent vintage, or some of their
> CPLDs, they will do exactly this.

Looks like we have an explanation then.  We're using an XC7A75T-CSG324,
a Xilinx Artix 7 series FPGA.  Thank you very much.